SILABS AN607 Humidity and temperature sensor designers guide Datasheet

AN607
Si70XX HUMIDITY
AND
TE M P E R A T U R E S E N S O R D E S I G NE R ’ S G U I D E
1. Introduction
This designer's guide describes the Si70xx family of humidity and temperature sensor products in a variety of
different applications. The first half provides a detailed description of the Si70xx family of humidity sensors
including specific application examples. System-level design considerations including sensor placement, system
calibration and product use are discussed. Finally, special handling considerations for both the sensor device and
the end product containing the sensor are discussed.
The second half begins with a brief description of humidity and the various ways it is quantified. It is important to
understand the terminology and the relative merits of each measurement. Next, methods for measuring humidity
and measurement challenges are presented. Finally, the impact of humidity on comfort is discussed.
“ Appendix A—Industry Specifications and Guidelines” contains valuable reference information about industry
standards applicable to the measurement and control of humidity. “ Appendix B—Equations for Vapor Pressure
and Humidity Calculations” contains a description of several equations useful for humidity related calculations. “
Appendix C—Term, Unit, and Coefficient Reference” contains a convenient reference containing unit conversions,
coefficients and a glossary of humidity related terms used in this document. “ Appendix D—Nonlinear Correction of
Voltage Inputs with the Si7013” explains the nonlinear correction of the Si7013 voltage input. “ Appendix E—
Thermal Model for a Sensor on a Paddle” presents a thermal model for a sensor on a paddle.
Figure 1. Si7013 Relative Humidity and Temperature Sensor Shown with and without
Hydrophobic Cover Installed
Figure 2. Si7053/4/5 Temperature Sensor
Figure 3. Si7034 Relative Humidity and Temperature Sensor
Rev. 1.7 3/15
Copyright © 2015 by Silicon Laboratories
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2. Si70xx Monolithic Humidity and Temperature Sensors
The Si70xx family uses state-of-the-art sensing technologies to create precise monolithic humidity and temperature
sensors. Temperature is sensed by a precision Vbe referenced circuit on the die. Humidity is sensed by measuring
the capacitance change of a low-k dielectric layer applied to the surface of the die. Consequently, both temperature
and humidity are precisely measured in extremely close proximity on the same monolithic device, which is critical
for accurate dew point measurement.
The Si7005 was the first-generation sensor in a 4 mm x 4 mm package. The Si7015 is a pin-compatible upgrade
(see “AN764: Upgrading from the Si7005 to the Si7015”). The Si7006, Si7007, Si7013, Si7020, Si7021, Si7022,
and Si7023 second-generation parts come in an industry-standard 3 mm x 3 mm package. The Si7034 is available
in an industry standard 2x2 mm package.
The Si7006, Si7013, Si7020, Si7021, and Si7034 are I2C parts that all have similar block diagrams. For example,
Figure 4 shows a functional block diagram of the Si7013 humidity and temperature sensor. Very few external
components are required. Communication with the device is via the I2C bus SDA and SCL pins. The Si7013 has an
optional analog input path for measurement of a remote thermistor or any other analog voltage. It also has a dual
function pin for I2C address selection and thermistor biasing. Depending on the microcontroller used in the
application, the 10 k pull-ups on the I2C bus may be included in the microcontroller. The only other component
required is the 0.1 µF power supply bypass capacitor.
VDDA
Si7013
8
TEMP
SENSOR
VINN
VDDD
9
HUMIDITY
SENSOR
VINP
VSNS
1.25V
REF
DIGITAL
LOGIC
5
2
ADC
6
7
AD0/VOUT
I2C
INTERFACE
ANALOG
INPUT
10
1
4
SCL
SDA
3
GNDA
TGND
(Exposed Pad)
GNDD
Figure 4. Si7013 Functional Block Diagram
The Si7050/3/4/5 temperature sensors are functionally identical to the I2C temperature and humidity sensors, but
there is no hole in the package, and only the temperature sensor functionality is supported.
Refer to the data sheets (e.g., Si7013) for detailed pin descriptions, register summary, and timing details.
The Si7007, Si7022 and Si7023 parts contain pulse width modulated (PWM) outputs. On one pin, the pulse width is
proportional to temperature while the pulse width on a different pin is proportional to humidity. There is a select pin
that can be used to interchange the assignment of the outputs pins. In most applications the PWM output of these
parts is filtered to give a voltage proportional to temperature or humidity. The functional block for all the PWM
output parts is the same, and this is shown in Figure 5 for the Si7022.
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Vdd
Si7022
Humidity
Sensor
1.25V
Ref
Calibration
Memory
Control Logic
ADC
SELECT
Temp
Sensor
PWM Interface
PWM1
PWM2
GND
Figure 5. Si7022 Functional Block Diagram
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2.1. Humidity Sensor Technology
Figure 6 illustrates the humidity sensor configuration. The dielectric layer is exposed to ambient air through the
package opening. Humidity in the air is exchanged with the dielectric material until equilibrium is reached. The
permittivity of the dielectric material is proportional to the amount of moisture it has absorbed. The capacitance
increases as the ambient air becomes more humid and decreases as the ambient air becomes less humid. If the
Si70xx is used at the dew point and condensation occurs on the sensor, the on-chip heater can be activated to dry
the sensor and restore operation once the sensor is above the dew point.
Moisture
from
Humid Air
Polimide Єr
A/D
Digital Output
Er increases as moisture is absorbed
Figure 6. Humidity Sensor Configuration
2.2. Temperature Sensor
The Si70xx parts have a very accurate (as good as ±0.3 °C over the full –40 to +125 °C temperature range)
temperature sensor integrated into the device. The Si70xx family of devices is designed to consume very little
power and does not artificially heat the temperature sensor unless, of course, the heater is turned on. The thermal
paddle on the bottom of the chip is electrically connected to ground inside the chip. The paddle can be soldered to
a ground plane on the PCB which can in turn be extended beyond the chip or can be left floating. In either case, the
ground pad is the thermal input to the on-chip temperature sensor. Care must be taken to isolate the chip from
thermal sources in the system in which it is included. Refer to the section on PCB layout guidelines for more details
on PCB layout and system design considerations to maximize sensor performance.
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3. System Design Considerations
3.1. Power Requirements
The Si70xx devices are inherently low power if the heater is not used. For example, Si7013 power consumption
during a conversion is 150 µA typical, and the power consumption in idle mode is less than 100 nA. This low power
consumption means that there is no significant self-heating in the conversion process. Because the Si7007,
Si7022, and Si7023 PWM output parts do humidity and temperature conversions twice a second to update the
PWM output the power consumption if these parts is higher at 180 µA typical. Generally this still does not cause
significant heating.
3.1.1. Battery Life Considerations
Normally, battery life is rated in milliamp-hours (mAh). For example, an Energizer® E91 battery is rated for
approximately 2500 mAh for light loads when allowed to discharge to 0.9 V.
Considering the case of an Si7013, which has a VDD range of 1.71 to 3.6 V, two AA batteries in series provide
3.0 V when charged and 1.8 V when fully depleted, so this is acceptable.
Again, for the case of an Si7013, a humidity and temperature conversion typically consumes 150 µA. In normal
mode, the total time for an RH and temperature conversion is 8.4 msec, and, in fast mode, this is reduced to
5.5 msec. A temperature conversion is performed every time an RH conversion is done (to allow for temperature
correction).
If a conversion is performed once per second, then 31.5x106 conversions occur per year. The number of milliamphours is as follows:
conversion current (mA)
milliamp hours (mAh) = number of conversions  conversion time (msec)  ------------------------------------------------------------------------------------- 1000msec/sec  3600sec/hour 
Plugging in values, this translates to 11.1 mA-hours for one year in normal mode and 7.2 mA-hours for fast mode.
The 100 nA quiescent current of the Si7013 will consume an additional number of milliamp hours, as shown below:
8760 (hours/year)milliamp hours (mAh) = quiescent current (nA)  ---------------------------------------------6
10 nA/mA
At 100 nA quiescent current, this is 0.876 mA-hours per year.
Even when targeting a 10-year battery life, the sensor itself would consume less than 120 mA hours of charge in
normal mode. This means that only about 5% of the available 2500 mAh is consumed by the sensor.
Of course, in a practical system, there are many other drains on a battery, and, often, coin cells, which have much
less capacity than AA cells, are used. On the other hand, it is generally acceptable to reduce the conversion rate
well below once per second, meaning that a sensor, such as the Si7013, will generally consume a small portion of
the available battery life.
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3.2. Temperature and Humidity Sensor Placement
The following are general guidelines for sensor placement:
For
accurate humidity and temperature measurement, keep heating from other components to less than
0.2 °C, or compensate for heating.
For rapid response to air temperature changes, keep the thermal mass that is attached to the sensor low,
and insulate the sensor from larger system thermal masses.
For rapid response to humidity changes, keep the sensor exposed to the ambient, or make the opening
comparable in size to the cavity in which the sensor resides.
Do not use materials, such as soft plastic, glue, or wood, in the vicinity of the humidity sensor since these
can absorb or emit moisture as well as give off fumes that affect the senor reading.
Protect the sensor against liquids and dust using the factory-installed ePTFE filter or similar cover.
Protect against ESD with exposed ground metal. For the humidity sensors, if the sensor is not in a cavity,
use the ePTFE filter cover to prevent ESD directly into the sensor area. Use conformal coating material on
the leads, or use ESD diodes on all pins. Unused pins may be connected to VDD for ESD protection.
For situations in which it is not possible to completely insulate the sensor for the system, insulate as much as
possible and use a temperature sensor connected to the system to allow compensation of residual heating. If a
thermal model can be developed for the overall system, it is also possible to apply an inverse filter and speed up
the response of the overall system to changes in the ambient. These are discussed in more detail below.
3.2.1. Place the Sensor Away from Heat Sources
As air is heated or cooled, humidity will be reduced or increased by approximately 5% per degree. That is,
increasing temperature by 1 °C will reduce the relative humidity of 100% humid air to 95%RH or 50% humid air to
47.5% RH. This “rule of thumb” is useful for estimating the effect of small temperature increases.
The first consideration in trying to measure humidity or temperature outside of an enclosure is to place the sensor
away from any heat sources internal to the enclosure or to thermally insulate the sensor from the internal heat
source so it is better connected to the ambient environment than internal heat sources. For accurate humidity
measurement, heating from other sources should be limited to no more than 0.2 °C.
In some cases, where the amount of heating is known or can be measured, it is possible to compensate for the
heating or cooling. For larger temperature increases, the “Magnus” equation can be used to more accurately
calculate the change of humidity for a change in temperature (see also appendix B).
