Most of heat treatment processes include the heating of a metal alloy at a high temperature. In order to obtain the optimum properties with many of the modern alloys, a fairly accurate control of their heat treatment temperatures is necessary. Pyrometry, or the measurement of high temperatures, is therefore, one of the most important aspects.
1. Electrical Resistance Pyrometers:
These make use of the fact that the electrical resistance of metal varies as its temperature varies. Generally platinum in the form of a very fine wire wound on to a mica former and inserted into a refractory tube is used. The variation in resistance is determined by a Wheatstone-bridge type of circuit and the temperature is calibrated from a previously made calibration chart.
Although the platinum resistance thermometer gives accurate temperature readings, it is not used for furnaces because it is bulky, fragile and the characteristics of coil are altered by the action of any reducing gases in the furnace atmosphere. It is, therefore, used principally as a master instrument in calibration of other pyrometers.
2. Thermo-Electric Pyrometers:
These work on the principle that if two dissimilar wires are joined together at both pairs of free ends and one junction is heated to a higher temperature than the other, an electromotive force is set up in the circuit. If the wires be separated at the cold junction and connected instead to a sensitive millivoltmeter, the resultant e.m.f. in the circuit due to the hot junction can be measured, which is directly proportional to the temperature.
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Such a pyrometer is known as thermocouple and comprises two dissimilar wires of suitable composition, which will produce a circuit e.m.f. large enough to be measured and be able to function at very high temperatures.
Other important features include i.e., Undue oxidation or fluctuation of electrical output, refractory insulation to prevent the wires from touching except at the hot junction; a refractory sheath to protect the couple from corrosive slags and injurious gases; a suitable measuring instrument.
In thermocouples, the wires used are comparatively expensive and the indicating instrument is located at a considerable distance from the furnace. Thus, the thermocouple wires are run only upto the terminal head of the pyrometer and beyond that joined upto the indicating instrument by compensating leads which taken as a pair, have nearly the same thermo-electric constant as the couple wires themselves.
Any variations in ambient temperature of the indicating instrument are compensated for by means of a bimetalic strip attached to the pointer of the indicating instrument.
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Errors introduced by the variation in resistance of the compensating leads with the temperature (if they are long) can be taken care of by incorporating a ballast resistance in the millivoltmeter so that the resistance of the latter will be made large in comparison with that of the external circuit. The ballast resistance is usually of some alloy, such as manganin, which has low temperature coefficient of resistance.
The resultant e.m.f. produced in the pyrometer circuit can be measured more accurately by employing a potentiometer system.
3. Radiation and Optical Pyrometers:
For measuring high temperatures it becomes necessary to use an instrument which need not be in contact with the hot body. Such pyrometers depend on the radiant energy emitted by the hot body and obey certain laws, which apply to a perfect ‘black body’. A ‘black body’ may be defined as one absorbing all radiations falling on it, without loss by reflection or transmission.
The radiation from the interior of a chamber at uniform temperature approaches closely to the ideal conditions. On the other hand, only a fraction of the theoretical energy is received from a body in the open, and this fraction (always less than unity) is called the emissivity, which depends upon (a) wavelength, (b) temperature of surface, (c) character of surface. A few values are given in the table 2.1.
For bodies in the open, corrections have to be made.
4. The Radiation Pyrometer:
The radiation pyrometers used under ‘black body’ conditions, are subject to the Stefan- Boltmann Law which may be stated as- “an increase of one per cent in the absolute temperature of the radiating body results in an increase of four per cent in the energy emitted”.
Stated mathematically the law is,
E = K(T4 – T04)
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where K = 1.34 x 10 12 calories per square centimeter per second, E is total energy radiated by body at absolute temperature T to surroundings at absolute temperature T0.
Since T0 is small compared with T, we can write,
E = KT4
For bodies in the open we can use the following formula to find true temperature T.
where e is total emissivity, and S is apparent temperature indicated by pyrometer.
