THERMOSTAT 
GALVANOMETER 
RESISTANCE 
BRIDGE 
Figure 7. Thermostat and resistance bridge. 
uses for which the tank was designed the bridge 
will be used only between 25 and 30 ohms and it 
is safe to assume that temperatures can be measured 
to an accuracy of +0.002. The bridge is periodi- 
cally checked against a standard 10.0100-ohm re- 
sistor, certified by the Bureau of Standards, and over 
a period of three years there has been no measur- 
able divergence from this standard. 
In addition the standard thermometer is checked 
periodically against the ice point provided by dis- 
tilled water ice. 
CALIBRATION OF REVERSING 
THERMOMETERS 
Although the tank can be used to calibrate any 
instrument capable of being inserted into it, it is 
largely used for reversing thermometers. The step- 
by-step procedure used in reversing-thermometer 
calibration is presented here. 
Step 1: Fill Tanks 
The system is first filled with water to which is 
added 15 gallons of alcohol. This amount of alcohol 
permits temperatures to be lowered to —3°C with- 
out ice forming on the refrigerating coils. The solu- 
tion is checked periodically with an alcohol hydrom- 
eter and more is added as needed to replace that 
lost by evaporation. Alcohol was chosen as being 
the simplest and cleanest antifreeze to use, as well 
as the least expensive. 
Step 2: Set Temperature 
Ordinarily the desired temperature is dialed on 
the thermostat the night before calibration work is 
to start. The whole system is then put on a time 
switch, so that it will be at working temperature in 
the morning. 
For the highest speed of operation, the calibrating 
run is usually started at the lowest temperature de- 
sired and then increased through the necessary range. 
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