Pyrometry: A Practical Treatise on the Measurement of High Temperatures

by Charles R. Darling

Fig. 17.—Darling’s Compensator, fitted to Galvanometer.

An automatic compensator for use with base-metal pyrometers has been devised by the author, and is illustrated in [figs. 17] and [18]. A spiral made of a compound strip of two metals is attached to the needle of the indicator, and coils or uncoils when cooled or heated, thereby moving the pointer over the scale. The length of the spiral is such that an alteration of a given number of degrees in its temperature moves the pointer by the same number of degrees on the scale—or, in other words, the temperature scale of the pyrometer is identical with that of the spiral. The metals forming the junction are continued, in the form of wires, to the interior of the galvanometer, where a cold junction is formed, which will always possess the same temperature as the spiral. The scale is constructed to represent differences of temperature between the hot and cold junctions, and before coupling up the pyrometer the pointer indicates the temperature of the spiral; that is, of the cold junction. On connecting the thermocouple the pointer is moved by the coil of the indicator through an amount represented by the difference in temperature between the two junctions, and therefore finally indicates the temperature of the hot junction.

Fig. 18.—Indicator fitted with Darling’s Compensator.

Example.—If the cold junction were at 20°, the pointer, before connecting the couple, would indicate 20° on the scale. If the hot junction were 580° hotter than the cold, then on completing the circuit the pointer would move 580 additional degrees along the scale, so that the figure indicated would be (20 + 580) = 600°, the temperature of the hot junction. If now the indicator were heated by 10°, the spiral would tend to augment the deflection by 10°, but simultaneously the deflection due to the junctions would fall off by 10°, and the reading would still be 600°.

This method of compensation renders the readings independent of the cold junction, and, in addition to its use for high temperatures, enables ordinary and low temperatures to be read simply and correctly, as will be shown later. The spiral is located in the tower rising from the top of the indicator in [fig. 18].

In Paul’s method of compensation the thermocouple and indicator are placed across a Wheatstone bridge, two arms of which contain resistances of copper, whilst the resistances in the other two arms are of manganin. Any change in temperature at the cold junction is shared by these four resistances, and, whilst affecting the resistance of the copper parts, no change is caused in the manganin parts, as this alloy has a negligible temperature coefficient. If, therefore, the bridge were initially balanced at 20° C., and the temperature rose to 30°, the increased resistance of the copper would destroy the balance, and permit of a small current passing through the indicator. A fall to 10°, by diminishing the resistance of the copper, would cause an equal current to pass through the indicator in the opposite direction. The amount of this current is arranged so as to add the rise in temperature of the cold junction to the reading of the indicator in the one case, and to subtract the fall in the other, thus retaining true readings for the cold-junction temperature at which the couple was standardized.

Constant Temperature Cold Junctions.—If the cold junction can be kept at a steady temperature, compensators are unnecessary, but no good practical means of achieving this end has yet been devised. Water-cooled heads have already been referred to; but in many situations the connecting-pipes entailed would be objectionable, and hence this arrangement is not greatly used. An alternative method, suggested by Prof. A. Zeleny, is to bury the cold junction in the ground. Recent experiments, conducted at Cambridge by R. S. Whipple, showed that a junction buried 10 feet deep did not vary in temperature by more than 2° C. over a period of three years. This has led to the adoption of buried junctions in special cases; but it is probable that much greater variations would be experienced in the ground beneath large furnaces, in which case the advantages of this procedure would be lost. A common workshop method is to locate the cold junction in a thermos flask filled with oil, when a temperature constant to 2° C. may be secured, although the changes in the temperature of the surrounding atmosphere may be as great as 150 C. For special work, ice may be used in the thermos flask, thus securing absolute constancy; but this procedure is not feasible in ordinary works practice.

Special-Range Indicators.—When the working range of a pyrometer is from 600° C. upwards, it is evident that the part of the scale occupied by the first 600° is useless, and that it would be an advantage if the whole scale could be utilised for the special working range, so as to secure more exact readings. This may be accomplished by a “set-up” against the movement of the pointer caused by the thermocouple, so as to prevent any motion over the scale until an assigned temperature is reached. For example, a junction developing 12 millivolts at 1000° C. may be coupled to an indicator in which the full-scale deflection of the pointer is produced by 6 millivolts. If an E.M.F. of 6 millivolts be opposed to the junction, no deflection will occur until the temperature at which the couple develops 6 millivolts is reached—when the opposing E.M.F. will be overcome. This temperature may be 500° C., so that the whole scale may be divided up between 500° and 1000°. The length of the indicator scale is thus effectively doubled; and by using different values for the set-up, it is evident that any desired range may be obtained within the limits of sensitivity of the indicator. The method of procuring the opposing E.M.F. varies with different makers. The Cambridge and Paul Instrument Company employ a dry cell and a series resistance, connected so as to oppose the thermocouple; and by adjusting the resistance any desired set-up may be obtained, the value of which, in degrees, may be read off by connecting the cell and resistance to the indicator, the couple having been switched out of the circuit. Thus, to adjust for a range of 500°-1000° on an indicator giving full-scale deflection for 500°, the resistance is regulated so that the cell alone causes the pointer to move to the end of the scale. The method adopted by Paul consists of suitable resistances inserted in a Wheatstone bridge, which may be thrown off the balance, and thus cause an opposing E.M.F. of the correct amount at the terminals of the indicator.