What Is Pressure?
Pressure is defined as force per unit area that a fluid exerts on its surroundings. You can measure this force by detecting the amount of deflection on a diaphragm positioned inline with the fluid. Given the known area of the diaphragm, pressure can then be calculated. Pressure sensors are packaged with a scale that provides a method to convert to engineering units. The SI unit for pressure is the Pascal (N/m2), but other common units of pressure include psi, atmospheres, bars, inches of mercury, millimeters of mercury, and torr.
Pressure Measurement Methods
There are three methods for measuring pressure: absolute, gauge, and differential. Absolute pressure is referenced to the pressure in a vacuum, whereas gauge and differential pressures are referenced to another pressure such as the ambient atmospheric pressure or pressure in an adjacent vessel.
Figure 1. Pressure Sensor Diagrams for Different Measurement Methods
The absolute measurement method is relative to 0 Pa, the static pressure in a vacuum (shown as REF in Figure 1). The pressure being measured is being acted upon by atmospheric pressure in addition to the pressure of interest. Therefore, absolute pressure measurement includes the effects of atmospheric pressure. This type of measurement is well-suited for atmospheric pressures such as those used in altimeters or vacuum pressures.
Gauge and differential measurement methods are relative to some other dynamic pressure. In the gauge method, the reference is the ambient atmospheric pressure. This means that both the reference and the pressure of interest are acted upon by atmospheric pressures. Therefore, gauge pressure measurement excludes the effects of atmospheric pressure. These types of measurements are easy to identify in examples such as tire pressure and blood pressure measurements.
Differential pressure is very similar to gauge pressure; however, the reference is another pressure point in the system rather than the ambient pressure. You can use this method to maintain a relative pressure between two vessels such as compressor tank and associated feed line.
Pressure Sensor Types
There are a variety of pressure sensor designs due to different measurement conditions, ranges, and materials used in construction of the sensor. Common pressure sensor types are bridge-based, amplified, and piezoelectric sensors. Pressure is measured by converting the physical phenomenon to an intermediate form, such as displacement, which can be measured by a transducer.
Wheatstone bridge- or strain-based transducers are a common way of measuring displacement. Sensors using this type of design meet a variety of requirements such as accuracy, size, cost, and ruggedness. Bridge sensors are used for high- and low-pressure applications, and can measure absolute, gauge, or differential pressure. Bridge-based sensors use a strain gage to detect the deformity of a diaphragm subjected to the applied pressure (see Figure 2).
Figure 2. Cross Section of a Typical Bridge-Based Pressure Sensor 
A change in pressure causes the diaphragm to deflect, corresponding to a resistance change of the strain gage. This can be measured by a conditioned DAQ system. Foil strain gages can be bonded directly to a diaphragm or bonded to an element that is connected mechanically to the diaphragm. Silicon strain gages are sometimes used as well. Using this method, resistors are etched onto a silicon-based substrate and transmission fluid is used to transmit the pressure from the diaphragm to the substrate.
Capacitive Pressure Sensors
Figure 3. Capacitance Pressure Transducer
A variable capacitance pressure transducer measures the change in capacitance between a metal diaphragm and a fixed metal plate. The capacitance between two metals plates changes if the distance between these two plates changes due to applied pressure.
Piezoelectric Pressure Sensors
Figure 4. Piezoelectric Pressure Transducer
Piezoelectric sensors rely on quartz crystals rather than a resistive bridge transducer. Electrodes transfer charge from the crystals to an amplifier built into the sensor. These crystals generate an electrical charge when they are strained. Piezoelectric pressure sensors do not require an external excitation source and are very rugged. The sensors, however, do require charge amplification circuitry and are very susceptible to shock and vibration.
Amplified Pressure Sensors
Sensors that include integrated circuitry, such as amplifiers, are referred to as amplified sensors. These types of sensors may be constructed using bridge-based, capacitive, or piezoelectric transducers. In the case of a bridge-based amplified sensor, the unit itself provides completion resistors and amplification necessary to measure the pressure directly with a DAQ device. While excitation must still be provided, the accuracy of the excitation is less important. The SCXI-1520 offers a solution designed for this type of sensor. However, you could use other NI C Series and PXI modules to measure the output of the amplified sensor when combined with a power supply to provide excitation.
Optical Pressure Sensors
Pressure measurement using optical sensing has many benefits including noise immunity and isolation. Read Fundamentals of FBG Optical Sensing for more information about this method of measurement
Choosing the Right Pressure Sensor
Bridge-based or piezoresistive sensors are the most common type of sensor because of the simple construction and durability. This translates to lower cost and makes them ideal for higher-channel systems. They offer the flexibility of using a variety of signal conditioners based on your performance needs. Bridge sensors are used for high- and low-pressure applications, and can measure absolute, gauge, or differential pressure.
