Types of thermometers
Liquid/vapour thermometers rely on the expansion or contraction of liquid in a very fine bore calibrated tube or capillary. It is important to ensure that the liquid column in the instrument is continuous, otherwise the reading will be inaccurate.
Bi-metallic thermometers consist of two metals with different coefficients of expansion. Bimetallic thermometers are susceptible to vibration and should only be installed in positions that are free from this effect.
Thermocouples rely on heat applied to the junction of two dissimilar metals generating a very small voltage which can be measured. A change in voltage output will indicate a change in temperature.