Fig. 15.—Calibration Curves for Two Thermo-electric Pyrometers.
[Fig. 15], A, is a calibration curve for thermocouple 1, connecting deflections with corresponding differences between the temperatures of the hot and cold junctions. In order to read from this curve the temperature of the hot end, the reading corresponding to the observed deflection is added to the existing temperature of the cold junction. Thus if a deflection of 56 divisions were obtained with the cold junction at 25°, the temperature of the hot junction would be (540 + 25) = 565° C. The advantage of this method of calibration is that it is unnecessary to take precautions to keep the cold junction at a steady temperature; and when a single cold junction is used, as in [fig. 6], this plan should always be followed. It will be noted that this curve passes through zero, as no deflection represents no difference of temperature.
[Fig. 15], B, represents the calibration curve for pyrometer 2, and is such that direct readings may be obtained corresponding to any given deflection, for a cold junction temperature of 25°. This curve, therefore, cuts the axis of zero deflection at 25°, as no deflection corresponds to the condition when both hot and cold junctions are at 25°. This method of calibration may be used with advantage for couples of the type shown in [fig. 4], where two cold junctions exist in the head, and the simple rule of adding the cold junction temperature does not apply. Many suggestions have been made for correcting for alterations in the temperature of the cold end of such a couple, but none are accurate, and it is necessary to keep this part at the temperature of standardization to secure correct readings. In both of the above calibrations the galvanometer used possessed a scale divided into 100 equal arbitrary divisions.
In making permanent temperature scales from these curves to attach to the existing galvanometer scale, intervals of 100° may be taken and marked opposite to the corresponding divisions on the existing scale. Each 100° may then be equally subdivided into as many parts as the length of scale permits, and numbered at suitable intervals. If the junction used yield a calibration curve departing greatly from a straight line, every 50° interval should be taken, or, if necessary, every 25°. In the examples given both curves are nearly straight lines in the working region, viz. 400° to 800° for the iron-constantan junction, and 500° to 1100° for the platinum-iridioplatinum.
One precaution necessary in standardizing an indicator by this method is to ensure that the metals used are pure, as impurities lower the melting points. If ordered as “pure” from any dealer of repute, the metals will generally be found satisfactory. The common salt used should be the ordinary salt sold in blocks, and not a prepared table salt. A second precaution, when observing melting points, is to guard against a possible error due to the substance becoming “surfused” or “overcooled”; in which case the temperature falls below the ordinary freezing point before solidification commences. When freezing occurs, however, the temperature rises to and remains at the true melting point, and an increase of deflection following a gradual fall always indicates overcooling. The higher deflection then attained is the true freezing point. Antimony frequently overcools to 600° before freezing, but on setting rises to the correct figure—631°. All metals and salts are liable to overcooling occasionally.
Standardization by Measurement of E.M.F.—It has been found, as the result of experiments, that the relation between the E.M.F. developed by a junction and its temperature—under constant conditions of the cold junction—may be expressed approximately by a formula as under:—
log E = A log t + B (Holman’s formula),
where E = electromotive force in microvolts, t = temperature in Centigrade degrees, and A and B are constants depending upon the junction. With certain junctions this formula may be applied over the working part of the scale with an error not exceeding 2° C., but with others the discrepancy is greater. In order to determine the constants A and B, it is necessary to measure the E.M.F. at two known temperatures, which should be chosen as far apart as possible in the working region. When these constants are known, a measurement of E enables the temperature t to be found by calculation.