TABLE OF CONTENTS The methods of measuring temperature can be categorized as follows:
Expansion of materials;
Electrical resistance change;
Thermistors;
Thermocouples;
Pyrometers;
Semiconductors.
Thermometer is often used as a general term for devices for measuring temperature. Examples of temperature measuring devices are described below.
Liquid in glass thermometers using mercury were, by far, the most common direct visual reading thermometer (if not the only one). Mercury also has the advantage of not wetting the glass; that is, the mercury cleanly traverses the glass tube without breaking into globules or coating the tube. The operating range of the mercury thermometer is from ?30 to +800 F (-35 to +450 C). The freezing point of mercury ?38 F (-38 C). The toxicity of mercury, ease of breakage, the introduction of cost-effective, accurate, and easily read digital thermometers, has brought about the demise of the mercury thermometer for room and clinical measurements. Other liquid in glass devices operate on the same principle as the mercury thermometer. These other liquids have similar properties to mercury (e.g., have a high linear coefficient of expansion, are clearly visible, are nonwetting), but are nontoxic. The liquid in glass thermometers are used to replace the mercury thermometer, and to extend its operating range. These thermometers are inexpensive, and have good accuracy (<0.1 C) and linearity. These devices are fragile, and used for local measurement. The operating range with different liquids is from ?300 to +1,000 F (-170 to +530 C). Each type of liquid has a limited...
