Chapter-7 | Temperature Measurement

Temperature is one of the seven fundamental units of measurement, unit of temperature in Kelvin(K).

We use two types of scale in measuring temperature that is –

1.Relative scales [Fahrenheit (°F) and Celsius (°C)]

2. Absolute scales [Rankine (°R) and Kelvin (K)].

The relation between the above two is described by the following relations –

F = 1.8C + 32

C = (F-32)/1.8

R= F+ 460

K=C + 273

Temperature Gauges


Thermocouple consist of two dissimilar conductor wire, made from different metals. These wire are welded together at one end, form a junction used to measure temperature.


A thermometer is a simple device of measuring normal temperature. It may be Liquid-in-glass thermometer for measuring the temperature of our environmental condition or a digital type, used mostly by doctors.


Thermistors have a high coefficient of resistivity. Thermistors are made of semiconductor of solid type.

The suitable temperature measuring range for the thermistor is -100 degree Celsius to 300 degree Celsius. Some special type thermistor can measure up to 600 degrees of temperature.

The change in temperature is measured by changes in its resistance. So, the Wheatstone bridge circuit is used for it.

Also, the thermistor can convert changes in temperature to corresponding changes in voltage as a current.

Methods/Instrument of Measuring Temperature

Basically, there are two types of sensor used in the measurement of temperature, contact type, and non-contact type.

Contact type sensor is –

  • Thermocouples
  • Liquid-in-glass thermometers
  • Resistance temperature detectors (RTDs)
  • Thermistors
  • Bimetallic strip thermometers
  • Pressure thermometers

In the case of non-contact-type sensors, the radiant power of the optical radiation received by the object or system is measured directly.

Some non-contact-type sensors are –

  • Radiation Pyrometers
  • Optical Pyrometers
  • Fibre-Optic Thermometer

Next, Chapter-8 | Pressure Measurement

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