1. How do dynamic pressure sensors work?
Dynamic pressure sensors are instruments used for measuring rapid changes in pressure of liquids and gases. The name ‘dynamic pressure sensor’ is a misnomer because strictly speaking, dynamic pressure refers to the kinetic energy component of total pressure (see first paragraph here). However, dynamic pressure sensors do not detect dynamic pressure, they detect rapid changes in static pressure (e.g. compression of air inside an engine cylinder). The pressure sensitive element within the sensor is a piezoelectric crystal (also known as a piezoelectric transducer). Owing to the piezoelectric effect, the crystal generates an electrostatic charge in response to mechanical force. The electrostatic charge decays rapidly and so when the fluid is at a constant pressure the sensors output is zero. A sudden pressure rise causes the piezoelectric crystal to generate an electrostatic charge as a function of the increase in pressure. For example, if pressure rises from 100 to 150 kPa, the sensor will detect a pressure of 50 kPa. The response of a piezoelectric crystal to a step change in pressure is illustrated in the graph below. The signal’s rate of decay is defined by the discharge time constant of the crystal.
A typical configuration for a dynamic pressure sensor is illustrated below. The sensor is housed in a metal cylinder which is sealed by a thin stainless steel diaphragm. One end of the diaphragm is exposed to the process fluid and the other side is exposed to atmospheric pressure. The piezoelectric crystal is coupled to the back of the diaphragm so that the pressure felt by the diaphragm is transmitted to the crystal which generates an electrostatic charge in response. Since piezoelectric crystals are sensitive to vibrations, they are prone to misinterpreting external vibrations (e.g. of an engine) as pressure changes in the fluid. The effect of vibrations is cancelled out by a method known as acceleration-compensation in which an accelerometer, consisting of a seismic mass and an additional piezoelectric crystal, is used to measure the external vibrations. The measured acceleration is then subtracted from the output of the main piezoelectric crystal. Most dynamic pressure sensors include built in charge amplifiers and additional electronics for converting their outputs into standard signal forms such as 0-10 V or 4-20 mA. Although piezoelectric crystals are passive devices and therefore do not require an external supply voltage, the built in electronics do require a supply voltage.
2. General characteristics of dynamic pressure sensors
Dynamic pressure sensors detect rapid changes in pressure of liquids and gases. They do so by utilizing a piezoelectric crystal, which generates a short lived electrostatic charge in response to mechanical force. Dynamic pressure sensors are extremely robust, and have very fast response times. This enables them to detect rapid changes in pressure e.g. inside an internal combustion engine cylinder. Many dynamic pressure sensors are compatible with fluid temperatures of up to 350 °C, with some compatible up to 700 °C. Most sensors include built in electronics for amplifying the output and converting it into to a standard output signal form (e.g. 0-10 V or 4-20 mA), though unamplified outputs are also available.
3. Input and output signals
Piezoelectric crystals are passive devices which generate an electrostatic charge in response to mechanical force. Unamplified dynamic pressure sensors are therefore able to operate without an external voltage supply. However, the output of an unamplified sensor must be connected to a charge amplifier before it can be read by a process controller. Most dynamic pressure sensors include built in charge amplifiers and additional electronics which convert the output into standard signal forms (e.g. 0-10 V or 4-20 mA). Sensors with built in electronics require a supply voltage, typically 5-30 V dc.
4. Applications of dynamic pressure sensors
Piezoelectric dynamic pressure sensors are used for measuring rapidly changing pressures with high temporal resolution. Typical applications include:
- Characterizing high speed valve performance.
- Characterizing pumps and air compressors.
- Monitoring internal combustion engine cylinder pressure.
- Monitoring gas turbines.
- Studying turbulent flows.
5. Typical specification
|Measurement range||0-0.1 bar to 0-8,000 bar|
|Fluid temperature||-50 to 350 °C (up to 700 °C available)|
|Natural frequency||30-400 kHz|
|Time response||1-500 µs|
|Crystal sensitivity||1-40 pC/bar|
|Long term stability||High|
|Supply voltage||5-30 V or passive|
|Output voltage||0-10 V, 4-20 mA or unamplified|
|Ingress protection||IP65 or IP67|
|Passive / active||Passive (excluding electronics)|
|Contact / non-contact||Contact|
6. Purchasing tips
- Natural frequency: The output amplitude of the piezoelectric crystal (for a fixed input excitation) is frequency dependent and increases significantly near to the crystal’s natural frequency. Conversely, the amplitude decreases at very low excitation frequencies. The sensor can only measure pressure changes at intermediate frequencies, where the sensors amplitude is not dependent on frequency. Sensors are typically available with natural frequencies of 30-400 kHz.
- Discharge time constant: The time taken for a 63.2% decay of the piezoelectric crystal output amplitude. Sensors with large time constants can measure low frequency pressure changes.
- Overload capacity: The overload capacity is typically 150-300% of rated pressure. This is the maximum pressure that can be applied without damaging the pressure sensor.
- Chemical Resistance: The manufacturer’s datasheet may provide information on chemical compatibility with highly corrosive fluids. Most diaphragms are made of various grades of stainless steel. Even if it appears that stainless steel is compatible with the fluid, minor corrosion of the diaphragm can still reduce accuracy (e.g. hydrogen embrittlement). Sensors with ceramic or ceramic coated diaphragms provide improved protection from highly corrosive fluids.
- Piezoelectric crystal sensitivity: Sensors are available with varying sensitivities, typically 1-40 Pico Coulombs per bar. High sensitivity sensors are less prone to disturbance from background electrical noise but more sensitive to external mechanical vibrations.
- Built in electronics: Most sensors include built in electronics for amplifying the output signal and converting it into a standard output signal form (e.g. 0-10 V or 4-20 mA). Sensors without built in electronics do not require a supply voltage because the piezoelectric crystals are passive components. However, sensors without built in electronics must be connected to external charge amplifiers.
7. Advantages of dynamic pressure sensors
Dynamic pressure sensors:
- Are extremely robust and capable of operating at high temperatures.
- Have extremely fast response times.
8. Disadvantages of dynamic pressure sensors
Dynamic pressure sensors:
- Are only capable of measuring changes in pressure, which severely limits their range of applications.
- Are not suitable for use with capillary tubes (which are used for cooling hot process fluids) because they dampen the pressure peaks.
9. Application tips
- Frequency response: As explained within the purchasing tips section, dynamic pressure sensors can only be operated within the frequency range for which their amplitude is not frequency dependent. The manufacturer can provide the recommended frequency range. It is also possible to experimentally characterize the sensors frequency response using an electrical impedance analyzer.
- Flush mounting: The sensor should preferably be mounted flush with the chamber or vessel to minimize the damping of pressure changes. Non-flush mounting of the sensor limits the maximum measurement frequency as a linear function of recess length. The maximum frequency (of the pressure change) is about 1/3 of the natural frequency of the recess and can be calculated using the equation below.
- Tightening torque: When installing the sensor, it is important to abide by the recommended tightening torque provided in the data sheet. Failure to abide my result in reduced measurement accuracy.
- Fluid sealing: Most dynamic pressure sensors have a flat base at the end of their screw thread. This enables a Dowty seal washer to be used for providing a leak tight seal. Dowty seals are easier to use and often provide a more reliable seal than Teflon tape and liquid sealants.
- External charge amplifier: External charge amplifiers (required for unamplified sensors) should be connected as close as possible to the sensor, preferably via EMI shielded cable, to minimize electrical noise.