Page 18 - Instrumentation and Measurement

2.3 Temperature Measurement

Temperature is a measure of the thermal energy in a body, which is the relative hotness or coldness
of a medium and is normally measured in degrees using one of the following scales; Fahrenheit (F),
Celsius or Centigrade (C), or Kelvin (K).

There are several methods of measuring temperature that can be categorized as follows:

1. Expansion of a material to give visual indication, pressure, or dimensional change
2. Electrical resistance change
3. Semiconductor characteristic change
4. Voltage generated by dissimilar metals
5. Radiated energy

2.3.1 Mechanical Thermometry

Mechanical thermometry is the measurement of temperature using mechanical means.

2.3.1.1 Liquid in Glass Thermometers

The liquid in glass thermometer involves a liquid expanding up a capillary tube. The height to which
the liquid expands is a measure of the temperature. With mercury as the liquid, the range possible is
-35 ͦ C to +600 ͦ C, with alcohol -80 ͦ C to +70 ͦ C, with pentane -200 ͦ C to +30 ͦ C. Such thermometers
are direct reading, fragile, capable of reasonable accuracy under standardised conditions, fairly slow
reacting to temperature changes, and cheap.

2.3.1.2 Filled-System Thermometers

Many physical properties change with temperature, such as the volume of a liquid, the length of a
metal rod, the electrical resistance of a wire, the pressure of a gas kept at constant volume, and the
volume of a gas kept at constant pressure. Filled-system thermometers use the phenomenon of
thermal expansion of matter to measure temperature change.

The filled thermal device consists of a primary element that takes the form of a reservoir or bulb, a
flexible capillary tube, and a hollow Bourdon tube that actuates a signal-transmitting device and/or a
local indicating temperature dial. A typical filled-system thermometer is shown in Figure 2.9. In this
system, the filling fluid, either liquid or gas, expands as temperature increases. This causes the
Bourdon tube to uncoil and indicate the temperature on a calibrated dial.

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