Thermometers: Bimetal, Liquid Expansion, Mercury Types

Study Notes

These thermometers are inexpensive, rugged, and easy to use.
They have a wide temperature range and a nearly linear response.
They can be equipped with a recorder for continuous monitoring.
They are limited in accuracy, must be installed locally, and cannot measure remote temperatures.
Calibration can shift if the thermometer experiences rough handling.
They are suitable for measurements where local temperature readings are needed and the measurement points are accessible.

Liquids expand proportionally to temperature increases.
This expansion can be used to measure temperature.
The relationship between volume and temperature is described by an equation with coefficients of volumetric expansion.

They consist of a glass stem with a bulb and a capillary tube.
The bulb has a larger volume than the capillary.
The capillary is often made of a different glass than the bulb for better workability.
The stem has a lens-shaped front for magnification and a white-enameled back for contrast.
A restriction in the capillary prevents mercury from flowing back into the bulb.
The open end of the capillary is sealed under vacuum and sometimes contains an inert gas to increase the usable temperature range.
Industrial thermometers often have a metal case to protect the bulb and are placed in a thermal well to minimize breakage.
Mercury is commonly used as the liquid, but its temperature range is limited by its freezing and boiling points.
By pressurizing the thermometer with a gas like nitrogen, the upper temperature limit can be extended to about 538 °C.

Podcast