3.2.2. Thermal Mass and Thermal Resistance
When the IC is soldered down, it becomes thermally connected to the printed circuit board on which it is mounted.
The printed circuit board is, in turn, thermally connected to the system it is mounted within. The time constant to
respond to changes in ambient air temperature depends on the effective thermal mass the device is connected to
as well as the effective thermal resistance.
If the entire system (such as a thermostat) can be expected to go up or down in temperature along with the ambient
it is measuring, then separating the sensor from the system is not necessary, but response time will generally be
slow. A general model for this is shown in Figure 7.
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Tambient
R3
Sensor
R2
R1
Tambient
System
C1
C2
Figure 7. General Thermal Model for Sensor Placement
If the system has a large thermal mass (C1) or other internal heat sources the thermal resistance from the sensor
to the system (R2) should be much larger than the thermal resistance of the sensor to ambient (R1). For rapid
response, the thermal mass connected to the sensor (C2) should be minimized. A practical example of this along
with some specific numbers is discussed in "Appendix E—Thermal Model for a Sensor on a Paddle" on page 38.
3.2.3. Optimizing Humidity Response
The humidity sensor element of the Si7006, Si7013, Si7020, Si7021, and Si7034 devices have a response time of
17–18 seconds at room temperature. For the Si7007, Si7022, and Si7023 devices, the humidity sensor response
time is six seconds at room temperature. If the humidity sensing element is placed within a cavity, this will be
affected by the following factors.
The
temperature of the cavity compared to the air temperature
As
discussed in "2.1.Humidity Sensor Technology" on page 4, as air is heated or cooled, the air humidity will
decrease or increase by 5% per degree of heating or cooling. The sensor and surrounding air must track the
ambient temperature in order to accurately measure humidity.
The
size of the cavity versus the size of the opening
Generally,
the opening should be comparable in size of the cavity in order to avoid slowing of the response.
Porous
materials within the cavity can absorb or emit humidity and dramatically affect the response.
Organic vapors, such as those from glue, can also produce a response. These effects are worse if the
cavity opening is small compared to the cavity size. The cavity should be made of materials, such as hard
plastic, metal, or hard rubber. Any sealants should be fully cured.
3.2.4. Protection Against Liquids and Dust
Like most other IC type humidity sensors, the Si70xx humidity sensors measure humidity by the change in
dielectric constant of a porous polyimide film. Conductive dust particles will cause errors in the readings,
particularly if they become lodged in the humidity sensing film. Organic solvents can cause a semi-permanent shift
in humidity readings. Even liquid water can have a permanent effect on humidity response if it leaves a mineral
residue after evaporation.
Generally, a porous cover is used to protect against dust and liquids. Typically, the cover is made of expanded
polytetrafluoroethylene (ePTFE). Polytetrafluoroethylene is often known under the name brand Teflon®, and
ePTFE is often known under the name brand Gore-Tex®.
The effectiveness of the cover is expressed by the ingress protection or IP rating. A cover with a rating of IP67 is
recommended, which means the cover is completely dustproof and can withstand water immersion to a depth of at
least one meter.
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Most of the Si70xx humidity and temperature devices are offered with a factory installed cover that is rated IP67.
The cover is solder resistant (it can withstand a peak temperature of 260 °C) and has a pore size of 0.25 µm, so it
blocks all dust while passing water vapor. The cover will block liquid water at pressure equivalent to over one meter
depth.
As a practical test of the effectiveness of the cover, Si7021 devices were subjected to a cigarette smoke test. In this
test, Si7021 devices were placed in a 3 liter jar with 20 lit cigarettes. During the time the cigarettes were burning, a
small air gap was allowed so the cigarettes would not extinguish. After burning was complete, the air gap was
removed, and the parts were allowed to sit in the smoke for 24 hours. Use of the cover reduced the maximum shift
from this extreme exposure from almost 40% RH to about 6% RH. In another test in which 100 cycles of
condensation were allowed to form on the parts, the cover reduced the shift in reading from an average of 2.0% to
1.57%. The cover has a minimal effect on response time.
3.2.5. Compensation for Heat Sources and Optimizing Transient Response
In some cases, it is not possible to completely isolate the sensor from the system for aesthetic reasons, and it is
desirable to still obtain a fast and accurate measurement of the ambient temperature and humidity by
compensating for the effects of the system.
In these cases, it is best to optimize the sensor location as much as possible using the above guidelines. To
compensate for the system effects, either the system must have a known effect, or an additional temperature
sensor (typically a thermistor) is needed. The thermistor should be placed to measure the temperature of the
system in the place where it is having the most influence on the humidity and temperature sensor.
In these cases, the Si7013 sensor is a good choice because it has an auxiliary A/D for digitizing the thermistor
voltage and a linearization engine for converting this to temperature.
3.3. Dealing with Condensation and High Humidity
Prolonged exposure to high humidity will cause gradual drift of the humidity sensor readings. All members of the
Si70xx family have on-chip heaters that can be used to heat the chip to counter local high humidity and reduce this
drift.
Condensation will also cause erroneous readings. If the condensation forms on the polyimide film, there can be
permanent shifts in sensor accuracy due to residue left after evaporation. The hydrophobic filter prevents liquid
water from penetrating, so condensation on the outside of the part will generally not result in condensation on the
polyimide sensing film of the Si70xx. However, if condensation forms on top of the filter, readings will be high until it
evaporates. Also condensation on the PCB can affect reliability and signal integrity. Turning on the heater will
reduce the chance of condensation forming and will also evaporate condensation.
However, turning on the heater will affect the local relative humidity (see also “Estimating RH with Heating” on page
28. For example, turning on the heater with the control setting 0x3 heats the sensor about 5 °C (depending on PCB
design and airflow), which results in a ~30% drop in local RH. However, due to variability in air flow and heater
current, the Si70xx heating can vary ±2 °C making RH readings with the heater on unreliable. Depending on the
nature of the application, there are several ways of dealing with this:
The
amount of heating can be measured or characterized. For example, turn the heater on and off a few
times, and use the on-chip temperature sensor to measure the amount of heating.
The air ambient temperature can be sensed with a separate sensor, and RH can be calculated
While the RH reading is not accurate, the dew point reading is fairly accurate (although generally about
1 °C low) with the heater on. If dew point is the only concern, it can be calculated from the humidity and
temperature and then subtract 1 °C.
The Si7006, Si7013, Si7020, Si7021, and Si7034 have options for increasing the heater current up to 94 mA with
VDD = 3.3 V. Depending on the PCB layout and thermal design, it is possible to get junction temperatures well in
excess of 100 °C. Shifts in sensor readings from previous exposure to high humidity can be reversed by turning on
the heater with a sufficiently high setting to get the chip temperature over 100 °C for approximately 24 hours.
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3.4. PCB Layout
The Si70xx should be thermally isolated from the equipment connected to it to prevent heat from the equipment
from affecting RH. The Si70xx should be thermally immersed in the ambient environment it is intended to sense.
One strategy for accomplishing this is to put the Si70xx on a small PCB and run a ribbon cable to the host
processor. The small PCB should be placed away from heat sources and should be placed in the ambient
environment as much as possible. That said, even with the hydrophobic filter, keeping dust, liquids and cleaning
agents away from the sensor is required.
3.5. Design and Bring-Up Checklist
Be
sure the sensor is placed away from heat sources and exposed to the environment being measured.
Prevent the active area of the sensor from being exposed to liquids, dust, and other contaminants as well
as sunlight or other UV sources. The optional filter cover available with Si70xx parts serves this purpose
and is compatible with soldering.
Generally, avoid the use of ground planes around the Si70xx, which could conduct heat from external
sources. Route the ground connection.
not connect unused pins. Make sure the CSb pin of the Si7015 is low prior to starting I2C
communications.
Do
Si7005 should not be on the same bus as other I2C devices when it is active. It acknowledges data
bytes that match its address. This issue has been resolved with other members of the Si70xx family
Be sure to meet all of the timing and level requirements of the device. The Si7005 can tolerate SDA or SCL
higher than VDD and has 8.5 mA drivers. The Si7006, Si7013, Si7020, and Si7021 have 2.5 mA drivers
The
and do not tolerate I2C pins higher than VDD. The Si7034 is a 1.8 V part but can tolerate 3.3 V on its inputs.
It's drive strength is 1.5 mA. See also application note, “AN883: Low-cost I2C Level Translator” for a low
cost I2C level translator circuit.
the I2C signals away from analog nodes and noisy digital nodes.
Use 0.1 µF bypass capacitors on VDD placed close to the sensor.
Route
careful attention to I2C protocols, such as start and stop conditions, the repeated start of a read
transaction, and proper treatment of the Acknowledge bit.
Allow adequate time for initialization (per data sheet).
If the optional thermistor sensing of Si7013 is used, make sure the thermistor is thermally isolated. If there
are long leads to the thermistor, use a twisted pair. Avoid noise pick up; use either a shield or capacitive
filter.
Pay
3.6. Si70xx Self Test
The following steps define a reliable test of the Si7006, Si7013, Si7020, Si7021, and Si7034 family that uses the
integrated heater:
1. Read and write all I2C registers checking for expected values and capability of modifying where
appropriate
2. Perform an RH and temperature measurement.
3. Turn on the heater and wait 60 seconds.
4. Check for delta temperature with heater on. This can be adjusted changing the heater setting. A setting of
0x3 will give over 3 °C.
5. Check for delta RH is > RHinitial 4x (delta temperature in °C).
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3.7. ESD Considerations
It is desirable to expose the Si70xx sensor to the environment. For the sensor to respond to the environment there
must be a way for the air being sensed to reach the sensor (the environmental access port). This means that the
sensor may also be exposed to ESD as specified in IEC 61000-4-2 with ESD peak voltage of up to ±15 kV.
For the humidity sensors, when the cover is not used and the ESD source is directly over the package opening, it is
possible for ESD to arc into the sensor area and cause damage. This can be avoided by using of the Silicon Labs
filter cover or by placing the Si70xx so that the sensor opening is offset from the environmental access port.
The above approaches will prevent ESD discharge into the sensor area, but ESD discharge to the leads may still
be possible. The best practice for ESD protection of the leads is to arrange the sensor placement and
