Introduction to Dynamic Pressure Sensors
Piezoelectric pressure sensors measure dynamic pressure. They are typically not suited for static pressure measurements. Dynamic pressure measurements including turbulence, blast, ballistics, and engine combustion require sensors with special capabilities. These capabilities include fast response, ruggedness, high stiffness, extended ranges, and the ability to measure quasi static pressures. These are standard features associated with PCB® quartz pressure sensors.
PCB® manufactures two types of pressure sensors. Charge mode pressure sensors generate a high impedance charge output. ICP® (Integrated Circuit Piezoelectric) voltage mode sensors feature built-in microelectronic amplifiers that convert the high impedance charge signal into a low impedance voltage output.
Piezoelectric pressure sensors are available in various shapes and thread configurations to allow suitable mounting for various types of pressure measurements. Quartz crystals are used in most sensors to ensure stable, repeatable operation. The quartz crystals are usually preloaded in the housings to ensure good linearity. Tourmaline, another stable naturally piezoelectric crystal, may be used in PCB sensors where volumetric sensitivity is required. Figure 1 is a general purpose pressure sensor with built-in electronics.
Figure 1 – Typical quartz pressure sensor cross section
When a positive pressure is applied to an ICP pressure sensor, the sensor yields a positive voltage. The polarity of PCB charge mode pressure sensors is the opposite: when a positive pressure is applied, the sensor yields a negative output. Charge output sensors are usually used with external charge amplifiers that invert the signal. The resulting system output polarity of a charge output sensor used with a charge amplifier produces an output that is the same as an ICP sensor. Reverse polarity sensors are also available.
Most PCB piezoelectric pressure sensors are constructed with either compression mode quartz crystals preloaded in a rigid housing or unconstrained tourmaline crystals. These designs give the sensors microsecond response times and resonant frequencies in the hundreds of kilohertz, with minimal overshoot or ringing.
The mechanical structure of the pressure sensor will impose a high frequency limit. The sensitivity begins to rise rapidly as the natural frequency of the sensor is approached. The increase in sensitivity is illustrated in Figure 2.
It is generally acceptable to use sensors over a range where the sensitivity deviates by less than ± 5%. The upper frequency limit occurs at approximately 20% of the sensor resonant frequency.
The high frequency response can be limited by drive current, cable length and cable capacitance. For more detailed information on driving long cables refer to the PCB Driving Long Cables webpage.
The low frequency response of a charge mode pressure sensor is determined by the charge amplifier. The discharge time constant (DTC) of the amplifier that sets the low frequency response can be very long or very short depending on the charge amplifier model used. A longer DTC allows for lower frequency measurements. A shorter DTC will limit the low frequency response.
Internal resistance and capacitance values set the discharge time constant and the low frequency response of ICP® pressure sensors. The discharge time constant establishes the low frequency response analogous to the action of a first order R-C high pass filter. The DTC of the signal conditioner should also to be taken into consideration. It influences the low frequency response of the overall system. Refer to the Low Frequency Response of ICP® Sensors section in the PCB General Signal Conditioning guide for more detailed information.
The quartz crystals of a piezoelectric pressure sensor generate a charge when pressure is applied. Even though the electrical insulation resistance is quite large, the charge eventually leaks to zero. The rate at which the charge leaks back to zero is dependent on electrical insulation resistance.
In a charge mode pressure sensor with a voltage amplifier, the leakage rate is fixed by capacitance and resistance values in the sensor, low noise cable, and the external source follower voltage amplifier. When a charge mode pressure sensor is used with a charge amplifier, the leakage rate is fixed by the electrical feedback resistor and capacitor in the charge amplifier.
The resistance and capacitance of the crystal and the built-in electronics normally determine the leakage rate in an ICP® pressure sensor.
The output characteristic of piezoelectric pressure sensor systems is that of an AC coupled system. Repetitive signals decay until there is an equal area above and below the original base line. As magnitude levels of the monitored event fluctuate, the output remains stabilized around the base line with the positive and negative areas of the curve remaining equal. Figure 3 represents an AC signal following this curve.
In this example, a 0 to 2 volt output signal is generated from an AC coupled pressure application with a one second steady state pulse rate and one second between pulses. The frequency remains constant, but the signal quickly decays negatively until the signal centers around the original base line (where area A = area B). Peak to peak output remains the same.
Figure 3 - Typical AC coupled piezoelectric system output signal.
Precision mounting of pressure sensors is essential for good measurements. Always check the installation drawings supplied in the manual with the sensor, or contact PCB to request detailed mounting instructions. Use good machining practices for the drilling and threading of mounting ports, and torque the sensors to the noted values. Mounting hardware is supplied with PCB sensors. Various standard thread adaptors are available to simplify some sensor installations.
