Recorders for Radiation Pyrometers.—Any of the thermo-electric recorders described in Chapter II may be applied to radiation pyrometers, the chart being suitably divided according to the fourth-power law. When taking a record, the pyrometer is fixed on a stand or bracket and focused on the desired spot. [Fig. 54] is an example of a record taken with a Thread recorder and Féry pyrometer, in which the division of the temperature scale according to the fourth-power law will be noticed. It is possible to arrange that the working temperature shall lie on the open part of the scale, by adjusting the sensitiveness of the galvanometer accordingly before calibrating.
Management of Radiation Pyrometers.—It is not advisable to place a radiation pyrometer in the hands of an unskilled observer, as intelligent oversight is required if good results are to be secured. Care must be taken to adjust the galvanometer needle to zero before taking a reading, and the needle should always be locked during transit. When focusing on an object in a furnace it is necessary to make certain that the red image seen is actually that of the object, which may be done by moving the pyrometer until the side of the object, or some special feature, is visible in the eye-piece, when the pyrometer may be moved until the image surrounds the junction. Occasions may arise, as in taking the temperatures of various zones of a rotary cement-kiln or other furnace, in which it is required to focus the mirror for a specified distance; in which case the author has adopted the plan of placing a fixed pointer opposite the milled head which controls the mirror (P, [fig. 44]) and focusing the bars of a window at measured distances, marking the same on the milled head opposite the pointer; and it would be a convenience if all radiation pyrometers were thus marked initially. A good check to correct focusing in the case of a heated object is to alter the focus in both directions, and finally to adjust to the maximum reading, which should correspond to the true focus.
Great care should be taken not to damage the mirror. If, in a workshop, the surface become covered with dirt, this should be removed by gentle brushing with a camel-hair brush or by blowing air over the mirror. The focusing device should never be strained beyond its working limits; when these are reached, the pyrometer should be moved bodily until the object can be correctly sighted within the ordinary limits of the movement of the milled head. If metallic fumes or dense smoke intervene between the furnace and the pyrometer, the radiations will be impeded and the temperature recorded will be too low; and in such cases the pyrometer should be placed at the open end of a tube and sighted upon the closed end, which should terminate at the spot under observation.
In all cases it must be borne in mind that the indications only apply to black-body conditions. If a block of steel be sighted inside a furnace, and then be removed to the exterior and again sighted, the external reading will be much less than the internal, owing to the inferior radiating power of the surface, which now derives no assistance from the furnace. All readings should therefore be taken whilst the object is still in the furnace, or (as in taking the temperature of molten metal in a ladle) a fireclay tube with a closed end inserted in the mass may be used, and readings taken through the open end. Statements are sometimes made that the difference between external readings and black-body readings is constant for a given surface, and that the one may be translated into the other; but this is true only for unchanging surfaces, such as platinum, and seldom applies to ordinary working surfaces. As black-body conditions are so easy to ensure, it is simpler and safer always to arrange to take observations under such conditions, rather than to trust a relation seldom constant in practice.
When using a radiation pyrometer for a number of furnaces, fireclay tubes, closed at one end, may be inserted in each, so that the closed end terminates at the working spot, the open end being left flush with the exterior of the furnace. The diameter of such tubes will depend upon the length and also upon the make of the pyrometer; in all cases the image of the closed end must be large enough to overlap the receiving junction or spiral. Information on this point can always be obtained from the makers, or can be discovered by trial with openings of known diameter. When using the pyrometer to obtain temperatures in the interior of the tube of an electric furnace, such as that illustrated in [fig. 29], a solid object, such as a short fireclay cylinder, or a piece of graphite, should be placed in the middle of the tube, and focused on the junction.
Special Uses of Radiation Pyrometers.—For regular use at temperatures above 1000° C. or 1850° F. the radiation pyrometer will be found to be more useful than instruments of the thermo-electric or resistance type as the latter undergo deterioration owing to the continuous action of the furnace gases, which becomes more marked as the temperature increases. Examples of industrial processes in which 1000° C. is considerably exceeded are the manufacture of glass, pottery, and cement, the treatment of special steels, and the casting of metals and alloys. Even for temperatures between 750° and 1000° C. a radiation pyrometer may be used, but is not so convenient for this range as a thermo-electric instrument. There is no upper limit to the instrument, which may be calibrated by the fourth-power law to the highest temperature attainable, that of the electric arc, which has been found to be 3720° C. by the use of a Féry radiation pyrometer. Measurements may therefore be made beyond the limits of thermal junctions, such as the temperature of electric furnaces and of thermit in the mould, and of molten steel before pouring, thus opening out the possibility of accurate control at extremely high temperatures. There is always a danger, however, of the cold junction becoming unduly heated when near to large masses at very high temperatures, and serious errors may arise from this cause. Two examples may be cited to illustrate the usefulness of the radiation pyrometer in practice: (1) the hardening of steel projectiles; and (2) the determination of the temperature of the clinkering zone in a rotary cement kiln. In (1) the projectile is brought to a given spot near the brink of the furnace, where it is in the focus of a radiation pyrometer, and when at the specified temperature is raked out of the furnace and drops into an oil-trough. It has been found that a difference of 10° C. from the standard temperature at which the projectiles should be quenched may cause a serious lowering of the penetrative power of the finished projectile; and hence a radiation pyrometer, which may readily be sighted on each individual shell, is the best to use for this purpose. In (2) the hottest spot may be found by focusing the pyrometer to different distances up the kiln, and, by taking a record, any fall in temperature due to defect of coal or air supplies, or to excessive feed of raw material, may be detected, thus furnishing information from which the process may be regulated to the best advantage. At the temperatures prevailing in such kilns—1300° to 1450° C., or 2370° to 2640° F., according to the nature of the kiln—a Féry radiation pyrometer is quite sensitive to changes of 10° C. or 18° F., and the author has found it to be entirely satisfactory in this connection. The adaptability of radiation pyrometers to all temperatures above a red heat, combined with the absence of deterioration, renders these instruments of great value, and the possibility of obtaining records is a further recommendation. The radiation method, however, is not suited to the purposes of an installation, as even if mirrors and junctions could be constructed so as to be identical, the arrangement would be very costly. A cheap adaptation of the radiation principle, by means of which a number of furnaces, such as a set of cement-kilns, could be controlled from a centre, would be of great advantage, and would add further to the general utility of this class of pyrometer.
CHAPTER VI
OPTICAL PYROMETERS
General Principles.—When a solid is heated to 450° C., it commences to send out luminous radiations and appears a dull-red colour in a darkened room. As the temperature rises, the luminous radiations become more intense; the colour changes to a lighter red, then to orange, yellow, white, and finally to a dazzling white. Attempts have been made to assign temperatures to specified colours, and Pouillet, in 1836, introduced a table which purported to give the relation between colour and temperature. The following table, published by Howe in 1900, differs considerably from that of Pouillet, who had no accurate means of measuring the temperatures he assigned to the colours:—