Application advantages of LBA differential pressure

Application advantages of LBA differential pressure sensors due to
their very high pneumatic impedance
1
MEASURING PRINCIPLE
First Sensors LBA differential pressure sensors
measure ultra low air or gas pressures from 25
Pa (0.1 inH2O) full scale. The sensors are
based on a new and innovative MEMS
technology which integrates a micro flow
channel within the silicon sensor chip. At the
same time the sensors use the proven principle
of inferring differential pressure from a thermal
mass flow measurement (see Fig. 1). A heating
element is located between two temperature
sensitive resistors. A gas flow transfers heat
from the upstream to the downstream resistor
causing a temperature difference between them
and as a result, a voltage signal proportional to
mass flow is generated. Because the flow is
caused by the pressure difference between the
two sensor ports, the output signal is also a
measure of the applied differential pressure.
Fig. 2 shows the unamplified output of a basic
LBA sensing element as a function of differential
pressure. The LBA technology features high
dynamic ranges and high sensitivities for low
pressures especially around zero. The LBA
sensors offer analog signal conditioning for
calibration, temperature compensation and
amplification. They can be optimised to different
application requirements depending on whether
a high sensitivity, a high dynamic range or a
linear output signal is needed.
200
Vout (mV)
100
Flow
0
-4000
-2000
0
2000
4000
∆p (Pa)
-100
Temperature sensitive
resistors
Heating element
-200
Fig. 1: Principle of thermal mass flow measurement
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Fig. 2: Characteristic curve of an unamplified basic
LBA sensing element
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Application advantages of LBA differential pressure sensors due to
their very high pneumatic impedance
2
SENSOR CONSTRUCTION
The LBA sensor is based on a silicon sensor
chip, only about 4 mm2 (0.006 in2) in size, which
contains the micro-flow channel, the sensing
element and a fully analog CMOS signal
conditioning circuitry. By integrating the
miniaturised flow channel on the sensor chip
level (see Fig. 3), First Sensors LBA pressure
sensors can achieve very high pneumatic
impedances up to 200,000 Pa/(ml/s), which is
up to 1000 times higher than comparable
sensors. This reduces the gas flow through the
sensor to an absolute minimum and allows
unique application advantages in dusty and
humid environments as well as when using long
connection tubes or filters (detailed explanation
in chapters 4 to 6).
In conventional flow-based differential pressure
sensors the flow channel and gas flow through
the sensor is determined by the geometry of the
plastic housing. In contrast, the micro-flow
channel of the LBA device is defined on the
silicon chip level. This allows advantages in the
construction of the sensor housing such as a
high design flexibility, very small and stable
packages as well as reduced manufacturing
costs. Further, the semiconductor technology
used for the LBA silicon sensor chip enables
extremely low production tolerances together
with cost effective mass production.
Top wafer
3
FLOW MEASUREMENT WITH
DIFFERENTIAL PRESSURE SENSOR
Differential pressure sensors with very low
pressure ranges of only a few millibar (a few
inches of water column) are often used for
volumetric flow measurement in tubes and pipes.
Examples are respiration flow measurement in
medical devices as well as air flow measurement
or filter control in HVAC applications. An
artificial flow restriction, e.g. by means of a
baffle, orifice or laminar flow element induces a
pressure drop to the flow which is a measure of
•
the volumetric flow rate (V) and can be detected
with a differential pressure sensor (see Fig. 4).
First Sensors flow-based LBA sensors are
calibrated to differential pressure and can
detect the pressure drop across a flow element
in a bypass configuration. Due to its very high
pneumatic impedance the flow through the
sensor is limited to max. 120-180 µl/min. This
means that the LBA sensor almost behaves like
a membrane-based differential pressure sensor
which does not allow any flow-through.
However, it still shows the high measuring
sensitivity for low pressures which is typical for
the thermal mass flow sensing principle.