environmental access so that high-level ESD events will preferentially be directed to ground (i.e., have an exposed
ground trace or ground shield closer to the environmental access port than the sensor). If grounded, a metal case
is used; this is also effective for ESD protection.
If it is not possible to protect the leads of the device from ESD, unused leads should be connected to VDD. Highquality ESD protection diodes can be used on leads that have signals on them. The ESD protection device should
be rated for more than 15 kV immunity and should limit ESD voltage peaks to less than 10 V. Some examples that
have been tested include Vishay MSP3V3 and COMCHIP CPDQ3V3U-HF.
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4. Humidity and Temperature Sensor Special Handling Considerations
4.1. Product Storage
The Si70xx are shipped in sealed anti-static bags. The sensors may be stored in a humidity and temperature
controlled (RH: 20% to 60%, Temp 10 °C to 35 °C) environment for up to one year after being removed from the
bag prior to assembly (the moisture sensitivity rating is MSL2). Do not store the humidity sensors in polyethylene
bags (typically blue, yellow, or pink) because these emit gases that can affect the sensor. Metallic, anti-static,
sealable, moisture-barrier bags are recommended for storage. Do not use sealants or tapes to seal inside the
packaging.
4.2. Use of Conformal Coating and Under-Fill Materials
Use of conformal coating or under-fill material is possible with the following cautions:
Conformal
coatings and under-fill material must not be allowed to come directly in contact with the humidity
sensing layer of the Si70xx as they will adhere to the polyimide film and cause a permanent shift in the
humidity sensor readings. Even very small particles can have a significant effect.
Generally, materials that outgas or give off an odor have the potential to affect sensor performance.
Following are general recommendations to avoid humidity sensor drift from fumes:
Cover
the Si70xx during the application process with a cover that forms a seal on the device such as Kapton™
KPPD-1/8 polyimide tape.
The optional Silicon Laboratories protective cover will not block fumes, but it will block liquids and particles. It may
be effective if the fume concentration is not high.
Use low volatile organic compound (VOC) materials.
Immediately cure the material in a well-ventilated environment.
We recommend that a test run be performed to measure humidity in a controlled chamber before and after the
coating process to make sure no shifts are occurring. If a humidity-controlled test chamber is not available, perform
a side-by-side test with one board coated and one board not coated. These cautions are consistent with all
polyimide-based relative humidity sensors. In general, if a process has been qualified with a similar part from a
different vendor, it will be acceptable for the Si70xx devices.
4.3. Assembly Flow
Limit soldering iron rework to five seconds per lead, for compete rework use a new sensor as manual removal and
soldering can shift senor accuracy outside of data sheet limits. Avoid the use of hot air rework tools.
5 seconds
maximum
Figure 8. Limit Solder Rework to Five Seconds or Less
The humidity sensor opening should generally be covered during soldering to prevent flux from getting on the
sensor surface. Further it is recommended that the Si70xx only be soldered with standard reflow (no hand
soldering or hot air tools). The hydrophobic filter is compatible with standard reflow soldering. If the hydrophobic
filter cover is not used, Kapton tape will serve the same purpose, although it has to be removed after soldering.
Soldering iron touch-up is possible if liquid flux is not needed and care is taken to avoid excessive heating (five
seconds per lead). If complete rework is needed, the recommended method is to use a new part and reflow the
entire board.
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The Si70xx filter cover is compatible with standard reflow soldering. It provides lifetime protection against dust and
liquids and should not be removed after soldering.
Do not remove
protective filter
Figure 9. Do Not Remove Si70xx Protective Cover
The reflow should follow JEDEC standards for lead free solder and reflow with a peak temperature of less than
260 ºC. “No clean” solder paste should be used. The use of an ultrasonic bath with alcohol for cleaning after
soldering is specifically not recommended. The Si70xx sensor opening must be kept clean and free of particulates
during assembly; the pre-installed white filter cover will protect the opening from particulates. Ensure the sensor
opening does not come into contact with conformal coatings. Do not expose the Si70xx to volatile organic
compounds or solvents. If installed, do not remove the white filter cover from the devices.
The use of water-soluble flux and water rinse after soldering is permissible if done with care. The use of DI water is
recommended. If the hydrophobic cover is used, a spray pressure of less than 40 PSI will prevent water entry into
the cover. Without the cover, care would need to be taken to avoid particles from the water or from blow drying to
contaminate the sensor area.
The high-temperature soldering process will introduce a recoverable shift in the sensor indication. Generally,
accuracy will be back within tolerance limits within 48 hours of soldering if the sensor is stored in normal ambient
conditions with ~50%RH.
4.4. Sensor Sensitivity to Chemicals and Vapors
The Si70xx is sensitive to many chemicals and fumes. Notably, household cleaning agents, such as ammonia, are
known to cause sensor readings to drift. To maintain accuracy of the SI70xx, avoid exposure to chemical fumes
and contaminants.
Inert
dust (e.g., talc) is essentially benign.
amounts of dust can slow response.
Contaminants or particles embedded in the polyimide can affect the RH accuracy.
Certain polyethylene bags will outgas and damage the sensor.
Bleach, hydrogen peroxide, ammonia, and other chemicals can affect or damage the sensor.
Excessive
4.5. Recovering Calibration after High Humidity or Chemical Exposure
Typically, initial accuracy can be recovered by baking the sensor at 125 °C for 12 hours followed by ~2 days
storage period in normal ambient conditions with ~50%RH. High RH exposure (i.e., 75% RH for 12 hours) will
accelerate the post-bake recovery, but, after high RH exposure, approximately two days at normal RH is still
recommended for the device to fully recover its accuracy.
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4.6. Relative Humidity Sensor Accuracy
To determine the accuracy of a relative humidity sensor, it is placed in a temperature and humidity controlled
chamber. The temperature is set to a convenient fixed value (typically 25–30 °C) and the relative humidity is swept
from 20 to 80% and back to 20% in the following steps: 20% – 40% – 60% – 80% – 80% – 60% – 40% – 20%. At
each set-point, the chamber is allowed to settle for a period of 30 minutes before a reading is taken from the
sensor. Prior to the sweep, the device is allowed to stabilize to 50%RH. The solid trace in Figure 10, “Measuring
Sensor Accuracy Including Hysteresis,” shows the result of a typical sweep.
Figure 10. Measuring Sensor Accuracy Including Hysteresis
The RH accuracy is defined as the dotted line shown in Figure 10, which is the average of the two data points at
each relative humidity set-point. In this case, the sensor shows an accuracy of 0.25%RH. The Si70xx accuracy
specification includes:
Unit-to-unit
and lot-to-lot variation
of factory calibration
Margin for shifts that can occur during solder reflow
The accuracy specification does not include:
Accuracy
Hysteresis
(typically ±1%)
from long term exposure to very humid conditions
Contamination of the sensor by particulates, chemicals, etc.
Other aging related shifts ("Long-term stability")
Effects
Variations
due to temperature. RH readings will typically vary with temperature by less than  0.05%  C.
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4.7. Hysteresis
The moisture absorbent film (polymeric dielectric) of the humidity sensor will carry a memory of its exposure
history, particularly its recent or extreme exposure history. A sensor exposed to relatively low humidity will carry a
negative offset relative to the factory calibration, and a sensor exposed to relatively high humidity will carry a
positive offset relative to the factory calibration. This factor causes a hysteresis effect illustrated by the solid trace
in Figure 10. The hysteresis value is the difference in %RH between the maximum absolute error on the
decreasing humidity ramp and the maximum absolute error on the increasing humidity ramp at a single relative
humidity setpoint and is expressed as a bipolar quantity relative to the average error (dashed trace). In the
example of Figure 10, the measurement uncertainty due to the hysteresis effect is ±1.0%RH.
4.8. Prolonged Exposure to High Humidity
Prolonged exposure to high humidity will result in a gradual upward drift of the RH reading. The shift in sensor
reading resulting from this drift will generally disappear slowly under normal ambient conditions. The amount of
shift is proportional to the magnitude of relative humidity and the length of exposure. In the case of lengthy
exposure to high humidity, some of the resulting shift may persist indefinitely under typical conditions. It is generally
possible to substantially reverse this effect by baking the device as described in the following section.
4.9. Bake/Hydrate Procedure
After exposure to extremes of temperature and/or humidity for prolonged periods, the polymer sensor film can
become either very dry or very wet, in each case the result is either high or low relative humidity readings. Under
normal operating conditions, the induced error will diminish over time. From a very dry condition, such as after
shipment and soldering, the error will diminish over a few days at typical controlled ambient conditions, e.g., 48
hours of 45 ≤ %RH ≤ 55. However, from a very wet condition, recovery may take significantly longer. To accelerate
recovery from a wet condition, a bake and hydrate cycle can be implemented. This operation consists of the
following steps:
the sensor at 125 °C for ≥ 12 hours
Hydration at 30 °C in 75% RH for ≥ 10 hours
Following this cycle, the sensor will return to normal operation in typical ambient conditions after 48 hours.
Baking
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5. An Introduction to Humidity
Atmospheric air normally contains water vapor and can be thought of as a mixture of ideal gasses. Dry air (no
moisture content) is the combination of approximately (on a mole basis) 78.09% N2, 20.95% O2, 0.93% Ar and
0.03% CO2 and trace elements. The amount of water vapor found in air depends on available liquid water (or ice),
temperature, pressure and ranges from nearly zero to the point of saturation called “dew point (frost point)”. Water
vapor enters the air by evaporation due to the vapor pressure of water or ice.
5.1. Vapor Pressure
There are a few key concepts to keep in mind when discussing vapor pressure. Pure water vapor pressure, p, is
due to water vapor pressure over water or ice without the presence of other gases such as air. In combination with
air, the actual water vapor pressure is increased by a dimensionless factor referred to as a water vapor
enhancement factor, f. This factor is a weak function of temperature and pressure and is approximately 0.47% at
sea level and 20 °C. The actual vapor pressure of water vapor, p', is the pure water vapor pressure, p, multiplied by
the enhancement factor, f.
p = pf = p  1.0047 
Dalton's law states that the total pressure of a mixture of gasses is equal to the sum of the partial pressures of each
component gas and assumes the combination of gasses behaves like an ideal gas. In an ideal gas mixture such as
air, the total pressure is the sum of the partial pressures of each gas. The pressure of air at any point can be
calculated as follows.
p = p N2 + p O2 + p H2O + p Ar + p CO2
Note that pH2O is the actual vapor pressure of water in air referred to as p'.
