Preceding blogs have described a number of mechanical means for measuring temperature based on the expansion and contraction of liquids and solids. Another major classification of temperature sensors are based on electrical phenomenon.
A thermocouple sensor utilizes a junction of two dissimilar metals to measure temperature. The principle is based on the fact that an electrical potential (voltage) is created in any conductor exposed to a thermal gradient along its length. The two materials in a thermocouple are chosen such that an instrument (really a specialized voltmeter) can measure the difference in voltage produced by two conductors both subjected to the same thermal gradient.
Note – Contrary to popular belief, the voltage produced by a thermocouple is not actually created in the junction but rather along the length of the two conductors subjected to a thermal gradient.
In order to accurately measure this difference in voltage, the leads from the “hot junction” of the thermocouple (subjected to the temperature to be measured) must be joined to other conductors at what is called the “cold junction.” Although this secondary connection historically was made in an ice bath to assure that any voltage measured by the sensor was only the result of the thermal gradient produced by the temperature to be measured, instruments today usually incorporate a reference “cold junction” and do not require such measures. Thermocouple leads are connected directly to the instrument which is then able to convert the measured voltage to a temperature readout in the appropriate units. Thermocouple junctions are made using several different combinations of dissimilar metals based on the application. It is important that thermocouples and instruments indicating temperature be compatible to assure the accuracy of any measurement.
Resistance Temperature Detectors –
The resistivity of many materials changes with temperature. This change in resistivity can be used to measure temperature. Common forms of resistivity temperature detectors (RTD’s) use pure metals either deposited on a substrate or as wire wrapped around a non-conductive core to measure temperature. The measuring instrument senses the resistance of the device and converts it into temperature readings. In general, RTD’s are preferred over thermocouples due to their accuracy, long life and interchangeability.
Thermistors are made of semi-conductor materials which change resistivity with temperature. Their most common use is in applications where the change in temperature of the thermistor due to a flow of current through it results in a temperature increase. The resulting change in resistance due to an increase in temperature is used to control the flow of current through the device directly. Thermistors can be designed to either increase or decrease resistivity on heating. This makes them valuable in limiting in-rush currents and providing controlled heat in many applications.
All of the above can accurately measure temperature change but must be calibrated to measure absolute temperature. In the case of thermocouples, the reference or “cold junction” is produced within the measuring instrument but, generally, requires initial calibration to assure accuracy. Further calibration of thermocouple devices is required as changes in insulation characteristics and other factors may compromise accuracy over time. In the case of RTD’s, the calibration is performed in the manufacture of the sensing device and is permanent. Calibration of RTD’s is generally not required except for verification. The way a thermistor responds to heat is determined by the mixture of semiconductor materials used in the manufacturing process and may vary somewhat from one device to another. Small variances are of no consequence in many thermistor applications but, if they are used to measure temperature directly, proper calibration is required to produce an accurate reading.
Although there are other ways to measure temperature including non-contact infrared sensing devices, the types described in the last few blogs are those most commonly used in industrial cleaning equipment.
– FJF –