Air flow
Laminar flow element
Flow
p1
p2
p = p1- p2
Sensing element
Flow channel
Bottom wafer
Fig. 3: Principle construction of LBA differential
pressure sensor (cross section)
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Connecting
tube
LBA
sensor
V
p
Fig. 4: Typical volumetric flow measurement set-up
with differential pressure sensor
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Application advantages of LBA differential pressure sensors due to
their very high pneumatic impedance
4
IMMUNITY TO LONG CONNECTION
TUBES AND INPUT FILTERS
To measure volumetric flow, the flow-based
differential pressure sensor has to be connected
to the main flow channel e.g. via tubes as
shown in Fig. 4. Sometimes, additional filters
will be used in the bypass channel to protect the
sensor against dust, humidity or bacterial
contamination. However, any pneumatic
element between the main flow channel and the
bypass represents an additional flow resistance
which leads to a pressure drop. The pressure
sensor will therefore measure a differential
pressure which is lower than the one caused by
the flow restricting element in the main channel.
The result is an inaccurate measurement of the
volumetric flow rate in the main flow channel.
The higher the flow impedance of the connecting
tubes and additional filters compared to the
sensor, the more dominant is this effect.
Therefore, for conventional flow-based
differential pressure sensors a maximum
allowed tube length to the sensor is recommended
or respectively a correction formula is given to
compensate for the pressure drop in the
bypass.
A tube of 1 m (40 in) length with an inner
diameter of 1.6 mm (1/16 in) causes a
pneumatic impedance of approx. 120 Pa/(ml/s).
First Sensors LBA sensors feature pneumatic
impedances of up to 200,000 Pa/(ml/s). This
means that the bypass flow is almost exclusively
determined by the very high flow impedance of
the LBA device and influences of additional
components with resistance to flow can be
neglected. Therefore, LBA differential pressure
sensors can be used with long tubing, filters or
other pneumatic elements without losing its
calibration. Even if these elements change their
resistance over time, such as a clogging filter,
there will typically be no negative influence on
the measurement accuracy.
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5
IMMUNITY TO DUST
If flow-based pressure sensors are used for
volumetric flow measurement in dusty
environments such as e.g. HVAC applications
there is the danger that dust particles reach the
inside of the sensor and deposit on the walls of
the inner flow channel. This would lead to an
increase of the pneumatic impedance of the
sensor and therefore to a decrease in the
sensor output signal and a loss of calibration. In
a worst case scenario the flow channel will be
completely blocked which results in a total
failure of the sensor. Further, dust can cover the
sensitive measuring elements which also
degrades the sensor signal.
First Sensors LBA pressure sensors are highly
immune to applications in dusty environments.
Due to its very high pneumatic impedance, the
air flow through the sensor is extremely small.
This means that the total amount of dust-laden
gas which streams through the bypass channel
in a volumetric flow measurement set-up is
reduced to an absolute minimum compared to
conventional flow-based pressure sensors.
Additionally, the flow velocity is greatly reduced
so that the remaining dust quantity can
normally settle out before it reaches the input of
the sensor. In this way the LBA flow-based
pressure sensors prevent the ingress of dust
into the sensor and ensure highly accurate
measurements and very long sensor lifetimes.
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Application advantages of LBA differential pressure sensors due to
their very high pneumatic impedance
6
IMMUNITY TO HUMIDITY
In many medical devices such as respirators,
spirometers, sleep diagnostic equipment and
oxygen conservers the patient’s breathing is
controlled with the help of flow-based differential
pressure sensors. Since the respiratory flow
contains a considerable amount of humidity and
is often also warmer than the environment this
can lead to condensation inside the device.
Water droplets can condense onto the tubing
walls in the bypass line or in the sensor itself. If
the droplets exceed a certain size or accumulate
to larger droplets this can change the pneumatic
characteristics of the connecting tubes and the
sensor. This can result in an erroneous sensor
output signal and a loss of sensor calibration. In
a worst case scenario the flow channels will be
completely blocked which leads to a total failure
of the sensor.
First Sensors LBA pressure sensors are highly
immune to humid environments. Due to its very
high pneumatic impedance, the air or gas flow
through the sensor and its connecting tubes is
extremely small. This means that the total amount
of humid air which streams through the bypass
channel in a volumetric flow measurement, and
which can potentially condense, is reduced to
an absolute minimum compared to conventional
flow-based pressure sensors. Therefore, LBA
flow-based pressure sensors ensure highly
accurate measurements and very long sensor
lifetimes in typical applications with high
humidity.
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