Amplified sensors are built on many different technologies but offer built-in conditioning. This is helpful for lower-channel systems that do not warrant a dedicated signal conditioning system. Because the conditioning is built-in it, you can connect the sensor directly to a DAQ device as long as power is provided to the sensor in some way. For these reasons, amplified sensors are more costly.
Capacitive and piezoelectric pressure transducers are generally very stable and linear, but are sensitive to high temperatures and are more complicated to set up than most pressure sensors. Piezoelectric sensors respond very quickly to pressure changes. For this reason, they are used to make rapid pressure measurements from events such as explosions. Because of their superior dynamic performance, these types of sensors are the least cost-effective and care must be taken to protect the sensitive crystal core.
Designing the Right Measurement System
Bridge-based pressure sensors are by far the most common pressure sensor. The following section describes the necessary signal conditioning to make an effective bridge-based pressure measurement system. The basic signal conditioning requirements include amplification, filtering, excitation, offset nulling, and shunt calibration.
The output of the bridge is relatively small. In practice, most bridge-based sensors output less than 10 mV/V, which means 10 mV per volt of excitation. Therefore, bridge signal conditioners usually include amplifiers that boost the signal level to increase resolution and improve signal-to-noise ratios.
Bridge-based sensors are often located in electrically noisy environments. It is essential to be able to eliminate noise that can couple to strain gages. Lowpass filters, when used with strain gages, can remove the high-frequency noise prevalent in most environmental settings. Digital filtering offers very high levels of rejection with sharp roll-off characteristics without impacting accuracy.
Bridge-based signal conditioners typically provide a constant voltage source to power the bridge. Excitation voltage levels around 3 V to 10 V are common. Although a higher excitation voltage generates a proportionately higher output voltage, the higher voltage can also cause larger errors due to self-heating. It is important that the excitation voltage be very accurate and stable. Alternatively, you can use a less accurate or stable voltage, and measure the excitation voltage too. Some methods such as the ratiometric approach use both a precise excitation as well as feedback to the ADC to provide the highest level of performance.
When a bridge sensor is installed, it is very unlikely that it will output exactly zero volts when the structure or fluid is at rest. Slight variations in resistance among the bridge arms, installation conditions, and lead resistance will generate some nonzero initial offset voltage. You can perform offset nulling by either hardware or software. In software compensation, you take an initial measurement before strain input is applied, and use this offset to compensate subsequent measurements. The hardware balancing method uses an adjustable resistance, a potentiometer, to physically adjust the output of the bridge to zero.
Shunt calibration is the procedure used to verify the output of a bridge-based measurement relative to some predetermined pressure. Shunt calibration involves simulating the input of pressure by changing the resistance of an arm in the bridge by some known amount. This is accomplished by shunting, or connecting, a large resistor of known value (Rs) across one arm of the bridge, creating a known ΔR. The output of the bridge can then be measured and compared to the expected voltage value. The results are used to correct span errors in the entire measurement path, or to simply verify general operation to gain confidence in the setup.
Figure 5. Shunt calibration involves connecting a large resistor of known value (Rs) across one arm of the bridge, creating a known ΔR.
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 that prevents pressure spikes in the event of water hammer. A snubber is a good choice to protect your sensor in certain applications, but the peak impact pressure is sometimes the region of interest. In such a case, you would want to select a pressure sensor that does not include overprotection.
NI Measurement Systems for Pressure Sensors
Pressure measurements are one of the fastest growing types of sensor measurements, and National Instruments provides many options for measuring pressure sensors. From 1 to 1,000+ channels, National Instruments has the right platform for your pressure measurement system.
Figure 6. NI CompactDAQ is recommended for pressure measurements that require isolation.
NI CompactDAQ provides a portable and rugged solution that is recommended for pressure measurements requiring isolation. You can choose from different modules with up to 60 Vrms of channel-to-earth isolation and 24-bit resolution. The NI CompactDAQ platform offers up to four channels per module and 32 channels per chassis. The chassis can stream data through USB, Ethernet, or wireless.
Figure 7. The PXI platform is recommended for the most accurate pressure measurements.
The PXI platform provides a variety of chassis, controller, and module options that give you the power to create a measurement system that meets the specific needs of your application. The PXI platform is recommended for highly accurate pressure measurements. These solutions can scale to thousands of pressure measurement channels for large applications.
Figure 8. The SCXI platform is recommended for a cost-effective solution for pressure measurements.
NI recommends the SCXI platform for a cost-effective solution for high-channel-count pressure measurements. The SCXI platform provides up to eight pressure channels per module and offers options for isolated pressure measurements.
 Sensotec.com, “Honeywell Sensotec Frequently Asked Questions”, http://www.sensotec.com/pdf/FAQ_092003.pdf (current November 2003).