A quartz thermometer is a high-precision, high-accuracy temperature sensor. It measures temperature by measuring the frequency of a quartz crystal oscillator. The oscillator contains a specially cut crystal whose frequency has a nearly linear temperature coefficient, so temperature measurement reduces essentially to frequency measurement.
Quartz thermometers can achieve a resolution of 0.0001 °C and an accuracy of 0.02 °C over the range 0 to 100 °C. The high linearity allows high precision over the important temperature range with only one convenient reference point for calibration, the triple point of water. The triple point of water is a fixed point of the thermodynamic temperature scale at 273.16 K, or 0.01 °C.
The piezoelectric effect of quartz causes it to vibrate steadily under an alternating electric field. The resonance frequency depends on the crystal dimensions, cut orientation and elastic constants, and the elastic constants vary with temperature. By choosing a specific cut angle, such as the LC or SC cut, the frequency-temperature curve can be made nearly linear over a temperature range, converting temperature change into a precisely measurable frequency change.
The electronics count the oscillator frequency and compare it with a reference frequency to determine temperature. Because frequency can be counted very accurately, resolution is far higher than that of ordinary thermistors or thermocouples. Long-term stability is also good, because ageing and hysteresis of the quartz crystal can be controlled through material selection and packaging.
Hewlett-Packard introduced quartz thermometers in 1965, and its successor Agilent discontinued the Model 2804A quartz thermometer. The Model 2804A was a representative early commercial instrument and was widely used in metrology laboratories, temperature calibration and precision measurement.
Other manufacturers produce quartz crystals with nearly linear temperature coefficients for thermometers of similar performance. Modern quartz thermometers are usually combined with digital displays, computer interfaces and automatic calibration software for high-precision measurement in laboratories and industry.
Quartz thermometers are used for temperature metrology standards, precision baths, ocean temperature measurement, geophysical observation and industrial process control. In oceanography they can measure small changes in seawater temperature for studies of ocean heat content and climate change. In metrology they can serve as transfer standards, carrying national temperature standards to other sensors.
Limitations include a relatively narrow measurement range, sensitivity to mechanical vibration and mounting stress, slow response and higher cost than ordinary temperature sensors. They are best suited to applications requiring very high resolution and stability rather than rapidly changing temperatures. Crystal cut, packaging and counting electronics all affect final accuracy.