There are several useful equations when working with vapor pressure. The best one to use depends on the
available information you have about the problem you need to solve, the range of conditions for the problem the
degree of accuracy required and the computational resources available. These equations and their range of
application are discussed in detail in appendix B. Pressures referred to in this document are absolute (not gauge)
unless otherwise specified.
5.2. Temperature
The relative humidity value can change significantly with even slight variations in temperature. For example, a 1 °C
change in temperature at 35 °C and 75% relative humidity will introduce a –4% RH change. A higher temperature
increases the ability of air to absorb moisture and a lower temperature decreases the ability of air to absorb
moisture. Temperature changes can introduce moisture variations in an air mass if condensation occurs or through
secondary impacts such as changing the moisture absorption or desorption of environmental materials in an
enclosure. For humidity sensors that respond in proportion to relative humidity and not absolute humidity, the issue
of temperature measurement error is not significant unless conversion to dew point, absolute humidity or any other
measurement of water vapor in the air is required. In the case of a dew point calculation, a 1 °C error in the
measurement of the temperature will produce approximately a 1 °C error in the calculation of the dew point. This
temperature dependency not only emphasizes the importance of accurate temperature measurement, it also
highlights the necessity of thermal stability, which can be difficult to achieve. Even if the temperature and humidity
measurements are taken in relatively close proximity, there can be considerable differences in corresponding
levels of humidity and temperature. To achieve the most accurate measurement it is best if the humidity and
temperature measurements are taken as close as possible to each other—ideally co-located on the same chip.
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5.3. Evaporation
The average energy of molecules in a liquid is directly proportional to temperature. The vapor pressure of water will
increase as it is heated. When it reaches 100 °C at sea level, the vapor pressure will equal the atmospheric
pressure (approximately 760 Torr or 101.325 kPa) and the water will boil. The energy (vapor pressure) of water
molecules in an open partially filled container of liquid water (or ice), Figure 11, is such that some of the molecules
have enough energy to escape the attractive forces holding the water together and evaporate into the atmosphere.
Eventually all of the water will evaporate assuming the air is not saturated. If the container is sealed, Figure 12,
water molecules will evaporate into the air space above the water and some of the evaporated water molecules will
condense back into the liquid water. The rate of evaporation will exceed the rate of condensation until the air above
the water becomes saturated with water vapor. Once the air is saturated the rates of evaporation and condensation
will be equal (the vapor pressure will equal the saturation pressure inside the closed container). Evaporation takes
place at the surface of the liquid while boiling can take place throughout the volume of the liquid.
Evaporation > Condensation
Humidity in the open air is increasing
Figure 11. Water in Open Container
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Evaporation = Condensation
Saturated vapor
Figure 12. Water in Closed Container
Steam
p’>P
Evaporation >>> Condensation
Humidity in the open air is increasing
Figure 13. Water Boiling
Boiling starts at the liquid surface when the vapor pressure equals the atmospheric pressure. As temperature
increases, bubbles form below the surface as the vapor pressure increases to equal the atmospheric pressure plus
the additional pressure due to the weight of the column of liquid above the bubble. As temperature and vapor
pressure continue to increase, bubbles will form throughout the entire volume of liquid as shown in Figure 13.
Water vapor is completely absorbed into the air which can cause the air in close proximity to the boiling water to
become locally saturated. Visible steam consists of very small water droplets which have condensed out of this
localized super-saturated air.
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6. How Humidity is Quantified
Humidity represents the amount of water vapor contained in the air and can be quantified in many ways. Several of
the terms describing humidity are defined differently for meteorology applications and thermodynamics or chemical
engineering applications. For this reason, it is important to understand the context of the application. The following
is a brief description of the most common terms for quantifying humidity.
6.1. Absolute Humidity
Absolute humidity in the context of meteorological applications, sometimes referred to as “volumetric humidity”, is
defined as the mass of water vapor dissolved in a total volume of moist air at a given temperature and pressure.
Typical units are g/m3 or kg/m3. The value of absolute humidity defined in this manner changes with temperature
and pressure and is inconvenient to use in many engineering applications.
Absolute humidity for use in thermodynamics or chemical engineering applications is defined as the ratio of water
vapor mass to dry air mass. Typical units are kg/kg. Other names for this ratio include mass mixing ratio, humidity
ratio, mass fraction or mixing ratio. This quantity is simpler to use and more accurate for mass balance or heat
calculations. Due to the conflicting definitions of absolute humidity, caution is required when using this term.
6.2. Specific Humidity
Specific humidity, Yw, can be defined for meteorology applications as the ratio of water vapor mass per mass of
moist air expressed as g/kg or kg/kg. Specific humidity is constant with changes in temperature and pressure for
conditions above the dew point and is a useful quantity in meteorology. The rate of evaporation of water is directly
proportional to specific humidity.
6.3. Relative Humidity
Relative humidity is the ratio of actual water vapor present in air with the amount of water vapor that would be
present in air at saturation, expressed as a percentage. The official symbol for relative humidity is  although RH,
%RH, rh, or %rh are commonly used. Relative humidity can be expressed as the ratio of the actual vapor pressure,
p', to the saturation vapor pressure, ps’, at a constant temperature over a plane of liquid water.
RH (%) = p'   p s'   100
6.4. Dew Point
Upon heating, the capacity of air to absorb moisture increases. Consequently, the relative humidity of air
decreases as the air is heated. Conversely, as moist air is cooled, its capacity to absorb moisture decreases and
relative humidity increases. The dew point is the temperature, assuming constant pressure, moist air is saturated
(reaches 100% relative humidity) and cannot absorb additional water. As the temperature is decreased past the
dew point, moisture condenses until the air is saturated (reaches 100% relative humidity) at the new lower
temperature.
6.5. Frost Point
Frost point is the same as dew point over solid ice where the condensate is frost instead of liquid water.
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7. Humidity Measurement
For relative humidity measurements, it is not necessary to measure the ambient temperature unless you are using
a psychrometer. However, to determine the dew point or absolute humidity, the ambient air temperature is required.
Accurate air temperature measurements can be a significant challenge, since air is a poor thermal conductor and
the temperature at any given point can be impacted by air currents and temperature gradients. It is very important
to understand the dynamics of your measurement system and the dynamics of the environment being measured.
Before taking a measurement, you always need to wait long enough to ensure the temperature and humidity are
stable and the sensor(s) of the measurement instrument are in equilibrium with the ambient conditions to be
measured.
A wide range of techniques are employed to measure humidity. These range from simple mechanical indicators, to
highly complex and expensive analytical instruments. In general, measuring humidity (dew point, absolute
humidity, specific humidity, mixing ratio, relative humidity or equivalent wet bulb temperature) is not a trivial task.
Many of the instruments currently available have poor accuracy, narrow-bandwidth, contamination issues,
hysteresis and measurement drift over temperature and time. Regular calibration is required on some instruments,
which is both inconvenient and expensive. Some instruments are large, awkward, and expensive. Discussed
below, are different methods of humidity sensing. The new generation of humidity sensor technology used in
Silicon Labs’ solid state humidity sensors provide superior accuracy, minimum drift, low cost, low power, small size
and ease of use.
7.1. Psychrometer
A psychrometer is the oldest method for measuring humidity more commonly known as the wet bulb/dry bulb
method. A psychrometer consists of two thermometers, one with an ordinary dry bulb and the other with a moist
cloth covering the bulb (wet bulb). Evaporation from the moist cloth lowers the wet bulb thermometer's
temperature. The wet-bulb thermometer shows a lower temperature, Twb, than the dry-bulb thermometer, Tdb, if
the air is not saturated with water vapor. The temperatures are the same in saturated air. The amount of
evaporation is dependent on the relative humidity of the air (more evaporation and lower Twb with lower RH).
Given the dry bulb and wet bulb temperatures the relative humidity is looked up on a psychrometric chart. For
example, at sea level if Tdb = 25 °C and Twb = 18 °C, RH = ~50%. Looking up the %RH on a chart for every
measurement is both time-consuming and cumbersome but can be automated with a microcontroller. A
psychrometric sensor can achieve good precision with %RH resolutions of 0.01% for humidity ranges from
10–100% at temperatures from 0 to 60 °C, and accuracy of 1%. The disadvantages of a psychrometric sensor are
a slow response time, large physical size, the need to keep one thermometer bulb wet, the need to have airflow
around the wet bulb, and high cost.
7.2. Chilled Mirror Hygrometers
The chilled mirror hygrometer is considered the most accurate and reliable hygrometer. Chilled mirror hygrometers
use a cooled mirror with an optoelectronic mechanism to detect condensation on the mirror surface at an
accurately measured temperature. The system is configured to reflect LED light off a mirror at an angle of
approximately 45 degrees with a photo-transistor detecting the reflected light. The temperature of the mirror is
electronically controlled, typically with a Peltier-effect device. The system works by cooling the mirror's surface
below ambient temperature until condensation forms, causing the LED's light to scatter resulting in a sudden drop
in the output of the photo-transistor. The surface temperature of the mirror is read using an accurate temperature
sensor such as a thermistor. The temperature at which condensation forms is the dew point. All humidity values
can be calculated from the dew point. The mirror temperature can be controlled with a feedback loop to
continuously track the dew point. The chilled mirror is the most stable and accurate method to determine relative
humidity. However, it is crucial to keep the mirror clean, provide a method of clearing the condensation and to
ensure that the temperature sensor and mirror are of high quality. This method operates over the full humidity
range (0-100%RH) and can be used for numerous gases at many pressures. Chilled mirror hygrometer
instruments are bulky and very expensive.
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7.3. Mechanical Hygrometers
Mechanical hygrometers use sensing elements relying on a mechanical property of the sensor varying with
