calibration; constant sensitivity throughout its range of 

 use; continuous and permanent record of tank temperature 

 during calibration; and reduction of mechanical parts in the 

 control system that are subject to wear. 



The old control system/ retained as a backup, 

 uses a pair of thermistors mounted in the calibration tank 

 as temperature sensors. They comprise two arms of a 

 Wheatstone bridge, the other two being a fixed resistor and 

 a calibrated ten-turn potentiometer. The voltage resulting 

 from an imbalance of the bridge is introduced to an ampli- 

 fier which drives a servomotor. The rotational direction 

 of the servomotor is dependent upon the direction of im- 

 balance of the bridge. Coupled to the servomotor shaft is 

 a slip-clutch cam that closes either of two microswitches 

 mounted on opposite sides. These switches energize the 

 hot and cold solenoid valves respectively. This system is 

 limited by its design to a maximum of 35°C. The minimum, 

 with alcohol added, is -4°C. The system has no direct 

 readout and cannot be used for quick checks of the tank 

 temperature; however, it is capable of maintaining the bath 

 temperature to within ±0. 002°C of any desired point through- 

 out its range. 



THERMAL SYSTEM IMPROVEMENTS 



Improvements have continually been made to the 

 thermal system. The process of maintaining calibration 

 tank temperatures below, but near to, that of the room 

 requires the infrequent introduction of small quantities of 

 cold water. Originally, during the periods of zero flow, 

 the dormant water in the pipes absorbed heat from the room 

 throughout the length of pipe from cold tank to solenoid. 

 When the controller next opened the valve, water of the 

 wrong temperature was thus introduced. A similar but 

 opposite problem existed in the hot-water supply system. 

 To remedy these problems a recirculating line (fig, 7) was 

 installed in each of the external tanks. The pumps, which 

 are gravity fed, now continuously force water of the proper 

 temperature to a point as near as possible to the valves 

 regulating induction to the calibration tank. If the solenoid 

 valves are not open, the liquid is pumped back to the supply 

 tanks via the recirculating lines. 



15 



