Pressure Thermometers

About pressure thermometers

The sensing element in a pressure thermometer consists of a bulb containing gas. If the gas were not constrained, temperature rises would cause its volume to increase. However, because it is constrained in a bulb and cannot expand, its pressure rises instead. As such, the pressure thermometer does not strictly belong to the thermal expansion class of instruments but is included because of the relationship between volume and pressure according to Boyle's gas law: PV = KT.

The change in pressure of the gas is measured by a suitable pressure transducer such as the Bourdon tube. This transducer is located remotely from the bulb and connected to it by a capillary tube. The need to protect the pressure measuring instrument from the environment where the temperature is being measured can require the use of capillary tubes up to 5m long, and the temperature gradient and hence pressure gradient along the tube acts as a modifying input which can introduce a significant measurement error. Correction for this using the principle of introducing an opposing modifying input can be carried out.

This includes a second, dummy capillary tube whose temperature gradient is measured by a second Bourdon tube. The outputs of the two Bourdon tubes are connected together in such a manner that the output from the second tube is subtracted from the output of the first, thus eliminating the error due to the temperature gradient along the tube.

Pressure thermometers are used to measure temperatures in the range between -250°C and +2000°C. Their typical accuracy is ±0.5% of full-scale reading.

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