A radiation pyrometer is a non-contact type temperature measuring device. If a temperature sensor is brought in physical contact with a very hot body, the sensor will be damaged due to high temperature.

Hence, the higher temperatures are measured using a non-contact type temperature sensor (for example, radiation pyrometer). At high temperatures, a hot body emits radiations (i.e., electromagnetic radiations in the wavelength range of 0.1 - 100 Î¼m). These radiations due to temperature are known as thermal radiations.

A radiation pyrometer measures the temperature of very hot bodies by measuring the thermal energy of the radiations emitted by the hot body. As it is a non-contacting measuring device, it is used for measuring the temperature of moving bodies.

The temperatures that exceed the measuring range of thermocouples can be measured through a pyrometer. The radiation pyrometer utilizes the characteristics of a black body to measure radiant energy.

A black body is a perfect absorber and emitter of radiations (i.e., a black body emits/radiates a large number of thermal radiations than any object at a given temperature) and also it absorbs all the radiations that fall on it.

According to the principle used in radiation pyrometers (i.e., the way in which the temperature is measured), there are two types of radiation pyrometers.Total radiation pyrometer, andInfrared or Selective or Partial radiation pyrometer.

One of the principles is to measure the total energy radiated by the hot body. The pyrometer based on this principle is known as the total radiation pyrometer. In this type of pyrometer, in order to measure the total energy, the pyrometer should be designed to accept all energy of a broad range of wavelengths.

The total energy is determined by Stefan Boltzman's law equation, which states that the total thermal energy of the radiation emitted by a black body is proportional to the fourth power of the temperature of the hot body i.e., E ∝ T4.

In this pyrometer, the total radiation emitted from the hot body is made to fall on any of the temperature sensing devices such as thermocouple, bolometer, thermopile, etc as shown above. This device receives radiation from the hot body and generates an emf that gives the temperature of the hot body by calibration.

Infrared or Selective or Partial Radiation Pyrometer :

In the other type of principle, the spectral radiation intensity of radiation emitted by a hot body at a specific wavelength is measured. The radiation pyrometer based on this principle is known as selective or partial radiation pyrometer. This principle is based on Planck's law and according to this law, the energy of the radiation at a particular wavelength (Î») and at a given temperature (T) is given by the equation,

Infrared Pyrometers are also known as selective or partial radiation pyrometers. The temperature of a body determined by this device is based on the infrared radiations emitted by that body. The infrared radiation emitted from the body is proportional to the temperature of that body. The increase in temperature causes to increase in radiations emitted.

Generally, photoelectric transducers are used in infrared pyrometers to measure infrared radiations emitted from the body. A photovoltaic cell is placed in this pyrometer that senses and responds to radiations whose wavelengths falling in the infrared region. The below diagram shows the arrangement of the infrared pyrometer.

When the infrared radiations emitted from the hot body fall on the photovoltaic cell, it develops an emf proportional to the radiations received by cell. The value of emf developed by the cell gives the temperature of the hot body when calibrated.

The infrared filter prevents the cell from overheating that allows the radiations of 1000°C to 1200°C. The protecting window protects the filter from physical damage. The amount of radiation received by the photovoltaic cell depends upon the area of the diaphragms.

The radiation receiving elements convert the radiant energy into a form that represents the measure of temperature. The various types of radiation receiving elements used in radiation pyrometers are as follows,

Bolometer :

Bolometers are temperature measuring devices whose resistance increases when they are subjected to radiant energy (i.e., temperature). A Wheatstone bridge is used to measure the change in resistance. A bolometer makes use of two thin strips (one strip is exposed to radiation energy and the other strip is shielded).

These two strips are connected in two arms of the bridge. The resistance of the exposed strip increases when it absorbs the thermal energy of radiation whereas the shielded strip provides compensation for changes in ambient temperature. Bolometer has a fast response but, they are expensive and less rugged. Bolometers are made up of either a thin ribbon, platinum, or nickel.

Photoelectric Transducers :

These devices convert radiant energy incidents on them into a corresponding electrical signal (i.e., voltage or current). By measuring this electrical output signal, the radiant energy and hence temperature can be determined.

Examples of photoelectric transducers are photovoltaic cells, photoemissive cells, photoconductive cells. These photoelectric transducers are used in partial radiation or optical radiation pyrometers because these transducers are sensitive to a certain region of the spectrum.

Vacuum Thermocouple :

A vacuum thermocouple is a thermocouple that is constructed from two thin metal strips. The junction of the thermocouple used for detecting the temperature of radiant energy is coated with black color and the thermocouple is enclosed in a vacuum chamber. The chamber consists of a small window, so that the thermocouple junction may receive the radiation energy.

As the thermocouple is kept in a vacuum environment, there will be no loss of heat (due to conduction and convection) from the detecting junction and hence the thermocouple can detect even small radiant energy and also provide the corresponding emf. The vacuum thermocouple can detect rapid changes in radiant energy because of its less weight and vacuum environment.

Thermopile :

Thermopile is an assembly of small thermocouples connected in series in such a way that the resultant emf of the thermopile is the addition of EMFs of individual thermocouples. For complete absorption of the radiant energy by the thermocouple junctions, the junctions are flattened and a black coating is applied to them.

Thermopiles are highly sensitive devices and have a long time of response (for example, 2 sec or more than 2 sec). Usually, in most industrial applications response time of several seconds is desired.