Standardization may be effected either by subjecting the hot junction to several known temperatures, and noticing the deflections corresponding thereto; or by measuring the electromotive force developed by the junction, and calculating the corresponding temperature from a formula which is known to hold for the range comprehended by the instrument. The former method is simpler; and if carefully conducted is quite accurate. The latter method possesses the advantage that readings in millivolts may be translated directly into temperatures when the constants of a given thermal couple are known. It is now usual to mark indicators with a double scale, one reading millivolts and the other temperatures.
Standardization by Fixed Points.—Taking any millivoltmeter which, with a maximum of 20 millivolts at the terminals, will give a full scale deflection, the first step is to arrange that the pointer (or spot of light) shall just remain on the scale at the highest temperature to be attained by the junction. This may be done by placing the hot junction in boiling water and noting the deflection obtained, either in millivolts or equal arbitrary divisions, and also the temperature of the cold junction. The deflection observed is due to a difference of temperature (100-t) deg. C, where t is the temperature of the cold junction. If the highest temperature to be measured is 10 times (100-t), the deflection should be rather less than 1⁄10 of the scale, and similarly for any other required temperature limit. If the observed deflection exceed this proportion, a series resistance should be added until the correct value is obtained. This resistance is then permanently installed in the circuit for use with the junction under trial.
Before proceeding further it is necessary to consider whether the pyrometer is to possess a single cold junction of ascertainable temperature ([as in fig. 6]), or whether it will be arranged with two cold junctions in the head, as in [fig. 4]. In the former case it is simpler to prepare a “difference” scale; that is, one which reads differences of temperature between the hot and cold junctions, from which the temperature of the hot end may be obtained by adding to the difference that of the cold junction. In the latter case the cold end should be kept by artificial means at the temperature likely to be attained in practice—say 25° C.—a water-bath being suitable for this purpose. It is advisable to remove the shield of the pyrometer when standardizing, so as to expose the hot junction, as closer readings can then be taken.
A number of materials—preferably cheap—of known boiling points or melting points are then selected from a table of fixed points (page 16) so as to give about six points, distributed fairly evenly over the scale. As an example, if it were desired to prepare a temperature scale from 0° to 1000° C., the following might be chosen:—
| Substance and Condition. | Temperature. | |
| Water at boiling point | 100° C. | 212° F. |
| Tin at melting point | 232 | 449 |
| Zinc at melting point | 419 | 786 |
| Antimony at melting point | 631 | 1167 |
| Common salt at melting point | 800 | 1472 |
| Copper at melting point (covered with graphite) | 1084 | 1983 |
The hot junction is allowed to attain these temperatures successively, and the corresponding deflection in each case is noted. It is then possible to divide up the whole of the scale to read temperatures directly.
The first reading is taken by placing the junction in a vessel of boiling water, and for a locality near sea level it is not necessary in ordinary work to take account of fluctuations in the boiling point due to alterations of atmospheric pressure. To ensure that the other readings are taken when the substances are exactly at the melting point, the procedure is as follows: about 2-3 lb. of the substance are melted in a salamander crucible, and a small fireclay tube, closed at one end, is inserted in the molten mass. The hot junction is placed in the fireclay tube, and the intervening space filled with asbestos fibre. Great care must be taken not to let the junction touch the fused substance. The crucible is now allowed to cool, and a reading of the deflection taken every half-minute. When the substance is exactly at its solidifying point—identical in general with the melting point—the deflection remains stationary for several consecutive readings, owing to the liberation of latent heat of fusion in sufficient quantity to balance the loss by radiation. This stationary reading is noted for each substance, and represents the deflection given when the hot junction is at the temperature corresponding to the melting point, and the cold junction or junctions at the temperature existing when the observation is made. For melting the materials, a Davies furnace with a large Teclu or Meker burner is convenient up to 850° C.; but to melt the copper a blast lamp is requisite. The molten mass may be allowed to cool in the furnace.
From these observations a calibration curve may be drawn either for differences between hot and cold junctions, or for a steady temperature of the cold junctions. Two sets of data are appended to illustrate the procedure.
| Temperature of Hot Junction. | Pyrometer 1. Iron-constantan. | Pyrometer 2. | |||
| (Series resistance in | Platinum-iridioplatinum. | ||||
| galvanometer circuit.) | |||||
| Deflection. | Cold Junction. | Difference. | Deflection. | Cold Junction. | |
| 100° C. | 8·9 | 15° C. | 85° C. | 5·5 | ┐ |
| 232 | 21·8 | 17 | 215 | 15·6 | │ Constant |
| 419 | 40·6 | 19 | 400 | 29·4 | ├ at |
| 631 | 63·8 | 19 | 612 | 45·5 | │ 25° C. |
| 800 | 83·0 | 20 | 780 | 59·0 | │ |
| 1084 | . . . | . . . | . . . | 82·0 | ┘ |