Crystalline quartz thermometers and its working and construction


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The quartz crystal resonant frequency is temperature dependent, hence quartz resonant frequency measurement is a relation of temperature at which the crystal is. Quartz thermometers measure temperature automatically, quickly and with very high resolution.

Construction:

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The thermometer’s temperature sensing quartz resonator is located in a small sensor probe which connects through a length of cable to its oscillator. The oscillator is located in the main cabinet but can be physically removed as a unit to permit measuring temperatures at a distance from the cabinet. The cabinet, otherwise contains what is essentially a special frequency counter which displays the measured temperature directly. The temperature measurements are made automatically.

The outer shell of the probe is fabricated from stainless steel for chemical stability. The quartz wafer is situated parallel to and about 0.01 inch away from the end of the probe.

Working:

A quartz crystal, when coupled to conventional oscillator circuitry by replacing its resonant tank circuit, results in an oscillator whose frequency is controlled by the crystal as shown schematically:

The precise control depends on the crystal geometry and orientation with respect to the crystallographic axis referred to as a cut. The AT-cut, for example, is used in very stable frequency oscillators.

In a quartz thermometer, the transduction mechanism consists of the changes in the elastic and piezoelectric properties of the crystal as a function of temperature. These changes result in corresponding changes of the oscillator frequency. The frequency of an oscillator can be represented by a third-order polynomial in temperature.

The outputs from the two oscillators are mixed, and the output of the mixer is a much lower frequency but retains the linear relationship between frequency and temperature. The quartz thermometer measures temperatures in the range -80 to 250°C with an accuracy of ±0.075°C. The sensitivity is 1000 Hz/°C and the corresponding resolution is 0.0001°C.

Advantages:

  • Highly linear output

  • Long-term stability and reliability

  • High resolution of order 0.001°C

  • Excellent repeatability

Limitations:

  • Low-temperature range
  • Crystals have strong cross

Also read: WHAT IS RESISTANCE THERMOMETERS ?

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