INSTRUMENTATION & CONTROL

1. Radiation pyrometers

2. Optical pyrometers

Radiation Pyrometers
Operation of radiation pyrometer is based upon the measurement of radiant energy emitted by the hot body. Fig.1. shows a diagram of a radiation pyrometer. It consists of a lens to focus radiated energy from the body, whose temperature is required, on to a detector or receiving element. This receiving element may have variety of forms such as resistance thermometer, or a thermocouple or thermopile. A thermopile consists of several thermocouples connected in series. A temperature indicate, recorder, or controller is attached with the receiving element to indicate the temperature.

Advantages
Following are the advantages of radiation pyrometers:
1. They are able to measure high temperatures
2. In radiation pyrometers, there is no need for contact with target of measurement.
3. They possess fast response speed.
4. They have high output and moderate cost.

Disadvantages
Following are the disadvantages of radiation pyrometers:
1. Their scale is non-linear.
2. Errors due to presence of intervening gases or vapours that absorb radiating frequencies is possible in this pyrometers.
3. Emissivity of target material affects measurement.

Applications
Following are the applications of radiation pyrometers:
1. They are used for the temperatures above the practical operating range of thermocouples.
2. They can be used in the environments which contaminate or limit the life of thermocouples.
3. They are used for moving targets.
4. They are used for the targets not easily accessible, such as furnace interiors.
5. They are used for the targets which would be damaged by contact with primary elements like thermocouples and resistance thermometers.
6. They are used for the measurement of average temperatures of large surface areas.

Optical Pyrometers
Optical pyrometers provide an accurate method of measuring temperatures between 600 and 3000 °C and are very useful for checking and calibrating radiation pyrometers. But they are not suitable for recording or controlling temperatures.
The method of operation of optical pyrometers is based on the comparison of the intensity of the visual radiation emitted by the hot body with the radiation emitted by the source of known intensity. The brightness of radiation emitted by the hot body whose temperature is to be measured is matched with the brightness of a calibrated reference whose temperature is known. A typical industrial optical pyrometer is shown in figure-2(a) for the temperature range from 700 to 3000 °C. Figure-2(b) shows the schematic arrangement of an optical pyrometer.
An optical pyrometer consists of an incandescent lamp filament which is used as the reference source of radiation. This is arranging in the field of vision of a telescope through which both it and the hot body, are viewed simultaneously. The filament is heated by a two volt battery in series with a rheostat by which the temperature of the filament is adjusted. This filament is connected is one arm of a wheat-stone bridge circuit across which is connected a moving coil galvanometer. The electrical resistance of the lamp filament varies in accordance with its temperature. While the resistance in other arms of the bridge are of a material, the ohmic value of which does not alter with change of temperature. As the temperature of the filament is increased the bridge is progressively thrown out of balance. The degree of unbalance is shown by the magnitude of the galvanometer deflection which is the calibrated in terms of temperature.From the galvanometer provided that the rheostat is not moved after the temperature is obtained an absorption screen figure-2(B), is used between object and the filament that reduces intensity of the radiation from the object reading the filament so that the filament may be matched to a hot mass which is at considerably higher temperature than the filament its self. A monochromatic red screen fitted to the eye piece so that it may be brought in to the field of vision at will. Its function is to eliminate colour differences between the filament and the hot body to facilitate matching and also to prevent dazzle at the higher filament temperature.

Advantages
Optical pyrometers have a number of advantages which are given below:
1. It possesses flexibility, portability and is convenient for use.
2. It is light in weight.
3. It is useful for monitoring the temperature of moving objects and distant objects.
4. In this instrument there is no need for contact with target of measurement.
5. It is useful for high temperatures.
6. It has a good accuracy.

Disadvantages
Optical pyrometers have certain disadvantages:
1. It is not useful for measuring the temperatures of clean burning gases that do not radiate visible energy.
2. It is relatively expensive.
3. It is prone to human errors caused by operator adjustment of temperature dial.It is subject to emissivity errors.