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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Temperature Measurements
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