Quartzdyne's Pressure Sensor is a quartz resonator that changes frequency in response to pressure. The structure is a thickwalled hollow cylinder with closed ends. A thickness-shear-mode disc resonator divides the central portion of the hollow cylinder. Fluid pressure on the exterior hydrostatically compresses the quartz cylinder, producing internal compressive stress in the resonator. The vibrating frequency of the sensor changes in response to this stress.
The sensor is made of crystalline quartz , which is naturally piezoelectric. Quartz resonators use the inverse piezoelectric effect to induce the resonator to vibrate at its mechanical resonant frequency when electric fields are applied to its electrodes. An oscillator circuit supplies the power and allows the frequency to be measured.
Because frequency (and its inverse, time) can be measured with greater precision than any other parameter, the sensor's frequency output provides high resolution pressure measurement.
Quartz is a perfectly elastic material, providing sensor stability and repeatability. The quartz resonator has a high Q, which means that its vibration can be driven with very little electrical power. This provides long operating life in battery-powered applications.
THREE-CRYSTAL TRANSDUCER ASSEMBLY
The "transducer" is the entire assembly required to produce the frequency or digital output signals. Each Quartzdyne transducer relies on three thickness-shear-mode quartz resonators: the pressure sensor, a temperature sensor, and a reference crystal.
The quartz pressure sensor changes frequency in response to changes in pressure. Because the pressure crystal also exhibits a small change in frequency with temperature, a quartz temperature sensor is used to provide digital temperature compensation. We use a quartz crystal temperature sensor because it provides stability, shock resistance, and high resolution with low power consumption.
The reference crystal is the SC-cut crystal discovered by Quartzdyne co-founder Dr. Errol P. EerNisse. (The SC-cut is the preferred crystal for use in precision laboratory frequency counters.) The output signal of the reference crystal is mixed with the pressure and temperature signals to reduce their frequencies from the MHz range to the kHz range. (This simplifies the frequency counting requirements.) At room temperature and pressure, the frequencies of the pressure, temperature, and reference crystals are approximately 7.22 MHz, 7.15 MHz, and 7.20 MHz, respectively, providing pressure and temperature outputs of 20 kHz and 50 kHz. The pressure signal increases with pressure and the temperature frequency decreases with temperature.
The reference signal can also be used to count the two sensor frequencies. Since the reference signal is digitally-temperature compensated, it introduces no additional temperature error. Resolutions for pressure of 0.01 psi and temperature of 0.001 °C are easily obtained with one second gate times.
The metal housing of the transducer contains the high pressure fluid, and shock mounts the three crystals and the circuit. The housing design also ensures that the three quartz crystals are thermally coupled for temperature compensation.