Methods of measuring temperature

The temperature in a vessel or pipe is one of the most important parameters to monitor and control in any process. It may be measured by mercury-in-glass thermometers, bimetallic thermometers, pressure bulb thermometers, thermocouples, metal-resistance thermometers or thermistors. Accurate mercury-in-glass thermometers are used to check and calibrate the other temperature sensors.

1. Mercury-in-glass Thermometers - Mercury-in-glass thermometers may be used in small bench labs and institutes, because its fragility has restricted it use. In large fermenters it would be necessary to insert into thermometer pocket in the vessel, which introduces a time lag in registering the vessel temperature.
2. Electrical Resistance Thermometers -It is well known that the electrical resistance of metals changes with temperature variation. This property has been utilized in the design of resistance thermometers. The bulb of the instrument contains the resistance element, a mica framework (for very accurate measurement) or a ceramic framework (robust, but for less accurate measurement) around which the sensing element is wound. A platinum wire of 100-ohm resistance is normally used. Leads emerging from the bulb are connected to the measuring element. The reading is normally obtained by the use of a Wheat- stone bridge circuit and is a measure of the average temperature of the sensing element. They have a greater accuracy.
3. Thermistors - Thermistors are semiconductors made from specific mixtures of pure oxides of iron, nickel and other metals. Their main characteristic is a large change in resistance with a small temperature change. The change in resistance is a function of absolute temperature. The temperature reading is obtained with a Wheatstone bridge or a simpler or more complex circuit depending on the application. Thermistors are relatively cheap and have proved to be very stable, give reproducible readings, and can be sited remotely from the read-out point.