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Types of thermometer

The following properties or instruments have all been used to measure temperatures and are thus the basis of thermometers.
liquid in glass
platinum resistance
vapour pressure
bimetallic strip
liquid pressure
optical pyrometer

Liquid-in-glass thermometer

This type of thermometer is probably the most widely used and consists of a liquid in a thin-walled glass bulb to which is fixed a thin capillary tube (Figure 1). The property that varies with temperature is the volume of the liquid, which is measured by reading the length of the liquid thread, assuming the bore of the capillary tube to be uniform.

The ranges of the most common liquid-in-glass thermometers are as follows:

The mercury-in-glass thermometer has the advantages that it is direct-reading and that mercury is opaque, does not wet glass, is easily purified and does not distil at low temperatures. The relative expansion of mercury, though uniform, is fairly small, however, and the movement of the liquid thread is sometimes jerky. This thermometer is subject to various errors, of which the largest is due to the expansion of the glass bulb with rising temperature, which may be up to 10% of the mercury expansion, while with falling temperatures the glass contracts relatively slowly. There is also a small long-period expansion of the glass, known as the secular change (around 0.01 oC per year). Further errors may arise if the bore of the capillary tube is not uniform, in marking the fixed points and from internal and external pressure on the bulb.

Finally, a correction for the exposed mercury column may be required: this correction is equal to 0.000 16n(t tm), where n is the number of exposed Celsius degrees, t is the indicated temperature and tm is the mean temperature of the stem. The error may be as much as 10 oC at 400 oC.
© Keith Gibbs 2007