humidity. The most common example is the animal hair hygrometer, which uses a piece of animal hair kept under
tension. As humidity increases the hair becomes more flexible and stretches. A strain gage monitors the
displacement caused by the hair stretching with a change in the moisture content of the air. The output of the strain
gage is directly proportional to the relative humidity and is usually indicated on a mechanical meter. Mechanical
hygrometers are generally compact, light weight, reliable, and inexpensive. Accuracies, however are in the ±10%
range.
7.4. Electronic Humidity Sensors
Electronic humidity sensors typically use either a change in resistance or capacitance to measure humidity. These
sensors have become a popular choice because technology advances have made them accurate, compact, stable
and low power. A capacitive sensor consists of two electrodes, separated by a dielectric. As the water vapor in air
increases or decreases, the sensor's dielectric constant changes producing a higher (or lower) capacitance
measurement corresponding to the humidity level. A resistive sensor consists of two electrodes, separated by a
conductive layer. As the humidity in the air increases (or decreases) the conductivity of the sensing layer changes,
altering the resistance between the two electrodes. New techniques for producing thin films have made these types
of sensors, accurate, stable, and easy to manufacture in large quantities. The choice of material assures fast
response times with little hysteresis. The accuracy of electronic sensors is limited by their drift over time caused by
wide variations in temperature and humidity or the presence of pollutants.
The Si70xx humidity senors use a MiM capacitor as the reference and perform a high-accuracy 24-bit conversion
using a sigma delta conversion approach. Each part is factory calibrated for capacitance to RH with a minimum
offset and slope correction. Later members of the family include non-linearity correction and temperature
compensation.
Silicon Laboratories humidity sensors use the capacitance change due to moisture absorption of a polyimide film to
sense humidity. The polyimide film is deposited over a metal finger capacitor and exposed to the ambient via an
opening in the package. The polyimide material and sensing capacitor has been selected for excellent stability. An
optional expanded polytetrafluoroethylene (ePTFE) hydrophobic filter provides protection against dust and most
liquids.
The polyimide film is thin (<5 µm) and very responsive to humidity (response time of less than 10 seconds). The
hydrophobic filter has little impact on the response time.
While the Si70xx sensors are largely conventional, mixed-signal CMOS integrated circuits, relative humidity
sensors in general, and those based on capacitive sensing using polymeric dielectrics in particular, have unique
application and use requirements that are not common to conventional (non-sensor) ICs. Chief among those are:
The
need to protect the sensor during board assembly, i.e., solder reflow, and the need to subsequently
Rehydrate the sensor.
The need to protect the senor from damage or contamination during the product life-cycle.
The impact of prolonged exposure to extremes of temperature and/or humidity and their potential effect on
Sensor accuracy.
The effects of humidity sensor “memory”.
Each of these items is discussed in more detail in the following sections.
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8. Humidity Control for Thermal Comfort
8.1. General Considerations
Humans are sensitive to humid air, because the human body uses evaporative cooling as the primary mechanism
to regulate temperature. When the relative humidity and dew point are high, the rate of perspiration evaporation
from the skin decreases because the amount of water vapor in the air is already close to saturation. Because
humans perceive the rate of heat transfer from the body, rather than temperature itself, we feel warmer when the
relative humidity is high. Relative humidity is a useful indication of how hot the weather “feels” and is more intuitive
and easier to measure than other quantifications of water vapor in the air. Air conditioners are designed to maintain
between 40–60% relative humidity in the occupied space.
Table 1. Human Reaction to Humidity*
Dew Point Temp (°F)
RH at 90°F
Human Perception
> 75
> 62%
Extremely uncomfortable
70 to 74
52% to 60%
Quite uncomfortable
65 to 69
44% to 52%
Somewhat uncomfortable
60 to 64
37% to 46%
Comfortable but humid
55 to 59
31% to 41%
Comfortable
50 to 54
31% to 37%
Very comfortable
< 49
< 30%
A bit dry
*Note: See Lawrence, Mark G., “The Relationship between Relative Humidity and the Dewpoint Temperature in Moist Air—A
Simple Conversion and Applications”, American Meteorological Society February 2005.
Humans tend to react with discomfort to dew points >61 °F (16 °C). The body perspires and produces sweat to
cool down. High relative humidity and consequently a high dew point prevent the evaporation of sweat and reduce
evaporative cooling. The body may overheat, resulting in discomfort.
Lower dew points, <50 °F (10 °C), generally correlate with lower ambient temperatures requiring the body to be
less dependent on evaporative cooling. A lower dew point can be achieved with a high temperature at extremely
low relative humidity allowing for effective cooling.
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8.2. Heat Index
The heat index (HI), also called “apparent temperature” combines air temperature and relative humidity to estimate
how “hot” a human will perceive the ambient conditions to be. The heat index is calculated for ambient
temperatures above 27 °C (81 °F) and dew points above 12 °C (54 °F) (relative humidity above 40%). Heat index
is calculated with the following equation2.
–3 2
HI = – 42.379 +  2.04901523T  +  10.14333127RH  –  0.22475541TRH  –  6.83783  10 T 
–2
2
–3 2
–4
2
–6 2
2
–  5.481717  10 RH  +  1.22874  10 T RH  +  8.5282  10 TRH  –  1.99  10 T RH 
Where:
HI = Head Index in degress Farenheit
T = Ambient temperature in degrees Farenheit
RH = Relative humidity in %
1. www.shorstmeyer.com/wxfaqs/humidity/humidity.html
2. www.srh.noaa.gov/images/epz/wxcalc/heatIndex.pdf
3. www.crh.noaa.gov/pub/heat.php
8.3. Wind Chill
Wind chill, also called wind chill temperature, wind chill factor or wind chill index, expresses the perceived air
temperature on skin exposed to wind. Wind chill is defined for temperatures at or below 10 °C (50 °F) and wind
speeds greater than 4.8 kilometers/hour (3.0 mph). Wind chill is calculated by the following equations4.
T wc = 35.74 + 0.6215T a – 35.75V
0.16
+ 0.4275T a V
0.16
Where:
T wc = Wind chill temperature in degrees Farenheit
T a = Ambient temperature in degrees Farenheit
V = Wind velocity in miles-per-hour
Table 2. Human Reaction to Heat Index, HI, in Shady Light Wind Conditions*
Heat Index, HI, °F
Human Impact
80 to 90
Fatigue possible with prolonged exposure
and physical activity
90 to 105
Sunstroke, heat cramps and heat exhaustion are possible
105 to 130
Sunstroke, heat cramps and heat exhaustion likely. Heat stroke possible
> 130
Heat stroke is highly likely with continued
exposure
*Note: See www.crh.noaa.gov/pub/heat.php.
4. http://web.uvic.ca/~eos340/wind_chill.pdf
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8.4. Thermal Stress
Heat index and wind chill are a measure of how the temperature is perceived by humans and can be referred to
collectively as “apparent temperature” or “relative outdoor temperature”.
The human body looses heat by conduction and convection. The rate of heat loss by the body depends on the
amount of exposed skin and the wind speed. The human body responds to heat loss by attempting to maintain its
surface temperature. Rapid heat loss results in both the perception of lower temperatures and an actual greater
heat loss as the body attempts to maintain body temperature on exposed skin increasing the risk of hypothermia,
frostbite and death.
Table 3. Wind Chill Temperature Chart*
V\Ta
40 °F
30 °F
20 °F
10 °F
0 °F
–10 °F
–20 °F
–30 °F
10 mph
34
21
9
–4
–16
–28
–41
–53
20 mph
30
17
4
–9
–22
–35
–48
–61
30 mph
28
15
1
–12
–26
–39
–53
–67
40 mph
27
13
–1
–15
–29
–43
–57
–71
50 mph
26
12
–3
–17
–31
–45
–60
–74
*Note: The shaded area indicates a danger of frostbite.
8.5. Environmental Quality
Traditionally an environment was controlled based on a temperature measurement. In recent years the relative
humidity measurement has become just as important. Humidity control is especially important in living, storage,
and manufacturing sites. Control of temperature and relative humidity is also critical in the preservation of many
materials including medication, food, fabric and wood products. Unacceptable levels of temperature and/or relative
humidity contribute significantly to the breakdown of materials. Heat accelerates deterioration, whereas high
relative humidity provides the moisture necessary, to promote harmful chemical reactions and, in combination with
high temperature, encourages mold growth and insect activity. Extremely low relative humidity, which can occur in
winter in centrally heated buildings, may lead to desiccation of some materials causing them to become brittle.
Large fluctuations in temperature and relative humidity are damaging due to expansion and contraction which can
accelerate deterioration. Installation and operation of adequate climate controls to meet preservation standards will
retard the deterioration of materials considerably, while maintaining a comfortable environment for humans. For
example, luthiers recommend keeping fine wooden musical stringed instruments such as violins, cellos and guitars
at a RH between 45% and 55% and temperatures between 16 °C (55 °F) to 21 °C (70°F) to prevent warping and
splits in the wood. The ideal temperature and relative humidity will vary depending on the material and application.
Typically, a steady temperature of 16 to 21°C and a relative humidity between 30% and 60% suffices for many
applications. Low power battery operated temperature and humidity sensors can be used to monitor conditions
during shipment or storage for food and a variety of other materials.
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A PPENDIX A— I N D U S T R Y S PECIFICATIONS AND
G UIDELINES
A.1
ANSI/ASHRAE Standard 55
BS1339
BS 1339-1:2002 Part 1: Terms, definitions and formulae
BS 1339-2:2009 Part 2: Humidity calculation and tables - User guide
BS 1339-3:2004 Part 3: Guide to the measurement of humidity
U.K. National Physics Laboratory Guide to RH Measurement
http://www.npl.co.uk/publications/good-practice-online-modules/humidity/
The following topics can be found on Wikipedia:
A.2
Relative
Humidity
Dew Point
I2C
IP
Rating
Polymers
Wave Soldering
Si70xx certificate of compliance (with web link)
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A PPENDIX B— E QUATIONS F O R V APOR P R E S S U R E AND
H U MI D I T Y C ALCULATIONS
Many equations have been developed to express humidity parameter relationships. They can generally be broken
down into two groups, those developed from thermodynamic principles and equations empirically derived from
experimental data. The Clapeyron and Clausius-Clapeyron equations will be presented as examples of the first
group and the Sonntag, Magnus and Antoine equations as examples of empirically derived expressions. The
empirically derived expressions are generally easier to use but have limitations to the range of use and accuracy.
Approximations may be useful to further simplify calculations and conversions between humidity parameters as
long as the limitations imposed by the approximations are fully comprehended. The use of these equations and
approximations will be discussed below.
Clapeyron Equation
The Clapeyron equation is based on one of the Maxwell thermodynamic relationships contains no approximations
and provides an exact solution. It considers saturation pressure and temperature, the change of entropy
associated with a change of phase and the change in volume of the two phases and represents the slope of the
vapor-pressure curve. The Clapeyron equation can be expressed as follows:

p- = s
------------v
T
where:

p = Saturation Vapor Pressure
T = Temperature in °K
s = Entropy Change between the two Phases
v = Volume Change
since:
h
s = ------- Across a phase transition
T
where:
h = Enthalpy Change between the two Phases
The Clapeyron Equation can be rewritten as follows:

p
h-------- = ---------T vT
The Clapeyron equation is valid for all phase transitions (solid/liquid, solid/gas and liquid/gas) and represents the
slope of the phase change boundaries. The parameters in this equation that can be directly measured are
temperature, pressure and volume. Entropy and enthalpy can only be measured indirectly in terms of the other
parameters.
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Clausius-Clapeyron Equation
The Clausius-Clapeyron equation modifies the Clapeyron equation with two simplifying approximations that make
this equation useful for ice to water vapor and liquid water to water vapor transitions. The first assumption is that
the change in volume from liquid water to gas (water vapor) or solid (ice) to gas (water vapor) is approximately
equal to the volume of the gas (water vapor).
V = V gas – V liquid  V gas
V = V gas – V solid  V gas
The second approximation is the gas (water vapor) can be treated as an ideal gas.
V gas = nRT
------------P
Incorporating these assumptions in the Clapeyron equation yields the following.
h
h  p
-------- = ----------------= ---------- T 
2
nRT
RT
T ----------------------p
p
Rearranging:
h dp
--------
---------dT
2
 p  =
RT
Integrating as an indefinite integral and assuming h is constant:
dp = h
1- dT
-------  ----R T2
– h 1
ln  p  + C 0 = ----------  --- + C 1
R  T
 ------p
Where:
h = enthalpy change for phase change which varies between
2.501 x 106 and 2.257 x 106 J/kg in the range of 0–100 °C
R’ = universal gas constant
R = specific gas constant for water which is 461.5 J/(K – kg)
Rearranging:
p = C 2 e
–--------h-  --1-
R  T
Accuracy is best around the temperature used to calculate C2.
For example C2 = 2.53 x 1011 Pa at 0 °C
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Humidity-Related Calculations
While the above expressions are physically based, they are difficult to solve and manipulate. Hence, many
approximate formulas have been developed. The two most common are the Antoine Equation and the Magnus
equation. Many of these formulas contain coefficients that can vary depending on their source, its age and, in some
cases, the context of the equations use.
Antoine Equation
This equation calculates saturation vapor pressure. Additionally; coefficients are available for this equation for a
wide variety of vapors other than water. The coefficients used in the following equation are for an air-water system
and are optimized for use over the temperature range 0 to 100 °C.
3830 -
p s = exp  23.19 – ----------------------T – 44.83
3830
t dp = -------------------------------------- + 44.83
23.19 – ln  Ps 
Where:
–2
Pressure in Pascals  Nm 
Temperature in °K (°C + 273.15)
Magnus Equation
This equation calculates saturation vapor pressure as does the Antoine Equation. It has the advantage that it can
be easily manipulated to find the dew point (tdp) or frost point.
Ps = C 1 e
A 1 T -
 --------------- B + T
1
Where:
P’s is pressure in Pascals Nm–2
T is temperature in °C
Over the range of –40 to +50 °C, the best fit constants are as follows:
A1
B1
C1
17.625
243.04
610.94
Reference
Alduchov, Oleg A., Robert E. Eskridge, 1996: Improved Magnus Form
Approximation of Saturation Vapor Pressure. J. Appl. Meteor., 35, 601–609
For air with a vapor pressure P’, the dew point is defined as the temperature at which the water vapor would be
saturated. Thus,
P
In  -------
C 
1
T d = B 1  -------------------------------P

A1 – ln -------
C 
1
Utilizing the relationship
RH = 100 x P’/P’s
Rev. 1.7
27
AN607
A1 T
RH
B 1 In  ---------- + --------------- 100 B + T
1
Td = -------------------------------------------------------A1 T
RH