For example, if the instrument reads 900°C, when sighted on iron in the open, the true absolute temperature is,
Most of the radiation pyrometers use either a lens or a mirror to concentrate the heat rays emitted by the hot body on to a small thermo-couple, and E.M.F. developed is measured by a calibrated millivoltmeter; ranges are usually 550° to 2000°C, but there is really no upper limit.
Fixed focus and focusing instruments are used, as illustrated in Fig. 2.24 (a) and (b). In the Fery pyrometer the inclined mirror (M) in front of the thermocouple is used to indicate when correct focus has been obtained, namely, the image of the hot body appears broken in halves, when out of focus [Refer X in Fig. 2.24 (b)].
Such radiation pyrometers require about 15 seconds to take a reading, but recording can be made in the same way as a thermoelectric pyrometer. It is essential that the image of the hot body be sufficiently large to cover the sensitive thermo-element, in which case the distance from the hot objective is immaterial.
When measuring furnace temperatures, the gases should be turned off, since the higher temperature of the gases may produce an error. Water vapour absorbs certain radiation and gives a low reading.
Improved designs are now available. One, which is suitable for measuring surface temperature quickly (5 sec), uses a hemispherical gilt reflector placed on the hot surface to provide black body conditions. To measure gas temperatures a suction pyrometer is desirable, consisting of a thermocouple surrounded by concentric radiation shields. The gas is aspirated past the couple at a high velocity.
The pyrometer, however, uses a bimetal spiral with a pointer and compensation spiral. The heat rays are concentrated by a lens on to the bimetal spiral which unfolds to indicate temperature on a scale.
5. Optical Pyrometer:
In these pyrometers the intensity of light from the hot body is compared with the intensity of light from some standard source, and both are matched in the instrument to one specific wavelength, usually red light (0.00065 mm) and even a colour-blind man can do this. The commercial instruments differ in the optical devices used to make this comparison of the two intensities of light.
In a standard commercial pyrometer a polarising device compares the red ray from the hot body with the ray of the same wave-length from an electric lamp the intensity of which is calibrated by an amyl acetate lamp.
In the disappearing filament pyrometer the intensity of a standard filament lamp is varied until it disappears against light from the hot body. Fig. 2.25 shows the construction of a typical instrument. The lamp is placed inside a telescope the focusing of which forms an image of the hot body in the plane of the filament.
The filament and the image are viewed through the eye-piece of the telescope, adjusted to suit the operator’s eye. An electric current is passed through the lamp filament so that it is just hot enough to disappear against the image of the hot body. This current can be measured by an ammeter, and, if previously calibrated, can be used to indicate temperature of the hot body.
In the simple electric circuit using an ammeter a large portion of the scale is useless, owing to the measurement of current used in heating the filament to 750°C. In some instruments the voltage drop across the lamp is measured, but the most satisfactory method of using the whole of the scale without the ‘set back’ of the zero is to make the lamp filament one of the arms of a wheat stone bridge and calibrate the galvanometer in degrees, with a zero reading corresponding to about 750°C.
The resistances in the circuit are so arranged that balance is obtained when the filament is at 750°C. As the filament becomes hotter its increase in resistance throws the bridge out of balance and the galvanometer pointer moves over the scale.
To cut out colour difference a red glass filter is placed in front of the eyepiece. For temperatures above 1400°C an absorbing screen is placed between the hot body and the filament so that the latter is never overheated, but this of course necessitates recalibration.
The principle on which the instrument is based is Wien’s Law, which states that the intensity of any radiation (I) of wave-length (λ) emitted from a body at an absolute temperature (T) is given by:
I = c-5 eK/λT
where c and K are constants, and e is the base of Naperian logarithms.
For Non-Black body conditions:
where S is apparent temperature and E is emissivity of a non-transparent material at wave-length l.
The reading obtained from an object in the open will depend upon its reflecting power and its surroundings. An oxide-free metal may reflect considerable sunlight into the instrument and give a false reading.
The optical and radiation pyrometers can be calibrated against a thermocouple or the melting of a palladium wire in an electric furnace arranged to approximate a “black body” by a series of diaphragms suitably disposed. Alternatively, the instruments can be focused on the bottom of a re-entrant tube in a crucible containing a pure metal and the freezing- point determined.