Humidity Sensor
Application note
2012/10/15
Page 1/7
Alps Humidity Sensor
Application Note
www.alps.co.jp
Humidity Sensor
Application note
2012/10/15
Page 2/7
-CONTENTS1. Overview........................................................................................................................... 3
2. Basic Information about Humidity .............................................................................. 3
2.1. Relative Humidity.................................................................................................. 3
2.2. Absolute Humidity ................................................................................................ 3
2.3. Dew Point................................................................................................................ 4
3. Design Guide................................................................................................................... 4
3.1. Air Circulation........................................................................................................ 4
3.2. Isolation from Heat Source ................................................................................. 5
4. Measuring Outside Humidity........................................................................................ 5
5. Re-hydration .................................................................................................................... 6
6. Contamination................................................................................................................. 6
7. Module .............................................................................................................................. 6
8. Application Example...................................................................................................... 7
8.1. Discomfort Index................................................................................................... 7
9. Legal Disclaimer ............................................................................................................. 7
www.alps.co.jp
Humidity Sensor
Application note
2012/10/15
Page 3/7
1. Overview
ln( Pmax )= a / Tk + b + cTk + dTk + e × ln(Tk )
2
(2)
Humidity is one of representative parameters for
environment. In this document, basic knowledge
and formulas are introduced. And important
information
when
the
humidity
sensor
Where, a to e are constant values.
is
a = −6096.9385
b = 16.635794
implemented in final product is also covered. This
document is applied to HSHC series humidity
c = −2.711193×10−2
sensor.
d = 1.673952×10−5
e = 2.433502
2. Basic Information about Humidity
2.1. Relative Humidity
Where, T is temperature of the environment in K.
Relative humidity is the ratio of the actual amount
Using formula (1), water vapor pressure is:
of water vapor in the air to the maximum amount at
P = Pmax ×
the temperature. ALPS’ humidity sensor output is
(3)
Based on ideal gas law, volumetric humidity Hv [g/m3]
proportional to the relative humidity.
P
RH [%]=
×100
Pmax
RH
100
is obtained as:
(1)
H v = 217 ×
P
T
(4)
Where, RH is relative humidity, P is water vapor
Volumetric humidity is ratio of the mass of water
pressure in hPa and Pmax is saturated vapor pressure
vapor and a unit volume of air.
in hPa.
Mass mixing ratio (also called as mix ratio) Hs
2.2. Absolute Humidity
[kg/kgDA] which is the ratio of the mass of water
vapor and dry air is obtained as:
Absolute humidity is the amount of water vapor
present in a unit volume of air. There are mainly 2
Hs =
kinds of way to express absolute humidity. At first,
water saturated vapor pressure is calculated by
following formula. Several ways are discovered to
≈
M water × P
M air × (Pair − P )
18
P
×
29 (Pair − P )
(5)
calculate saturated vapor pressure. In this document,
Where, Pair is bolometric air pressure of the
Sonntag’s formula which is used by WMO (World
environment in hPa, and Mwater and Mair are mass of
Meteorological Organization) is introduced. This
the water vapor and dry air in kg respectively. DA in
formula and constant values are valid within -100ºC
the unit means Dry Air.
to 100ºC.
www.alps.co.jp
Humidity Sensor
Application note
2012/10/15
Page 4/7
2.3. Dew Point
Air flow
Dew point is the temperature which water vapor
begins to condense into liquid water. Dew point is
measured by dew point meter, or obtained by
calculating the temperature which makes present
water vapor pressure as saturated vapor pressure.
Present water vapor pressure is:
P = Pmax ×
RH [%]
100
Outer shell of
the device
Sensor
(6)
Fig. 1 Wide window
Then calculate:
P
y = ln
611.213
PWB
(7)
Air flow
Dew point Td [º C] is:
Td = ay + by 2 + cy 3 + dy 4
(8)
Where a to d are constant values. If y ≥ 0,
a = 13.715
b = 8.4262 × 10 −1
c = 1.9048 × 10 −2
Fig. 2 Multiple windows-1
d = 7.8158 × 10 −3
If y ≤ 0,
Air flow
a = 13.7204
b = 736631 × 10 −1
c = 3.32136 × 10 −2
d = 7.78591 × 10 −4
3. Design Guide
3.1. Air Circulation
Fig. 3 Multiple windows-2
In order to monitor outside humidity by using the
To open wide window helps to introduce out side
sensor mounted in the device, introducing outside air
air to the sensor. And multiple openings are much
to the sensor element is very important. Here, we
better to circulate air around the sensor. Placing the
look at some design examples.
