Pressure Measurements With DAQ
Systems
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This application note describes pressure sensors and explains how they work. It also details the
requirements for measuring pressure and the signal conditioning required for the measurement. It
also explains how to use NI DAQ systems to measure bridge based sensors, and recommends
three different starter kits for pressure measurements. Finally, an appendix is included explaining
other less common pressure sensors and their areas of application.
Table of Contents:
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What Is Pressure?
Pressure Sensors
Signal Conditioning For Pressure Sensors
DAQ Systems for Measuring Pressure with Strain Based Pressure
Sensors
Appendix - Pressure Sensors for Special Applications
References
What Is Pressure?
Pressure is defined as force per unit area that a fluid exerts on its
surroundings.[1] There are two pressure measurement types: absolute and
gauge. Absolute pressure is referenced to the pressure in a vacuum, whereas
gauge pressure is referenced to another known pressure level, usually the
ambient atmospheric pressure. The SI unit for pressure is the Pascal (N/m2), but
other common units of pressure include atmospheres, bars, inches of mercury,
millimeters of mercury, and Torr.
Pressure Sensors
Because of the great variety of conditions, ranges, and materials for which
pressure must be measured, there are many different types of pressure sensor
designs. Often we measure pressure by converting it to some intermediate form,
such as displacement, which we can then measure with a sensor. Different
methods are available for measuring pressure depending on whether the
pressure being measured is greater or less than atmospheric pressure. Of all the
pressure sensors, Wheatstone bridge (strain based) sensors are the most
common, offering solutions that meet varying accuracy, size, ruggedness, and
cost constraints. For information on other types of sensors, refer to the Appendix
at the end of this document.
Bridge sensors are used for high and low pressure applications, and can
measure absolute, gauge, or differential pressure (the difference in pressure
between two different points of interest). All bridge sensors make use of a strain
gauge and a diaphragm (See Figure 1).
Figure 1. Cross Section of a Typical Bridge-Based Pressure Sensor.[2]
When a change in pressure causes the diaphragm to deflect, a corresponding
change in resistance is induced on the strain gauge, which can be measured by
a Data Acquisition (DAQ) System. Sensors that use foil strain gauges can be
bonded directly to a diaphragm or bonded to an element that is connected
mechanically to the diaphragm. There are also silicon based sensors, wherein
the strain gauge is etched onto a silicon substrate and a transmission fluid is
used to transmit the pressure from the diaphragm to the substrate. In general, foil
strain gauges are used in high pressure (70-700 MPa) applications. They also
have a higher operating temperature than silicon strain gauges (200° C versus
100° C, respectively), but silicon gauges do offer the benefit of larger overload
capability. A silicon strain gauge can sustain up to 400% of its rated pressure
without being damaged, whereas a foil strain gauge can only typically sustain a
50% overload. Because they are more sensitive, silicon gauges are also often
preferred in low pressure applications (~2kPa).[2]
Once you have chosen the type of material for your pressure sensor, you must
determine the type of pressure measurement you will be making. There are three
types of pressure measurements: absolute, gauge, and differential. As
mentioned earlier, an absolute pressure measurement includes atmospheric
pressure in the measurement; its reference point is 0 Pa, the pressure in a
vacuum (See Figure 2).
Figure 2. Diagram of a Absolute Pressure Sensor.[3]
Gauge pressure is measured relative to ambient atmospheric pressure, so it
does not include atmospheric pressure (See Figure 3).
Figure 3. Diagram of a Gauge Pressure Sensor.[3]
Differential pressure is similar to gauge pressure, but instead of measuring
relative to ambient atmospheric pressure, differential measurements are taken
with respect to a specific reference pressure (See Figure 4).
Figure 4. Diagram of a Differential Pressure Sensor.[3]
A common cause of sensor failure in pressure measurement applications is
dynamic impact, which results in sensor overload. A classic example of
overloading a pressure sensor is known as the water hammer phenomenon. This
occurs when a fast moving fluid is suddenly stopped by the closing of a valve.
