Experimental Technique - Temperature Measurement

SME1912 EXPERIMENTAL TECHNIQUES Chapter 3 – Part 1 Measurement of Temperature CONCEPT AND DEFINITIONS • Temperature is related to heat • Measuring temperature means measuring heat • Heat is measured using the concept of heat equilibrium • i.e heat on the measuring equipment is brought to the same heat of the measured item SOME HISTORY In 1593, Galileo Galilei invented a rudimentary water thermometer (using the contraction of air to draw water up a tube). HOW TO MEASURE TEMPERATURE Six types with which the engineer is likely to come into contact are: 1. thermocouples, 2. resistive temperature devices (RTDs and thermistors), 3. infrared radiators, 4. bimetallic devices, 5. liquid expansion devices, and 6. change-of-state devices. TEMPERATURE SCALE The two scales commonly in use today date from the eighteenth century and are named after Gabriel Daniel Fahrenheit and the Swedish astronomy professor Anders Celsius. Fahrenheit designed his scale to have two reference points that could be set up in his workshop. He originally chose the melting point of pure ice and the temperature of a normal human body, which he took as being 32° and 96° respectively. These conveniently gave positive values for all the temperatures he encountered. Later he changed to using the boiling point of water (212°) as the upper fixed point of the scale. TEMPERATURE SCALE contd. Celsius also used the ice and steam points, but took them to be 0 °C and 100 °C respectively. Although the Celsius scale has taken precedence over the Fahrenheit scale, the latter is still familiar in weather reports in the United Kingdom: a summer’s day temperature of 75 °F seems much more pleasant than one of 23 °C! TEMPERATURE SCALE A third, fundamental, temperature scale was proposed in 1854 by the Scottish physicist William Thomson, Lord Kelvin. It is based on the idea of the absolute zero, the point of no discernible energy, which is independent of any particular material substance. The Kelvin scale is widely used by physicists and engineers to determine and apply fundamental laws of thermodynamics. Thermocouple Temperature Measurement Sensors Thermocouples consist essentially of two strips or wires made of different metals and joined at one end. Changes in the temperature at that juncture induce a change in electromotive force (emf) between the other ends. As temperature goes up, this output emf of the thermocouple rises, though not necessarily linearly Thermocouple Temperature Measurement Sensors Resistance Temperature Devices Resistive temperature devices capitalize on the fact that the electrical resistance of a material changes as its temperature changes. Two key types are the metallic devices and thermistors. As their name indicates, RTDs rely on resistance change in a metal, with the resistance rising more or less linearly with temperature. Thermistors are based on resistance change in a ceramic semiconductor; the resistance drops nonlinearly with temperature rise. Resistance thermometer construction Thermistor; The Theory If we assume that the relationship between resistance and temperature is linear (i.e. we make a first-order approximation), then we can say that: ∆R = k∆T where ∆R = change in resistance ∆T = change in temperature k = first-order temperature coefficient of resistance Thermistors Thermistor symbol Thermistor, bead type, insulated wires Infrared Temperature Measurement Devices Infrared sensors are noncontacting devices. They infer temperature by measuring the thermal radiation emitted by a material. RADIATION PYROMETER Inventor of Radiation Pyrometer Pieter van Musschenbroek Bimetallic Temperature Measurement Devices Bimetallic devices take advantage of the difference in rate of thermal expansion between different metals. Strips of two metals are bonded together. When heated, one side will expand more than the other, and the resulting bending is translated into a temperature reading by mechanical linkage to a pointer Fluid-Expansion Temperature Measurement Devices Fluid-expansion devices, typified by the household thermometer, generally come in two main classifications: the mercury type and the organic-liquid type. Versions employing gas instead of liquid are also available. Fluid-Expansion Temperature Measurement Devices Mercury is considered an environmental hazard, so there are regulations governing the shipment of devices that contain it. Fluid-expansion sensors do not require electric power, do not pose explosion hazards, and are stable even after repeated cycling. On the other hand, they do not generate data that is easily recorded or transmitted, and they cannot make spot or point measurements. Change-of-State Temperature Measurement Devices Change-of-state temperature sensors consist of labels, pellets, crayons, lacquers or liquid crystals whose appearance changes once a certain temperature is reached. They are used, for instance, with steam traps - when a trap exceeds a certain temperature, a white dot on a sensor label attached to the trap will turn black. Change-of-State Temperature Measurement Devices Response time typically slow, so these devices often do not respond to transient temperature changes. And accuracy is lower than with other types of sensors. Furthermore, the change in state is irreversible, except in the case of liquid-crystal displays. Even so, changeof-state sensors can be handy when one needs confirmation that the temperature of a piece of equipment or a material has not exceeded a certain level, for instance for technical or legal reasons during product shipment.