sensor close to the window and making the cavity
around the sensor small helps faster response time.
www.alps.co.jp
Humidity Sensor
Application note
2012/10/15
Page 5/7
3.2. Isolation from Heat Source
4. Measuring Outside Humidity
Saturated vapor pressure (formula (2)) is a
In general, humidity sensor will be mounted on
function of temperature. Temperature change makes
the circuit board and enclosed in the housing.
relative humidity change. In order to measure
Following previous section, sensor is placed away
humidity with decent accuracy, we need to apply
from heat source,. Windows for circulation are
measuring object air to the sensor with keeping
opened as many as possible. However still sensor
humidity and also temperature. Therefore, isolating
reading may have offset from weather station on you
the sensor from heat source is very important. To
desk. Internal heating should be suspected. If you
isolate the sensor from heat source, we have to be
are using digital humidity sensor (HSHCAL series),
careful about circuit board design. There are several
the sensor has temperature measurement capability.
heat sources in a smart phone or other kind of
Logging temperature and humidity is recommended
devices. Batteries for hand held devices tend to
with following steps.
occupy large area and generate heat especially
1.
during charging. Processors, memories and voltage
regulators also generate heat. And, user’s hands are
No airflow, not direct sun light
2.
another large area heat source. Based on formula (1)
and (2) above, sensor reading in the closed device
Put your device in temperature controlled room.
Wait until device temperature becomes room
temperature.
3.
could be different from out side air humidity.
Turn on the device and start humidity and
temperature logging.
Mounting humidity sensor away from heat sources in
4.
Continue until sensor reading is saturated.
a small device would be difficult. We will cover how to
5.
Plot the data and read the sensor reading drift.
compensate the temperature effect in the following
section.
If the environment (temperature and humidity) is
stable and the device has internal heating,
temperature reading goes higher value and humidity
reading goes lower value. It is because of saturated
vapor pressure is changed by temperature.
To compensate this effect, following formula can
be used:
RH out = RH in ×
Pmax in
(9)
Pmax out
Where, RHout and RHin are relative humidity out side
Fig. 4
Sensor location away from
heat source
and inside of the device, respectively. Pmax out and
Pmax in are saturated vapor pressure out side and
inside of the device, respectively. Saturated vapor
www.alps.co.jp
Humidity Sensor
Application note
2012/10/15
Page 6/7
pressure is obtained by formula (2) above.
Temperature reading from the sensor can be used
for calculation of Pmax in. For Pmax
out,
5. Re-hydration
additional
The sensing element is capacitor with dielectric
temperature sensor which read out side temperature
material. Environment air hydrates or dehydrates the
is required. And, in this way, there is no humidity
dielectric material in the capacitor, and it makes
generation in side the device is an important
capacitance change. This hydrate and dehydrate
assumption.
Following charts are examples of
process has hysteresis to extreme conditions, such
device inside relative humidity for 2 different out side
as soldering. During reflow soldering process, the
humidity conditions.
sensor is in more than 100 ºC for over a minute and
100
peak is about 250 ºC or more. The sensor is over
90
dehydrated and humidity reading will drift to lower
80
side. Then it will gradually recover to original state. Its
60
50
40
Out side Temp [C]
70
duration is related to temperature and humidity
condition. ALPS recommend re-hydration which is 85
ºC and 85%RH for 6 hours after reflow soldering.