A 1 – In ---------- – --------------- 100 B + T
1
Equation Comparison
The following graph compares the above equations:
Mark Lawrence Rule of Thumb*
This easy to use equation provides an estimate of change in dew point for a change in relative humidity or can be
easily reversed to estimate a change in relative humidity for a known change in dew point. This approximation is
valid for RH>50%. This rule of thumb says that the dew point temperature decreases approximately 1C for every
5% decrease in RH starting at tdp = t and RH=100%. This relationship is very handy if very little computational
capability is available and the accuracy limitations and range of applicability are acceptable.
*Note: See Lawrence, Mark G., “The Relationship between Relative Humidity and the Dewpoint Temperature in Moist Air - A
Simple Conversion and Applications”, American Meteorological Society February 2005.
28
Rev. 1.7
AN607
Estimating RH with Heating
Equation Development
The Magnus equation for partial pressure of water in air is:
RH
P =  ----------  C 1 e
100
T
A 1  -------------------- B1 + T 
for a given relative humidity RH in percent and temperature in ºC. Typical values for A1, B1, and C1 are 17.625,
243.04, and 610.94, respectively.
If the air is heated, the partial pressure does not change, and the apparent relative humidity drops according to:
RH
P =  ----------  C 1 e
 100 
 T + T 
A 1  ---------------------------------- B 1 + T + T 
or
A1  T
------------------- B1 + T 
e
RH = RH --------------------------------------- T + T 
e
A 1  ---------------------------------- B 1 + T + T 
this can be simplified to:
RH = RHe
– A 1  B 1  T
------------------------------------------------------------  B 1 + T   B 1 + T + T  
Rev. 1.7
29
AN607
Linearization
The above equation is still too complex to be useful in simple systems; however, it can be noted that, over a narrow
temperature range, the relative humidity error is fairly linear with relative humidity for a given amount of heating and
ambient temperature.
The error is linear with RH and increases by about 5% RH per °C (this is the familiar “Mark Lawrence Rule of
Thumb”).
Thus, the actual RH can be estimated fairly accurately by:
RH measured
RH = -------------------------------- 1 – 0.05T 
The accuracy of this estimate can be improved by measuring the temperature of the RH sensor and correcting for
heating to get the ambient temperature.
T ambient = T measured – T
For 5 °C heating, the correction factor varies from 0.0598 at 0 °C to 0.0435 at 50 °C ambient or
CF = 0.0598 – 0.000346  T ambient
and finally,
RH measured
RH = ---------------------------------- 1 – CF  T 
30
Rev. 1.7
AN607
A PPENDIX C— TERM , U NIT , A N D C OEFFICIENT
R EFERENCE
Absolute
vapor pressure—A measure of the actual amount of water present in the air.
Point—For a given RH and temperature, the temperature at which condensation would form if the air
were cooled; meaningful as an indicator of comfort.
Hydrophobic—Water repellent/resistant.
IP Rating—Ingress Protection Rating; first digit indicates level of protection against particle; the second
digit represents level of protection against liquids.
IP67—An ingress protection rating indicating that the assembly is dust tight (6) and can withstand up to
1 m of water pressure (7).
Kapton—A polyimide film developed by DuPont that is stable over a wide temperature range (up to +400
°C). It is available in sheet, tape, and “dot” form and is used to protect selected components during solder
reflow.
Oleophobic—Oil repellent/resistant.
Relative Humidity—Absolute_Vapor_Pressure ÷ Saturated_Vapor_Pressure; expressed as a percentage.
Saturated vapor pressure—The maximum amount of water that the air can hold; dependent on
temperature.
Dew
Table 4. Common Pressure Unit Conversions
Multiply to Convert
Divide to Convert
Unit
Pascal
(Pa)
Hectopascal
(hPa)
Kilopascal
(kPa)
Bar
(bar)
Atmosphere
(atm)
Torr*
(torr)
lbf/in2
(psi)
Pa
1
1x10–2
1x10–3
1x10–5
9.8692x10–6
7.5006 x 10–3
145.04 x 10–6
hPa
100
1
1x10–1
1x10–3
9.8692x10–4
7.5006 x 10–1
145.04 x 10–4
kPa
1000
10
1
1x10–2
9.8692x10–3
7.5006
145.04 x 10–3
bar
100,000
1000
100
1
0.98692
750.06
14.50377
atm
101,325
1013.25
101.325
1.01325
1
760
14.696
torr
133.322
1.33322
1
19.337 x 10–3
psi
6.895 x 103
68.95
51.715
1
133.322 x 10–3 1.3332 x 10–3 1.3158 x 10–3
6.895
68.948 x 10–3 68.046 x 10–3
*Note: 1 torr ~= 1 mmHg
Temperature Conversions:
5
T °C = ---  T °F – 32 
9
9
T °F =  --- T °C + 32
 5
T °K = T °C + 273.15
T °R = T °F + 459.69
Rev. 1.7
31
AN607
Table 5. Humidity Terms and Definitions
Term
32
Definition
Units
p
Pure water vapor pressure (no air or other gas)
N/m2 or Pa
p’
Actual vapor pressure (water vapor in air)
N/m2 or Pa
ps
Pure water saturated vapor pressure (no air or other gas)
N/m2 or Pa
ps’
Actual saturated vapor pressure (water vapor in air)
N/m2 or Pa
p
Total atmospheric pressure
N/m2 or Pa
f
Water vapor enhancement factor
dimensionless
dv
Volumetric humidity mass of water vapor/volume of humid gas
kg/m3
Mg
Molal mass (molecular weight) of dry gas (air)
kg/mol
Mv
Molal mass (molecular weight) of water vapor
kg/mol
m
mass
kg
n
Amount of substance in moles
mol
R
Universal gas constant in joules per mole of air per K
J/(mol)(K)
R’
Universal gas constant in joules per kilogram of air per K
J/(kg)(K)
S
Percent of saturation
%

