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.