Optical Pyrometer – Construction & Working Principle

It is sometimes desirable to measure the temperature of a fuel bed or other body which may be at or above a red heat. For this purpose, a thermocouple is frequently not the best means either because the temperatures are too high or the position is inaccessible. Under such circumstances, optical pyrometers are used, and these may be employed at any temperatures up to 4,000°C. This type of instrument is frequently used by the steel smelters to ascertain the temperature of the furnace while melting steel.

What is an Optical Pyrometer ?

The optical pyrometer is a non- contact type temperature measuring device that operates on the principle of comparing the brightness of an object to the brightness of the filament inside the pyrometer and used to measures the temperature of furnaces, molten metals, and other overheated materials or liquids.

Simply, It is temperature measuring device which is used for measuring the temperature of molten metal’s, overheated material, furnaces otherwise liquid without contact. It is one kind of measuring device of temperature with a non-contact. 

Construction of Optical Pyrometer

The pyrometer has a cylindrical shape, and the inside parts of the optical pyrometer primarily include an

  • Eyepiece,
  • A power source,
  • An absorption screen,
  • Objective lens
  • Temperature source
  • And a red filter.

> On both sides of the device, an eyepiece and an object’s lens are arranged.

> A temperature bulb has a filament which is connected to a battery, millivoltmeter, and rheostat. The lamp is kept between the eyepiece and the lens.

> To increase the temperature range, an absorption screen is placed between the reference temperature lamp and the objective lens, and his range can be measured using the device.

> The filter helps in getting the monochromatic light. The red filter is placed between the lamp and the eyepiece so that the lamp only allows for a narrow band with a wavelength of 0.65mui.

It is cylindrical on the inside, with an objective lens and absorption screen on one side and a red filter and eyepiece on the other. The reference temperature lamp is located between the absorption screen and the red filter and is linked to a battery source via a rheostat and PMMC meter.

The intensity of the lamp can be controlled by varying the rheostat, or by varying the current flowing through the lamp’s filament. The red filter allows the observer looking through the eyepiece to see only a narrow band of wavelengths. The absorption screen broadens the temperature measuring range of the instrument.

Optical Pyrometer Working Principle

The measurement of temperature using an optical pyrometers works on the principle of brightness comparison. The brightness produced by the hot object (whose temperature is being measured) is compared with the brightness of the reference temperature lamp.

The brightness of the reference temperature lamp is adjusted until it equals the brightness of the hot body. The temperature of the hot body under measurement is given by the value of current passing through the filament of the lamp when both brightnesses are equal.

Working of Optical Pyrometer

The radiation emitted by a body is made to fall on the reference temperature lamp filament in order to measure its temperature. The brightness of the lamp is now varied by observing the filament through the eyepiece until the brightness of both the reference temperature lamp and the radiation emitted by the hot body becomes the same. When the brightness of the lamp’s filament and the hot body equalizes, the filament disappears.

Because the radiations emitted by the hot body, i.e. the brightness of the hot body depends on its temperature, and the brightness of the lamp depends on the current passing through the filament. The temperature can be determined by calibrating the optical pyrometer in terms of current flowing through the lamp’s filament. As a result, the temperature of the hot body is indicated by the PMMC meter, which measures the flow of current through the lamp’s filament.

Optical Pyrometer

The observer starts changing the rheostat values and the current in the reference lamp changes. This in turn, changes its intensity. This change in current can be observed in three different ways.

1. The Filament is dark : That is, cooler than the temperature source.

2. Filament is bright : That is, hotter than the temperature source.

3. Filament disappears : Thus, there is equal brightness between the filament and temperature source. At this time, the current that flows in the reference lamp is measured, as its value is a measure of the temperature of the radiated light in the temperature source, when calibrated.

Figure (iii) represents the disappearance of the lamp’s filament when the brightness of the hot body and reference temperature lamps is equalized. When the brightness of the lamp is lower than the brightness of the hot body, the filament of the lamp appears dark, as shown in figure I Similarly, when the brightness of the hot body is lower than the brightness of the reference temperature lamp, the lamp’s filament appears bright, as illustrated in figure (ii).

Advantages of Optical Pyrometer

  • The device’s simple assembly allows for easy use.
  • It is Capable of measuring high temperatures.
  • It is a portable instrument with lightweight.
  • The operation is simple
  • It has a very high accuracy of +/-5 degrees Celsius.T
  • here is no need for the optical pyrometer to make direct body contact with the object. As a result, it can be used in a wide range of applications.
  • As long as the size of the object whose temperature is to be measured corresponds to the size of the optical pyrometer, the distance between the two is not an issue. As a result, the device is suitable for remote sensing.
  • This device can be used to not only measure temperature but also to see the heat produced by the object/source. Thus, optical pyrometers can be used to measure and view wavelengths of 0.65 microns or less. A Radiation Pyrometer, on the other hand, can be used for high heat applications and can measure wavelengths ranging from 0.70 microns to 20 microns.

Disadvantages of Optical Pyrometer

  • It Cannot be used for continuous temperature measurement for small interval.
  • At low temperatures, it is less sensitive.
  • Optical pyrometers are expensive.
  • Incorrect rheostat adjustment and other thermal background radiations can have an impact on accuracy.
  • As the measurement is based on the light intensity, the device can be used only in applications with a minimum temperature of 700 degree Celsius.

Applications of Optical Pyrometer

  • It can be used to determine the temperature of furnaces.
  • It can be used to measure the temperature of heated materials as well as molten metals.
  • It can be used to rotating materials.
  • It can be used to hazardous materials, or high-electrical-field/high-voltage environments.

Frequently Asked Questions

In Optical pyrometer temperature is measured by

  1. Thermocouple effect
  2. Photoelectric (cell ) effect
  3. Comparing the brightness of the source with the brightness of a standard source
  4. None of the above.

Correct answer is 3.

Optical pyrometers operate within the visible spectrum to measure temperatures typically in the range from 700°C to 4,000°C by comparing the photometric brightness of the heated object against the brightness of a standard source, such as an incandescent tungsten filament. 

How temperature is measured in optical pyrometers

Temperature is measured by comparison of brightness of source with that of standard source.

Optical pyrometer is used to measure

As we know that, using contact type instruments, to measure temperature of the highly heated body is not possible. So this non-contact type device, optical pyrometer is used to measure the temperature.

For example :- rotating components, hazardous materials, or high-electrical-field/high-voltage environments. This type of instrument is frequently used by the steel smelters to ascertain the temperature of the furnace while melting steel.

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