(7) TEMPERATURE CORRECTION FOR DEPTH. A small temperature
correction may be applied to account for the head of the water above the center of
the thermometer element. The temperature, tC, at the level of the thermometer can
be determined from the equation:
tC = 0.0100 - 0.00000723 x h;
where h is the height in cm from the level at the center of the thermometer
element to the surface of the water in the glass cell.
(8) OPERATING PRECAUTIONS. The Equiphase Cell should be used in such a
manner that a minimum of heat is introduced into the cell from outside sources.
Heat may be conducted by thermometer leads or by the body of the thermometer
or thermocouples when placed in the well. This may be prevented by packing ice
around the upper portion of the well, and by chilling thermometers before
insertion. Heat may also be introduced by the power supplied to the platinum
resistance element. A self-heating error may be incurred if the thermometer is
operated at a current in excess of its calibration current. The glass cell itself is
transparent to heat and, if withdrawn from the insulating jacket during operation,
(fig. 12) cannot prevent radiant heat from room sources from affecting the
thermometer. This can be prevented by keeping the glass cell inside the insulating
jacket during operation. In no case will the triple point temperature in the glass
cell change. However, if the above precautions are not taken, and heat transfer
into the well is permitted, then the temperature of the contents of the well will rise
sufficiently to conduct this heat to the ice mantle. Then, in the case of devices
such as platinum thermometers in partially evacuated envelopes, deviations of
several millidegrees may be observed in the thermometer readings.
(9) TESTING THE EQUIPHASE CELL. The triple point of water is a fixed
point, the temperature of which is a matter of definition, and therefore cannot be
measured. There are tests, however, which can be used for a qualitative check of
the triple point cell.
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