The liquid in glass thermometer is the most common, easy to use and versatile thermometer. This type of thermometer consists of a thin glass tube inside a glass stem with a bulb at one end. When the temperature rises the liquid expands, so the length of the liquid in the capillary tube increases.
Each range of temperature to be measured requires a different liquid to be used. The most common liquid and the associated temperature ranges are shown below.
|
Liquid |
Temperature Range ( º C) |
|
Mercury |
-39 to 357 |
|
Pressurised Mercury |
-39 to 500 |
|
Pressurised Mercury (in quartz) |
-39 to 800 |
|
Alcohol |
-120 to 60 |
|
Pentane |
-200 to 30 |
Mercury has several advantages. It is opaque, does not wet glass, is easily purified and does not distil at low temperatures, and it is the most commonly used liquid. In addition, a correction for the exposed mercury column is easily made. It is
where
is the middle temperature of the stem.
Some of the errors of the liquid in glass thermometer are common to all liquid. The bulb expands along with the liquid and this tends to reduce the temperature measured. The expansion is up to 10 per cent of the liquid expansion when mercury is used. The capillary tube may not be uniform, the fixed points not precise, and over a long period of time the glass itself expands.