The fluid has momentum that is suddenly arrested, which causes a minute
stretching of the vessel in which the fluid is constrained. This stretching
generates a pressure spike that can damage a pressure sensor. To reduce the
effects of “water hammer”, sensors are often mounted with a snubber between
the sensor and the pressure line. A snubber is usually a mesh filter or sintered
material that allows pressurized fluid through but does not allow large volumes of
fluid through and therefore prevents pressure spikes in the event of water
hammer. A snubber is a good choice to protect your sensor in certain
applications, but in many tests the peak impact pressure is the region of interest.
In such a case you would want to select a pressure sensor that does not include
overprotection.[2]
Signal Conditioning For Pressure Sensors
Once you have chosen a sensor, you must connect it to your DAQ system. As
with any other bridge based sensor, there are several signal conditioning
considerations. Strain based sensors typically provide small signal levels. It is
therefore important to have accurate instrumentation to amplify the signal before
it is digitized by a DAQ device. Additionally, all bridge-based sensors require
voltage excitation to return a voltage representing strain. This voltage source
should be constant and at a level recommended by the sensor manufacturer.
Excitation and amplification are necessary to accurately measure the electrical
response of a sensor.
Once you have obtained a measurable voltage signal, that signal must be
converted to actual units of pressure. Pressure sensors generally produce a
linear response across their range of operation, so linearization is often
unnecessary, but you will need some hardware or software to convert the voltage
output of the sensor into a pressure measurement. The conversion formula you
will use depends on the type of sensor you are using, and will be provided by the
sensor manufacturer. A typical conversion formula will be a function of the
excitation voltage, full scale capacity of the sensor, and a calibration factor, e.g.
After you have properly scaled your signal, it is necessary to obtain a proper rest
position. Pressure sensors (whether absolute or gauge) have a certain level that
is identified as the rest position, or reference position. The strain gauge should
produce 0 volts at this position. Offset nulling circuitry adds or removes
resistance from one of the legs of the strain gauge to achieve this "balanced"
position. Offset nulling is critical to ensure the accuracy of your measurement,
and for best results should be performed in hardware rather than software.
DAQ Systems for Measuring Pressure with Strain Based Pressure Sensors
Using SCXI with Strain Based Pressure Sensors
National Instruments SCXI is a signal conditioning system for PC-based data
acquisition systems (See Figure 5). An SCXI system consist of a shielded
chassis that houses a combination of signal conditioning input and output
modules, which perform a variety of signal conditioning functions. You can
connect many different types of sensors, including absolute and gauge pressure
sensors, directly to SCXI modules. The SCXI system can operate as a front-end
signal conditioning system for PC plug-in data acquisition (DAQ) devices (PCI
and PCMCIA) or PXI DAQ modules.
Figure 5. A Typical National Instruments SCXI System.
SCXI offers an excellent solution for measuring pressure. The SCXI-1520
universal strain-gauge module is ideal for taking strain based pressure
measurements. It provides 8 simultaneous sampled analog input channels, with
programmable excitation (0-10 V), programmable gain (1-1000), and a
programmable 4-pole Butterworth filter (10 Hz, 100 Hz, 1 kHz, 10kHz) on each
channel. The SCXI-1314 terminal block provides screw terminals for easy
connections to your sensors.
Recommended starter kit for Pressure SCXI DAQ System:
1. PCI-6052E DAQ board
2. SCXI-1000 chassis
3. SCXI-1349 cable assembly
4. SCXI-1520 modules and SCXI-1314 terminal blocks
5. Refer to ni.com/sensors for recommended sensor vendors
Using SC Series DAQ with Strain Based Pressure Sensors
For high performance integrated DAQ and signal conditioning, the National
Instruments PXI-4220 (See Figure 6), part of the SC Series, provides an
excellent measurement solution. SC Series DAQ offers up to 333 kS/s
measurements with 16-bit resolution, and combines data acquisition and signal
conditioning into one plug in board. The PXI-4220 is a 200 kS/s, 16 bit DAQ
board with programmable excitation, gain, and 4-pole Butterworth filter. Each
input channel of the PXI-4220 also includes a 9-pin D-Sub connector for easy
connection to bridge sensors, and programmable shunt and null calibration
circuitry.
Figure 6. National Instruments PXI-4220.