% Relative humidity (%RH or %rh)
%
Y
Mixing ratio kg vapor/kg dry gas
kg/kg
Yw
Specific humidity kg vapor/kg humid gas
kg/kg
h
Enthalpy change between phases
hig
Ice gas
hif
Ice liquid
hfg
Liquid gas
s
Entropy change between phases
tdp
Dewpoint temperature
Rev. 1.7
AN607
A PPENDIX D— N ONLINEAR C ORRECTION OF VO L TAG E
I N P U TS W I T H T H E Si7013
The Si7013 has the capability to apply a lookup-table-based non-linear correction to voltage measurements. This
correction is invoked by writing a “1” to bit 5 of user register 1. Note that humidity measurements should not be
performed when this bit is set. In the discussion below, “input” refers to the A/D voltage measurement result, which
is a 16-bit signed integer, and “output” refers the output after the non-linear correction, which is assumed to be a
16-bit unsigned integer.
The non-linear correction is based on a 10-point table lookup linearization. Each point consists of the ideal output
for a given expected A/D measurement result. Table 6 is stored in the Si7013 memory, which must also have the
slope at points 1–9. Slope is multiplied by a scaler of 256.
 outputN + 1 – outputN 
slopeN = 256  ----------------------------------------------------------------- inputN + 1 – inputN 
Only nine of the input/output pairs need to be in the table because the 10th pair is determined by the slope
equation. Overall, the Si7013 has 27 16-bit numbers in its table (54 bytes). This table is stored in non-volatile
memory of the Si7013 and must be programmed based on the desired look-up table.
The actual output is determined by extrapolation:
If in >in2, out = out1+slope1 x (in-in1)/256
Else if in >in3, out = out2+slope2 x (in-in2)/256
Else if in >in4, out = out3+slope3 x (in-in3)/256
Else if in >in5, out = out4+slope4 x (in-in4)/256
Else if in >in6, out = out5+slope5 x (in-in5)/256
Else if in >in7, out = out6+slope6 x (in-in6)/256
Else if in >in8, out = out7+slope7 x (in-in7)/256
Else if in >in9, out = out8+slope8 x (in-in8)/256
Else out = out9+slope9 x (in-in9)/256
Rev. 1.7
33
AN607
The table must be arranged in order of decreasing input values. The table is entered into memory addresses 0x82–
0xB7 one byte at a time.
Table 6. Memory Location Descriptions
Memory Location
Description
0x82
MSB of in1
0x83
LSB of in1
…
…
0x93
LSB of in9
0x94
MSB of out1
…
…
0xA5
LSB 0f out9
0xA6
MSB of slope1
…
…
0xB7
LSB of slope9
The table itself is user-programmed, and, by default, all table values are 0xFF. It should be noted that, once the
non-linear correction data is saved to memory, it cannot be overwritten.
As an aid to calculation of the table, several tools have been developed.
A
spreadsheet, “Si7013 Thermistor Correction Calculator.xlsx”, is available under the Miscellaneous
section of the Sensors Documentation page at http://www.silabs.com/support/pages/documentlibrary.aspx?p=Sensors. This spreadsheet calculates the expected output of the A/D based on an
assumed thermistor and biasing circuit (the NCP18XH103F03RB thermistor used on Si7013 evaluation
boards with 24.3 k biasing resistors). Then, based on the desired output after linearization (In this case,
output = (temperature + 46.85) x 65536/175.72), the slope is calculated. Finally, the spreadsheet
calculates the hexadecimal values that should go in memory locations 0x82–0xB7 based on the input/
output and slope values.
The Si7013 evaluation board has the option of trying different values of linearization based on numbers
entered in a GUI. These values can be saved to file or burned into the Si7013 memory. For example:
For the Si7013 evaluation board with a 10 k thermistor and two 24.3 k bias resistors, and assuming the A/D
conversion is done using VDD as a reference with buffered inputs, the ideal input voltage version temperature is:
Vin = VDD x Rthemistor/(Rthermisor + 46.4 k)
Since VDD is also the reference, then the expected A/D conversion result is:
A/D counts = 32768 x Rthemistor/(Rthermisor + 46.4 k)
If it is desired to linearize this result for the same temperature representation as the onboard temperature sensor:
Temperature °C = (Output_Code x 175.72/65536 – 46.85)
Then, the desired output code is:
Output_Code = 65536 x (Temperature + 46.85)/175.72
Using thermistor data sheet values of resistance versus temperature and choosing to linearize at the points
(–15 °C, –5 °C, 5 °C, 15 °C, 25 °C, 35 °C, 45 °C, 55 °C, 65 °C, and 75 °C) results in the information in Table 7.
34
Rev. 1.7
AN607
Table 7. Example Non-Linear Correction to Thermistor Voltage Measurements
Thermistor Resistance
(from Data Sheet)
Vin/Vdd Assuming
24.3 k Bias Resistors
A/D Codes
Desired
(Temperature)
Code
Slope
53650
0.524694377
17193
11879
–256
33892
0.410851961
13463
15608
–294
22021
0.31181943
10218
19338
–364
14674
0.231912002
7599
23067
–476
10000
0.170648464
5592
26797
–640
6948
0.125081011
4099
30527
–877
4917
0.091877347
3011
34256
–1210
3535
0.067804738
2222
37986
–1684
2586
0.050521627
1655
41715
–2346
The values highlighted in gray would be the table entries for the Si7013.
Rev. 1.7
35
AN607
Entering the Table into Memory
The table is entered into memory addresses 0x82–0xB7, one byte at a time. For the above example, the values to
be written are listed in Table 8:
Table 8. Example Non-Linear Thermistor Correction Entries into Si7013 Memory
Table
Entry
Hex
Byte 1
Byte 2
Memory
Location
Table
Entry
Hex
Byte 1
Byte 2
Memory
Location
Table
Entry
Hex
Byte 1
Byte 2
Memory
Location
17193
4329
43
82
11879
2E67
2E
94
–256
FF00
FF
A6
29
83
67
95
00
A7
34
84
3C
96
FE
A8
97
85
F8
97
DA
A9
27
86
4B
98
FE
AA
EA
87
8A
99
94
AB
1D
88
5A
9A
FE
AC
AF
89
1B
9B
24
AD
15
8A
68
9C
FD
AE
D8
8B
AD
9D
80
AF
10
8C
77
9E
FC
B0
03
8D
3F
9F
93
B1
0B
8E
85
A0
FB
B2
C3
8F
D0
A1
46
B3
08
90
94
A2
F9
B4
AE
91
62
A3
6C
B5
06
92
A2
A4
F6
B6
77
93
F3
A5
D6
B7
13463
10218
7599
5592
4099
3011
2222
1655
36
3497
27EA
1DAF
15D8
1003
0BC3
08AE
0677
15608
19338
23067
26797
30527
34256
37986
41715
3CF8
4B8A
5A1B
68AD
773F
85D0
9462
A2F3
Rev. 1.7
–294
–364
–476
–640
–877
–1210
–1684
–2346
FEDA
FE94
FE24
FD80
FC93
FB46
F96C
F6D6
AN607
The command code, 0xC5, is used for programming; so, for example, to program a Si7013 at slave address 0x40
with the values above starting with 0x4C to memory location 0x82, one would write:
<Start Condition> 0x40 W ACK 0xC5 ACK 0x82 ACK 0x2E ACK <Stop Condition>
……
<Start Condition> 0x40 W ACK 0xC5 ACK 0xB7 ACK 0xD6 ACK <Stop Condition>
After writing all values, the chip should be reset by a power cycle or I2C reset command before attempting to use
the correction mode of the voltage-sensing circuit.
It is not possible to change the values once written. However, to verify the values were written properly, use
command 0x84. In the above example, to verify that 0x2E was written to location 0x82, use:
<Start Condition> 0x40 W ACK 0x84 ACK 0x82 ACK <Start Condition> 0x40R ACK 0x2E NACK <Stop Condition>
where 0x2E is the expected return value of the read transaction.
How to Choose Linearization Points
The table must be arranged in order of decreasing values for the input. The slope can be positive or negative, and
it is assumed that only positive output values are desired.
Generally, the input/output pairs should be chosen around the area where greatest accuracy is desired. If the
transfer function is highly nonlinear and the more closely spaced the points, the less error there will be from
interpolation. However, if accuracy is desired over a wide range, the input/output pairs should be spaced evenly
over the range of interest. The spreadsheet allows estimation of errors from the extrapolation process.
Other Register Settings
The voltage measurement configuration is determined by user register 2 of the Si7013. For thermistor
measurement, it is assumed that the A/D input buffers will be used and that the A/D reference is VDD. The buffers
are needed since the thermistor impedance is fairly large. VDD is used as a reference because the thermistor is
biased relative to VDD. Thus, for a “hold master” measurement (SCL is held low during the measurement phase),
Register 2 would be set to the following:
D7
D6
D5
D4
D3
D2
D1
D0
0
0
1
0
0
1
1
0/1
Not Used
Hold
Master
Enable
Buffer
VDD is
Reference
Switch High or Low to
Bias the Thermistor
Enable
7 msec
Not Used
Linearization Conversion Time
Noise and AC Pick Up
The A/D of the Si7013 is a delta sigma type converter, and the input is not sampled. Thus, it is assumed that the
input voltage is constant over the measurement period. Generally, some amount of analog filtering prior to the A/D
input is desirable. In the standard application circuit, this is accomplished with 0.1 µF capacitors. These capacitors
will form a filter at about 30 Hz, which is adequate for high-frequency noise pick up (e.g. am radio signals) but not
for 60 Hz. If 60 Hz filtering is desired, these capacitors can be increased to >1 µF, or the result could be digitally
filtered (average of several measurements). If the sampling can be synchronized to 120 Hz (use the faster
conversion time for this), then an average of just two samples would reject 60 Hz.
In the standard biasing circuit, the bias can be turned off between measurements to save power (change bit zero of
user register 2). If this is done, allow adequate settling time between enabling the bias and making the
measurement (use approximately 100 msec for the 0.1 µF filter, which has a time constant of 30 msec).
Rev. 1.7
37
AN607
A PPENDIX E — T HERMAL M OD EL FOR A S ENSOR O N A
P ADDLE
To illustrate some considerations for separating the sensor from the system, consider the following practical
example of the sensor on a 3 cm x 3 cm PCB connected by a 1 cm wide piece of PCB material 3 cm long:
3cm
1cm
3cm
Sensor
3 cm
Figure 14. Sensor on a 3x3 cm PCB
Referring to the thermal model discussed in “3.2. Temperature and Humidity Sensor Placement” :
Tambient
R3
Sensor
R2
R1
Tambient
System
C1
C2
Figure 15. General Thermal Model for Sensor Placement
38
Rev. 1.7
AN607
The thermal impedance to air for a standard FR4 PCB is about 1000 C/W per cm2 of PCB area. With two sides
exposed to the ambient a total of 18 cm2 is connected to the ambient. This makes R1 55.5 C/W.
The mass of 9 cm2 of PCB material is about 2.5 g (the specific gravity of FR4 is 1850 kg/m3 and assuming 1.5 mm
thickness) and the specific heat capacity of PCB material is 0.6 J/(g-C), so the heat capacity is 1.5 J/C.
The time constant R1 x C2 is 1.5 J/C x 55.5 C–second/J = 83 seconds.
This time constant is independent of the PCB area—more area means lower thermal impedance but higher
thermal mass. To improve the time constant beyond this thinner PCB material would have to be used or there
would need to be fins or airflow to reduce the thermal impedance.
Turning our attention to R2 the thermal conductivity of FR4 material in plane is around 1Watt/meter-C. So, for the
example of the connection of the sensor area to the rest of the system by 1cm wide 3cm long 1.5mm thick FR4:
Conductance = thermal conductivity x area/length = 1 Watt/meter-°C x 10–2m/cm x 1 x 0.15/3 = 0.05 x 10–2W/°C
Or thermal impedance (1/conductance) is 2000 °C/W (this is R2). This is assuming minimal copper routing on the
connector material.
With this design since R2 is 36 times R1, so the system heating will have a fairly minor effect. For example if the
system heating is 10 °C the sensor temperature would only increase 0.3 °C.
If more thermal connection is tolerable the connector area could be made shorter or wider or the PCB area the
sensor is connector to could be made smaller.
This example was intended to illustrate the thermal design considerations for good response to ambient conditions
and insulation from the system. In some cases, it is not possible to place the sensor in sufficient thermal contact
with the environment to shield it for the thermal mass and heat sources in the system. In this case, it is often
possible to compensate for the system by placing an additional temperature sensor in the system. However, in all
cases, the thermal contact of the sensor to the environment should be maximized, and the thermal contact of the
sensor to the rest of the system should be minimized.
Rev. 1.7
39
AN607
DOCUMENT CHANGE LIST
Revision 0.1 to Revision 1.0

Updated storage, handling, and assembly
instructions.
 Corrected Table 4, “Common Pressure Unit
Conversions,” on page 31.
Revision 1.0 to Revision 1.1

Multiple updates to include Si7013, Si7020, and
Si7021 parts.
Revision 1.1 to Revision 1.2

Added "4.2.Use of Conformal Coating and Under-Fill
Materials" on page 11.
 Corrected simplified Magnus equation on page 27.
Revision 1.2 to Revision 1.3

Added link to Si7013 Thermistor Correction
Calculation Table on page 31.
Revision 1.3 to Revision 1.4

Added sections 3.2.2 and 3.2.3.
Updated section 3.3.
 Added Appendix E.

Revision 1.4 to Revision 1.5

Revised to include Si7006, Si7007, Si7022, and
Si7023.
Revision 1.5 to Revision 1.6


Updated title to include temperature sensor.
Updated to include Si7050, Si7053, Si7054, and
Si7055.
Revision 1.6 to Revision 1.7

40
Added Si7034.
Rev. 1.7
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