Fig. 61.—Siemens’ Optical Pyrometer, Portable Form.
The adjustment in this pyrometer is simple, and the condition of equality sharply defined. Whereas, in matching the colours of two contiguous fields, separate observers may disagree to an extent representing 40° C. or more, a divergence of 10° C. is seldom exceeded when different operators adjust the tip of the filament to extinction. In a special test to decide this point, the author compared the observations of five persons, some trained and others untrained, with the result that all agreed to within 10° at a steady temperature in the vicinity of 1200° C.; and in this respect the Holborn-Kurlbaum pyrometer is superior to other forms of optical pyrometer. The continuous accuracy of the readings depends upon the permanence of the standard lamp, which is ensured by over-burning for 20 hours, after which the lamp may be used at its proper voltage for a long period without further change. As used for occasional readings in the workshop, such a lamp will last for a year or more without varying in brightness by an amount representing 10° C. at a temperature of 1800° C. When a new lamp is used, a fresh calibration is necessary; the makers, however, in such case send out a new temperature scale with the lamp.
Lovibond’s Pyrometer.—It is possible, by the use of coloured glasses superposed, to match closely any given colour; and Lovibond, whose tintometer for this purpose is well known, has applied this method to temperature measurement. Taking the case of a block of steel in a furnace, it is possible to arrange combinations of glasses which, when illuminated by a standard light, will give the same tint as the steel at any specified temperature. If it be desired to work the steel at 850° C., for example, glasses are provided which, when viewed by the light transmitted from a 4-volt glow-lamp, using a constant current, represent the tint of steel at 840°, 850°, and 860° respectively. The image of the steel is reflected by a mirror through one hole in a brass plate, which forms the end of a wooden box, at the opposite end of which an eye-piece is placed. A second hole in the brass plate receives light from the standard lamp, after passing through the glasses; and the appearances of the two lights are then compared. A skilled eye can readily detect a disagreement in the two fields corresponding to 10° C.; and by introducing the glasses in turn it can be observed whether the steel is within 10° C. of the temperature required. This instrument is cheap and simple, but is obviously only useful in deciding a pre-arranged temperature, as to take a measurement at an undefined temperature would involve an unwieldy number of glasses, and absorb a considerable time. The correct glasses to use for a given operation are decided under working conditions at temperatures measured by a standard pyrometer; after which any number of instruments may be made from glasses of the same colour and absorptive power as those used in the calibration. Correct matching is difficult below 700° C.
Mesuré and Nouel’s Pyrometer.—This instrument, shown in [fig. 62], consists of two Nicol prisms, between which is placed a piece of quartz cut perpendicularly to its axis. Light from the source, in passing through the first Nicol prism, is all polarised in the same plane; but on passing through the quartz is polarised in various planes, according to the wave-length. The colour seen after passing through the second prism, used as analyser, will depend upon the angle between this and the first or polarising prism. The analyser is connected to a rotating disc, divided into angular degrees; and on viewing the heated source the colour will appear red if the analyser be turned in one direction, and green if rotated in the opposite. The intermediate colour is a lemon-yellow; and the adjustment consists in rotating the analyser until this tint is obtained. The angular reading is then taken, and the temperature read off from a table prepared by making observations at known temperatures. Observers may disagree by as much as 100° C. in using this pyrometer, owing to differences in eyesight and judgment of the lemon-yellow tint; but a given operator, who has trained himself to the use of the instrument, may obtain much closer results with practice. The chief use of this device is to enable a judgment to be formed as to whether a furnace is above or below an assigned temperature, within limits of 25° C. on either side at the best; and hence it is convenient for a foreman or metallurgist to carry about for this purpose when other pyrometers are not in use. A great advantage is that the instrument is always ready for use, and has no accessories.
Fig. 62.—Mesuré and Nouel’s Pyrometer.
Colour-extinction Pyrometers.—Various attempts have been made to produce superposed glasses, or cells of coloured fluids, which will have the effect of extinguishing the colour of a heated source. As an example, three cells containing various dyes in solution may be prepared which, when looked through, will extinguish the colour at 840°, 850°, and 860° C. respectively. If it be desired to work at 850°, a difference of 10° on either side may be detected by a trained eye; but to follow a changing temperature a large number of cells would evidently be necessary. Heathcote’s extinction pyrometer, in its early form, consisted of an eye-shade in front of which two pairs of cells containing coloured fluid were mounted. In bringing a furnace to an assigned temperature, observation was made from time to time until a faint red image was perceived through one pair of cells, when the heat supply was regulated so as to maintain the existing temperature. When viewed through the second pair of cells, which contained a slightly darker fluid, no red image was to be seen at the correct temperature. With training, a workman could control a furnace to a fair degree of accuracy by this means, but the operation was tedious, and useful only for the attainment of a single temperature. In a later instrument, known as the “Pyromike” ([fig. 63]), Heathcote employs a single cell with flexible walls, so that by turning the screw-end, the length of the column of fluid interposed between the eye and the furnace can be altered. In taking a reading, the furnace is sighted and the screw turned so as to increase the length of the column of coloured fluid, until the image is no longer visible. A direct reading of the temperature is then obtained on a spiral scale marked on the cylindrical body of the instrument, over which the screwed portion rotates. This forms a simple and convenient temperature gauge for workshop use.