Temperature is a degree of how hot or cold is a body or amount of heat energy possessed by a body measured on a definite scale. Measurement of temperature is one of the important factors for various applications.
It is important to control and monitor the temperature in various processes. To meet this wide array there are a wide variety of temperature sensors developed by the process control industry. On application of heat on a body can cause various primary effects on it such as,
- There will be a change in the electrical properties of the body.
- Change in physical properties or dimensions of the body.
- Can lead to set up an electromotive force at the junction of two different metals.
- Variation in the intensity of the emitted radiations.
- Change in physical state.
- Change in the chemical state or properties of the body.
The above effects due to temperature can be used in temperature sensors for the measurement of temperature. The various types of instruments or sensors used for temperature measurements are based on the type of method used for measuring the temperature are as follows,
- Mechanical instruments (Non-electrical method)
- Electrical instruments (Electrical method)
- Optical instruments (Radiation method).
Mechanical Temperature Sensors (Non-electrical Method) :
Usually, the mechanical instruments that are used for temperature measurement are designed based on the principle of change of physical properties of a substance, when the substance is subjected to the temperature. The following are the mechanical type temperature sensors,
- Liquid in glass thermometer
- Liquid filled system thermometer
- Vapour pressure thermometer
- Bimetallic thermometer
- Solid rod thermometer.
In the above first two thermometers, the thermal expansion of the liquid with temperature is used to measure the temperature, whereas, in a vapour pressure thermometer, the temperature is measured by measuring the change of vapour pressure in the bulb, which occurs due to the change in temperature.
In bimetallic thermometers and solid rod thermometers, the expansion of the solid substance (i.e., metal rod or thin metal strips) is proportional to temperature and hence the thermal expansion of solid also gives the measure of temperature.
Electrical Type Temperature Sensors (Electrical Method) :
The electrical type temperature measuring devices are based on the variation of electrical parameters (such as resistance, voltage, frequency, etc.) with temperature. The electrical sensors have good sensitivity, high speed of response, and can measure a broad range of temperature when compared to mechanical instruments. The electrical instruments used for temperature measurement are as follows,
- Resistance thermometer
- Semiconductor thermometer
- Thermistors
- Thermocouples
- Quartz crystal thermometer.
The electrical resistance of an element either increases or decreases when the temperature increases. Thus, the temperature is determined by measuring the corresponding change in resistance of the elements, resistance thermometers, semiconductor thermometers, and thermistors use this principle.
In thermocouples, an emf is generated when a change in temperature is detected at the hot junction of the thermocouple. In a quartz crystal thermometer, the change in temperature changes the frequency of oscillation of the crystal oscillator.
Optical Type Temperature Sensors (Radiation Method) :
These instruments are based on the measurement of thermal energy of the radiations emitted by the hot body because at high temperatures a hot body emits radiations. An example of an optical temperature sensor is a radiation pyrometer.
A radiation pyrometer is a device used for measuring high temperatures. It measures the temperature of the body without making any physical contact with it. Because it provides a measure of the temperature by measuring the energy of the radiations generated by the hot body.
