ABSTRACT
A number of commonly used temperature-measuring devices utilize the physical phenomenon of thermal expansion. The expansion of solids is utilized in the bi-metallic thermometer by the differential expansion of bonded strips of two metals; the expansion of a gas is used in the gas thermometer; and the expansion of a liquid is used in the liquid-in-glass thermometer. The latter is the most common type of thermometer utilizing the thermal expansion principle. By using different liquids a range of liquid-in-glass thermometers are available for measuring temperatures in the range of about−70 to 540 ◦C.Mercury is themost common thermometer liquid used inHVAC environments as its temperature range (−39 to 537 ◦C) covers most temperatures experienced in the built environment. Thermometers are calibrated during manufacture for two or more temperatures,
which are usually the triple and boiling points of water for climatic temperature measurements. The distance between the calibration points is divided into a linear scale and a probable measuring error is plus or minus one scale division. However, the accuracy obtainable depends on the instrument quality and temperature range. For good quality calibrated thermometers the error can be as low as 0.2K over the range 0-100 ◦C. When measuring temperatures in pipes or ducts, accuracy can be improved by fitting the instrument in a thermometer well. A thermometer measuring air or gas temperatures can be affected by radiation from surrounding surfaces and this should be minimized if the temperature of the gas differs substantially from that of the surrounding surfaces. This may be achieved by shielding or aspiration. Shielding is accomplished by placing highly reflective surfaces between the thermometer bulb
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and the surrounding surfaces but care must be taken so that air movement around the bulb is not appreciably restricted. Aspiration is attained by drawing the surrounding air or gas over the bulb using a fan if necessary. Because of the relatively large thermal mass of the instrument, ample time is required to attain temperature equilibrium with the surrounding fluid before reading the temperature scale. Parallax may be avoided by keeping the eye level with the top of the liquid column.