Recommended starter kit for Pressure SC Series DAQ System:
1. PXI-1002 chassis
2. PXI-8176 embedded controller
3. PXI-4220 modules
4. Refer to ni.com/sensors for recommended sensor vendors
Using SCC with Strain Based Pressure Sensors
National Instruments SCC provides portable, modular signal conditioning for
DAQ systems (See Figure 7). SCC modules can condition a variety of analog I/O
and digital I/O signals, including bridge sensors. SCC DAQ systems include an
SC-2345 Series shielded carrier, SCC modules, a cable, and a DAQ device.
Figure 7. National Instruments SCC Carrier and Modules.
The SCC-SG24 Load Cell Input module accepts up to two full-bridge inputs from
load cells or pressure sensors. Each channel of the module includes an
instrumentation amplifier, a 1.6 kHz lowpass filter, and a potentiometer for bridge
offset nulling. Each SCC-SG24 module also includes a single 10 V excitation
source.
Recommended Starter Kit for Pressure SCC DAQ System:
1. PCI-6052E DAQ board
2. SC-2345 module carrier
3. SCC-SG24 modules (1 per 2 pressure sensors)
4. Refer to ni.com/sensors for recommended sensor vendors
Appendix - Pressure Sensors for Special Applications
Though standard bridge based pressure sensors are the most flexible sensor
type, there are nonetheless many applications that a basic bridge sensor will not
solve. When this is the case, other sensors must be investigated. The sensor
families described below meet conditions that a standard bridge sensor may not.
Some of the sensors (such as the Pirani gauge) are still based on a bridge
configuration, but the configuration has been specifically altered to meet specific
application constraints. The gauges described are divided into two groups: low
pressure (below atmospheric) and high pressure (above atmospheric).
Pressure Sensors (pressure < one atmosphere)
Thermocouple Gauges
To determine a chamber's pressure range between 10 and 10-3 Torr a gauge
measures the voltage of a thermocouple spot-welded to a filament exposed to
system gas (See Figure 1). A constant current supply feeds the filament, and the
filament reaches a temperature dependant on thermal losses to the gas. At
higher pressure, more molecules hit the filament and remove more heat energy,
changing the thermocouple voltage.[4]
Figure 1. Diagram of a Thermocouple Gauge.[4]
Pirani Gauges
In a Pirani gauge (See Figure 2), two filaments (platinum alloy in the best
gauges), act as resistances in two arms of a Wheatstone bridge. The reference
filament is immersed in a fixed-gas pressure, while the measurement filament is
exposed to the system gas.
A current through the bridge heats both filaments. Gas molecules hit the heated
filaments and conduct away some of the heat. If the gas pressure (or
composition) around the measurement filament is not identical to that around the
reference filament, the bridge is unbalanced and the degree of unbalance is a
measure of the pressure. In reality, modern Pirani gauges electronically adjust
the unbalance and use the current needed to bring about balance as a measure
of the pressure. This improves the linearity of measurement.
Any particular Pirani gauge has roughly the same dynamic range as a
thermocouple gauge but the measurement principle allows these gauges to
cover a greater total range (from 20 Torr to 10-5 Torr) than is available from the
thermocouple principle. Pirani gauges and their circuitry are typically ten times
faster than thermocouple gauges.[5]
Figure 2. Diagram of a Pirani Gauge.[5]
Ionization Gauges
Ion gauges allow measurement of pressure in vacuum chambers. There are two
types of tubulated hot filament ion gauge
· Bayard-Alpert (B-A) (See Figure 3) and
· Schulz-Phelps (S-P).
They differ only in the physical size and spacing of their electrodes. Both have
heated filaments biased to give thermionic electrons of 70e -- energetic enough
to ionize any residual gas molecules during collisions. The positive ions formed
drift to an ion collector held at about 150V. The current measures gas number
density, a direct measure of pressure. With a suitable controller, the commonly
available B-A ion gauges will measure pressures between 1 x 10-4 to 1 x 10-9
Torr. The electrode spacing of an S-P gauge can increase the upper pressure
measurement limit to 1 Torr.[6]
Figure 3. Diagram of an Ionization Gauge.[6]
Pressure Sensors (pressure > one atmosphere)
In general, the design of pressure sensors employed for measurement of
pressure higher than one atmosphere differs from those employed for pressure
less than one atmosphere. Most pressure sensors used this pressure range
require the transduction of pressure information into a physical displacement.
