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.
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.