For free-field blast applications, use aerodynamically clean mounts to minimize unwanted reflections from mounting brackets or tripods.
The sensing crystals of many pressure sensors are located at the diaphragm end of the sensor. Side loading of this part of the sensor during a pressure measurement creates distortions in the signal output.
It is important to avoid unusual side loading stresses and strains on the upper body of the sensor. Proper installation minimizes distortions in the output signal. Causes of side strains to the upper body include: a taut cable pulling at right angles to the electrical connector, and using a heavy adaptor with a cable attached to the small electrical connector in an environment with high transverse vibration.
In applications such as free-field blast measurements, a pressure sensor mounted in a thin plate can be subjected to side loading stresses when the pressure causes the plate to flex. Use an O-ring mount to minimize this effect.
Flush mounting of pressure sensors in a plate or wall is desirable to minimize turbulence, avoid a cavity effect, or avoid an increase in a chamber volume. Recessed mounting is more desirable in applications where the diaphragm end of the pressure sensor is likely to be subjected to excessive flash temperatures or particle impingement. Recessed mounting of pressure sensors will degrade the ability to measure high frequencies. The cavity effect of this type of mounting will typically reduce the sensor resonant frequency. For more detailed information review Introduction to Air Blast Measurements – Part 2: Interfacing the Transducer. See Figure 4 for typical flush mount installation. See Figure 5 for typical recessed mount installation.
Figure 5 - Typical recessed mount for 113B pressure sensor series.
Most PCB pressure sensors are supplied with seal rings for flush mounting. Certain models, such as Series 111, 112, and 113 can be provided with seal sleeves for recess mounting ports. Request seal sleeves when ordering. Order enough spare seal rings or sleeves for applications that require frequent removal and reinstallation of the pressure sensor. Before reinstalling a pressure sensor, check the mounting port to make sure that the old, distorted seal ring has been removed from the mounting hole. If you are using PCB® pressure sensors and have lost or misplaced the seals, call PCB to request replacement seals as no-charge samples.
PCB has various mounting adaptors that aid in pressure sensor mounting. Pressure sensors and adaptors with straight machined threads use a seal ring as a pressure seal. Pipe thread adaptors have tapered threads, which result in the threads creating the pressure seal. For more detailed information on pressure sensor adaptors and accessories refer to the PCB Accessories for Pressure Sensors webpage.
Control of the location of the pressure sensor diaphragm is achieved with a straight thread/seal ring mount. Pipe thread mounts do not allow precision positioning of the sensor depth because the seal is achieved through progressive tightening of threads in the tapered hole until the required thread engagement is reached. Pipe threads offer the convenience of an easier machined port than straight threads. Pipe thread mounts are well suited for some general applications.
Automotive in-cylinder pressures, ballistic pressures, and free-field blasts are examples of applications where thermal shock accompanies the pressure pulse. The thermal shock can be in the form of a radiant heat, such as the flash from an explosion, heat from convection of hot gasses passing over a pressure sensor diaphragm, or conductive heat from a hot liquid.
Virtually all pressure sensors are sensitive to thermal shock. When heat strikes the diaphragm of a piezoelectric pressure sensor that has crystals contained in an outer housing, the heat can cause an expansion of the case surrounding the internal crystals. Although quartz crystals are not significantly sensitive to thermal shock, the case expansion causes a lessening of the preload force on the crystals causing a negative signal output. To minimize this effect, various methods are used.
Certain PCB quartz pressure sensors feature internal thermal isolation designs to minimize the effects of thermal shock. Some models feature baffled diaphragms. Others that are designed to maximize the frequency response may require thermal protection coating, recess mounting, or a combination of these to lessen the thermal shock effects. Coatings include silicone grease, (may also be used to fill a recess mounting hole), RTV silicone rubber, vinyl electrical tape, and ceramic. The RTV and tape are used as ablatives, while the ceramic coating is used to protect diaphragms from corrosive gasses and particle impingement.
Crystals other than quartz are used in some PCB sensors. Though sensitive to thermal shock, tourmaline is used for shock tube and underwater blast sensors. In shock tube measurements, the duration of the pressure measurement is usually so short that a layer of vinyl tape is sufficient to delay the thermal effects for the duration of the measurement. In underwater blast applications, heat transfer through the water is not significant.
Thermal shock effects do not relate to the pressure sensor temperature coefficient specification. The temperature coefficient specification refers to the change in sensitivity of the sensor relative to the static temperature of the sensor. Since the thermal shock effects cannot be easily quantified, they must be anticipated and minimized by one of the techniques mentioned to ensure better measurement data.