30
20
6. Contamination
10
0
10
20
30
40
50
60
70
80
90
0
100
Inside Temp [C]
0 -10
60 -70
10 -20
70 -80
Fig. 5
20 -30
80 -90
30 -40
90 -100
40 -50
50 -60
sensing element. Its dimension is 200um x 200um. If
Color shows relative humidity inside the device in
the case of out side relative humidity 60%RH. Yellow area is
100%RH (condensation)
interference in organic solvents such as Toluene,
60
Xylene, Formalin, Acetone, Methanol and Ethanol for
40
Out side Temp [C]
70
50
30
0 -10
60 -70
Fig. 6
10 -20
70 -80
20 -30
80 -90
70
30 -40
90 -100
80
90
40 -50
0
100
50 -60
Color shows relative humidity inside the device
in the case of out side relative humidity 30%RH. Yellow area
is 100%RH (condensation)
positive %RH side. For any organic solvent, output
the baking, re-hydration is recommended.
10
40
50
60
Inside Temp [C]
200 hour or more, makes sensor output drift to
drift will recover by baking at 110ºC for 5 hours. After
20
30
And organic contamination makes sensor output drift
of organic solvents affect sensor output. Long term
80
20
the air flow and the sensor response time will be slow.
happen even the contamination is invisible and vapor
90
10
the contamination stuck into the opening, it will block
toward higher side, especially higher humidity. It will
100
0
Sensor has opening to introduce outside air to
7. Module
In order to solve sensor reading drift after reflow
soldering and contamination issue, ALPS has
capability to provide sensor module. It is a small
www.alps.co.jp
Humidity Sensor
Application note
2012/10/15
Page 7/7
PWB which has sensor, bypass capacitors and a
connector for wiring. Plastic cover is attached to
9. Legal Disclaimer
protect from contamination. Sensor is re-hydrated
before final test and delivered to end user as
THIS
INFORMATION
ON
THE
NOTE
ready-to-use products. This module is designed by
("INFORMATION") SHOULD BE USED ONLY AS A
request basis.
GUIDE.
ALPS MAKES, AND YOU RECEIVE, NO
WARRANTIES
OR
CONDITION,
EXPRESS,
IMPLIED, STATUTORY, OR OTHERWISE, AND
ALPS SPECIFICALLY DISCLAIMS ANY IMPLIED
WARRANTIES
OF
MERCHANTABILITY,
NON-INFRINGEMENT AND FITNESS FOR A
PARTICULAR PURPOSE.
ALPS DOES NOT WARRANT THAT THE USE
Fig. 7
Example of Module. Sensor is under black cover case.
OF
THIS
INFORMATION
WILL
BE
UNINTERRUPTED OR ERROR FREE OR THAT
8. Application Example
8.1. Discomfort Index
Human control its body temperature by
perspiration. Evaporation of the perspiration removes
THIS
INFORMATION
WILL
MEET
YOUR
REQUIREMENTS.
THE USE OF THIS INFORMATION IS
ENTIRELY AT YOUR RISK
temperature from body. But in higher humidity,
perspiration evaporates very slowly. It obstructs
IN NO EVENT WILL ALPS BE LIABLE FOR
cooling down the body temperature. Then human
SPECIAL,
INDIRECT,
WILLFUL,
PUNITIVE,
fells more heat in higher humidity, even in same
INCIDENTAL,
EXEMPLARY,
OR
temperature. Discomfort Index (DI) is the index to
CONSEQUENTIAL DAMAGES, DAMAGES FOR
express heat stress from temperature and humidity
LOSS OF BUSINESS PROFITS, OR DAMAGES
combination. DI is called as Temperature-Humidity
FOR LOSS OF BUSINESS
Index.
PARTY ARISING OUT OF THE USE OF THIS
OR ANY THIRD
INFORMATION , INCLUDING BUT NOT LIMITED
DI = 0.81 × Td + 0.01 × H (0.99Td − 14.3) + 46.3 (10)
TO THOSE RESULTING FROM DEFECTS IN THE
INFORMATION, OR LOSS OR INACCURACY OF
In Japan, it is said that 10% of population feel
DATA OF ANY KIND, WHETHER BASED ON
discomfort at DI=70. And risk of heatstroke rises
CONTRACT, TORT OR ANY OTHER LEGAL
above 80. DI value which people start to feel
THEORY, EVEN IF ALPS HAS BEEN ADVISED OF
discomfort is varies by race. We need to be noticed
THE POSSIBILITY OF SUCH DAMAGES.
that wind effect is not taken in to account in DI.
www.alps.co.jp