Basics of Temperature Measurement

The word temper was used in the seventeenth century to describe the quality of steel. It seems, after the invention of crude from of thermometer, the word temperature was coined to describe the degree of hotness or coolness of a material body. It was the beginning of seventeenth century when the thermometer – a temperature measuring instrument was first developed.

Galileo Galilei is credited with the construction of first thermometer, although a Dutch scientist Drebbel also made similar instrument independently. The principle was simple. A bulb containing air with long vertical tube was inverted and dipped into a basin of water or coloured liquid. With the change in temperature of the bulb, the gas inside expanded or contracted, thus changing the level of the liquid column inside the vertical tube. A major drawback of the instrument was that it was sensitive not only to variation of temperature, but also to atmospheric pressure variation.

Successive developments of thermometers came out throughout seventeenth and eighteenth century. The liquid thermometer was developed during this time. The importance of two reference fixed temperatures was felt while graduating the temperature scales. Boiling point of water and melting point of ice provided two easily available references. But some other references were also tried.

Fahrenheit developed a thermometer where, it seems, temperature of ice and salt mixture was taken as 0° and temperature of human body as 96°.These two formed the reference points, with which, the temperature of melting ice came as 32° and that of boiling water as 212°. In Celsius scale, the melting point of ice was chosen as 0° and boiling point of water as 100°. The concept of Kelvin scale came afterwards, where the absolute temperature of gas was taken as 0° and freezing point of water as 273° .

The purpose of early thermometers was to measure the variation of atmospheric or body temperatures. With the advancement of science and technology, now we require temperature measurement over a wide range and different atmospheric conditions, and that too with high accuracy and precision. To cater these varied requirements, temperature sensors based on different principles have been developed.

They can be broadly classified in the following groups:

1. Liquid and gas thermometer

2. Bimetallic strip

3. Resistance thermometers (RTD and Thermistors)

4. Thermocouple

5. Junction semiconductor sensor

6. Radiation pyrometer

Temperature is the most important process variable that requires continuous measurement and monitoring in a process industry. Among the different types of temperature transducers, the most commonly used ones are RTDs and thermocouples. Their popularity is mainly due to their ruggedness, repeatability and wide range of operation. Bare RTDs and thermocouples are rarely used in practice; instead, they are put in protective metallic sheaths.

The signal conditioning circuits should be properly designed, so as to avoid the errors due to lead wires in RTDs and variation of cold junction temperatures in thermocouples. There are several cases where the temperature to be measured is more than 2000°C, the conventional measuring techniques fail to measure the high temperature. Instead, the measurement is carried out from a distance. Radiation pyrometers are used in these situations. However, its principle of operation has not been included in this lesson.