Measurement of pressure requires techniques for producing the displacement
and means for converting such displacement into a proportional electrical signal.
Diaphragms, Bellows, and Bourdon Tubes
One common element used to convert pressure information into a physical
displacement is the diaphragm. A diaphragm is like a spring, and therefore
extends or contracts until a Hooke's law force is developed that balances the
pressure difference force. A bellows (See Figure 4) is another device much like
the diaphragm that converts a pressure differential into a physical displacement,
except that here the displacement is much more a straight-line expansion. Figure
4 also shows how an LVDT can be connected to the bellows so that the pressure
measurement is converted directly from displacement to voltage. In addition, the
displacement and pressure are nearly linearly related, and because the LVDT
voltage is linear with displacement, the voltage and pressure are also linearly
related.
Figure 4. Diagram of a Bellows Pressure Sensor.[7]
A common pressure-to-displacement conversion is accomplished by a specially
constructed tube (See Figure 5). If a section of tubing is partially flattened and
coiled as shown, then the application of pressure inside the tube causes the tube
to uncoil. This then provides a displacement that is proportional to pressure.
Figure 5. Diagram of a Coiled Tube Pressure Sensor.[7]
Many techniques are used to convert the displacements generated in the
previous examples into electronic signals. The simplest technique is to use a
mechanical linkage connected to a potentiometer. In this fashion, pressure is
related to a resistance change. Other methods of conversion employ strain
gauges directly on a diaphragm. LVDTs and other inductive devices are used to
convert bellows or Bourdon tube motions into proportional electrical signals.[7]
Solid-State Pressure Sensors
Integrated circuit manufacturers have developed composite pressure sensors
that are particularly easy to use. These devices commonly employ a
semiconductor diaphragm onto which a semiconductor strain gauge and
temperature-compensation sensor have been grown. Appropriate signal
conditioning is included in integrated circuit form, providing a dc voltage or
current linearly proportional to pressure over a specified range.
These devices are available for absolute-, gauge-, and differential-pressure
measurement. They are simple to use, often needing only three connections: a
dc supply, ground, and signal output. The connection to the measurement
environment is made through a fitting or welded-pipe connection.
Signal Conditioning for Special Sensors
The above sensors often require very specific signal conditioning, and in many
cases the sensors must be purchased with a sensor-specific controller. If your
sensor requires custom signal conditioning, integrating your measurement into
your DAQ system can often be accomplished by GPIB control. Many available
controllers are GPIB compliant, making it possible to automate your
measurements and integrate analysis and reporting with application software
such as LabVIEW or LabWindows/CVI.
References
[1] Johnson, Curtis D, “Pressure Principles” Process Control Instrumentation
Technology, Prentice Hall PTB.
[2] Sensotec.com, “Honeywell Sensotec Frequently Asked Questions”,
http://www.sensotec.com/pdf/FAQ_092003.pdf (current November 2003).
[3] Daytronic.com, “Strain Gauge Pressure Transducers”,
http://www.daytronic.com/products/trans/t-presstrans.htm (current November
2003)
[4] lesker.com, “Thermocouple Gauge Notes”,
http://www.lesker.com/newweb/Pressure_Measurement/Thermocouple/technical
_info.cfm?CFID=204107&CFTOKEN=10873921&section=thermo&init=skip
(current November 2003).
[5] lesker.com, “Pirani Vacuum Gauges Notes”,
http://www.lesker.com/newweb/Pressure_Measurement/Pirani/VacuumGauge_C
ontroller_TechInfo.cfm?CFID=204107&CFTOKEN=10873921&section=pirani&ini
t=skip (current November 2003).
[6] lesker.com, “Hot Filament Ion Gauges Notes”,
http://www.lesker.com/newweb/Pressure_Measurement/Hot_Filament/TechInfo.c
fm?CFID=204107&CFTOKEN=10873921&section=hotfilament&init=skip (current
November 2003).
[7] Johnson, Curtis D, “Pressure Sensors” Process Control Instrumentation
Technology, Prentice Hall PTB.