Thermal System 
The calibration tank uses external hot and cold 
tanks, the former maintained well above and the 
latter well below operating temperature. The transfer 
of heat is accomplished by pumping liquid from either 
tank to the calibration tank, with a return path 
furnished to provide a closed circulating system 
(fig. 2). The choice of hot or cold is made by the 
thermostat, which controls electrically operated sole- 
noid valves on the two systems and the pumping 
motors. In addition, there is a metering valve in each 
system to control the rate of flow of hot or cold 
liquid so as to maintain constant tank temperature 
despite unavoidable transfers of heat to the sur- 
roundings. When a macrochange of temperature is 
desired, the metering valves are opened fully to 
permit rapid transfer of a considerable volume of 
liquid from either the hot or cold external tanks. The 
use of external tanks avoids the thermal lags which 
would occur if the heating and cooling systems were 
installed in the actual body of the calibrating tank 
STIRRING MOTORS 
HOT WATER 
METERING 
VALVES 
CALIBRATION TANK 
itself. In the initial installation one pump was used 
for both hot and cold tanks, but on the first trial run 
it was found that because of the residual hot or cold 
liquid in the pump itself and despite the very short 
piping system from it to the tank, there was too much 
overshooting. A pump and metering valve have now 
been installed on each tank so that when the thermo- 
stat demands hot or cold it is provided instantly at 
the requisite rate of flow to allow full mixing, with 
minimum of overshoot. 
Hot Tank 
The hot tank measures 8 by 10 by 20 inches and 
is made of brass sheet Ye-inch thick, brazed at all 
edges (fig. 3). It is thermally insulated with 1-inch 
cork board held in place and surfaced by 3-inch 
Permacel industrial tape which is painted to form 
a strong continuous surface. Water is piped into the 
whole system through the rear end of this tank. Two 
1500-watt, 110-volt heating elements are mounted 
in it through the front, together with a thermometer 
COLD WATER 
TANK 
SOLENOID CONTROL 
VALVES 
Figure 2. Circulating system. 
