Pyrometry: A Practical Treatise on the Measurement of High Temperatures

by Charles R. Darling

Fig. 69.—Expansion Pyrometer.

The defect of pyrometers of this type is that the coefficient of expansion of the materials alters with prolonged heating, causing the readings to become erroneous. Re-adjustment in boiling water or other substance does not compensate properly for this alteration, as both materials are not equally affected. Again, the readings will be too low unless the whole of the expanding parts are in the interior of the furnace, in which respect this pyrometer is inferior to a thermo-electric instrument, which may be inserted at any convenient depth, and may therefore be used for a greater variety of purposes. The chief recommendation is cheapness; but an expansion pyrometer should never be used for work of precision. A graphite rod in an iron enclosure gives more consistent results than other materials.

Northrup’s Molten Tin Pyrometer.—Tin melts at 232° C., and boils at 2270° C. It does not give off vapour sensibly up to 1700° C., and expands with great uniformity. It is therefore suitable for measuring high temperatures on the same principle as an ordinary thermometer, and Dufour, in 1900, attempted to make a high-reading thermometer by enclosing tin in a silica bulb. Northrup has constructed an instrument in which the bulb and stem are of graphite, and the height of the molten tin is determined by lowering a nickel wire through a gland until it touches the tin, thereby completing an electric circuit and causing a bell to ring or producing a deflection on a galvanometer. The upper end of the nickel wire moves over a scale, which may be marked at two suitable fixed points, and the scale divided up as in the case of an ordinary thermometer. The durability of the graphite cover will determine the utility of this pyrometer, and protection by some good refractory will be essential to prevent oxidation. Such a pyrometer will not respond quickly to changes in temperature, but may prove useful in reading temperatures at ranges beyond the scope of present thermo-electric pyrometers. Northrup anticipates that this instrument may be used up to 1800° C.

Vapour-Pressure Pyrometers.—In these instruments mercury is placed in a stout steel tube, to which a pressure-gauge is attached, which registers the vapour-pressure of the mercury. Readings of pressure may be translated into temperatures by calibration with a standard pyrometer. The range of these instruments is limited—600° or 700° C.—and they are seldom used at present, having been superseded by more modern types.

Water-Jet Pyrometers.—In these instruments water is passed through a pipe placed in the furnace or hot space at a definite rate, and from the rise in temperature produced in the water that of the furnace may be obtained. An outfit of this kind entails the provision of a steady source of water-pressure, and the indications can only remain accurate so long as the bore of the pipe remains uniform. The calibration is made by comparison with a standard pyrometer. The drawbacks to the method are its inconvenience, and the necessity for continuous skilled supervision; and in consequence of these the arrangement is seldom used.

Pneumatic Pyrometers.—Attempts have been made to deduce furnace temperatures by blowing air at uniform pressure through a pipe located in the hot space, and noticing the increase in the temperature of the air. In the Uehling pyrometer, air from the hot space is drawn through an opening of fixed size by means of a steam-jet, which acts as an aspirator. The opening is placed at one end of a chamber, and the steam-jet aspirator at the other end; and a diaphragm with a central hole divides the chamber into two parts. The pressures existing in the two portions of the chamber vary according to the temperature of the air drawn in, and are measured by water-gauges, the readings of which may be translated into temperatures by calibration against a thermo-electric or other pyrometer. The method is ingenious, but is elaborate and costly; and is therefore little used.

Conduction Pyrometers.—If one end of a rod of metal be inserted in a furnace, heat will be conducted along it to the portion external to the furnace, and a steady condition will be obtained when the heat escaping from the external part of the rod, by convection and radiation, is equal to the quantity conducted along the rod. The hotter the portion in the furnace, the higher will be the temperature of all parts of the external length. A series of thermometers placed at intervals in the exterior portion would show a progressive fall in temperature along the rod; and the hotter the furnace the higher would be the reading on each thermometer. In applying this principle to the measurement of high temperatures, a bar of copper or iron is passed through the wall of the furnace, so that a length of 2 feet or more protrudes on the outside. Near the end of the external portion a hole is drilled to a sufficient depth to cover the bulb of a thermometer, which is inserted in the hole, into which a quantity of mercury is poured to make a metallic contact between the bulb and the bar. The reading of the thermometer furnishes an approximate clue to the temperature of the furnace, rising or falling with corresponding changes in the hot space. A calibration might be effected by comparison with a standard; but the method is only applied to the production of a prescribed condition, known by experience to be attained when the thermometer reading has a certain value—say 120° C. Changes in atmospheric temperature, or currents of air, seriously affect the readings, and the method at best is only approximate.

Gas Pyrometers.—Wiborgh, Bristol, and others have constructed pyrometers in which the pressure of an enclosed gas is recorded by a Bourdon pressure-gauge, the scale of which is calibrated so as to read temperatures. A porcelain bulb, terminating in a capillary tube which is connected to the gauge, is used to contain the air or other gas; but at temperatures above a red heat the readings become uncertain, owing either to leakages or the distortion of the bulb. The most suitable material for the bulb (alloy of platinum, 80 per cent., and rhodium, 20 per cent.) is too costly to use industrially, and would deteriorate under the influence of furnace gases. In the Bristol recording instrument the moving index of the pressure-gauge terminates in a pen, which touches a chart-paper revolving by clockwork. Good results are obtained up to 400° C., but beyond this the indications are uncertain, and the instrument is more correctly described as a recording thermometer.

Wiborgh’s Thermophones.—These consist of infusible clay cylinders, 2·5 cms. long and 2 cms. in diameter, which contain an explosive. When placed in a hot space, the explosion occurs after a definite time, the interval being less at high temperatures than at lower, as the rate at which heat is conducted through a solid varies directly as the difference between the external and internal temperatures. The interval elapsing between placing in the furnace and the subsequent explosion is noted on a stop-watch to the nearest 1/5 second, and from the observed time the temperature is obtained from a table, drawn up from the results of experiments under known conditions. If the cylinders be kept dry, an observer experienced in the use of thermophones may secure a reading to within 40° C.

Joly’s Meldometer.—This device, due to Dr Joly, is intended for laboratory determinations of melting points. It consists of a strip of platinum, heated by electricity, upon which a tiny fragment of the material is placed, which is viewed through a microscope. The temperature of the platinum is regulated by means of a rheostat in the circuit, and in making a test the temperature is gradually raised until the material is observed to become globular, or to flow over the platinum strip. The temperature at which this occurs is deduced from the linear expansion of the platinum strip, which is measured by a micrometer attached to the instrument. When carefully used, very accurate determinations may be made by the meldometer, the results, moreover, being obtained rapidly, and with the use of the minimum of material.