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:
- 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 SensorsBecause 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 SensorsOnce 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.
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
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”, (current November 2003).
[3] Daytronic.com, “Strain Gauge Pressure Transducers”, (current November 2003)
[4] lesker.com, “Thermocouple Gauge Notes”, (current November 2003).
[5] lesker.com, “Pirani Vacuum Gauges Notes”, (current November 2003).
[6] lesker.com, “Hot Filament Ion Gauges Notes”, (current November 2003).
[7] Johnson, Curtis D, “Pressure Sensors” Process Control Instrumentation Technology, Prentice Hall PTB.