APPENDIX: MULTIPLE CHANNEL TEMPERATURE 

 RECORDING UNIT 



General 



A sixteen-channel temperature sensing unit was developed and has been used 

 at NEL for several years. It consists essentially of thermistor beads cast in plastic 

 and attached to electric leads. The leads and beads are part of a bridge circuit 

 which feeds a recording-type potentiometer. The recorder prints numbered points 

 consecutively from 1 to 16 on a power-driven strip chart, the location of each 

 number indicating a particular temperature. In normal operation, a full cycle 

 of recordings requires approximately half a minute. This time varies, depending 

 upon the range of temperatures to be recorded. Since the temperatures measured 

 are of sea water near the surface, they may vary considerably as the elements go 

 through the thermocline. However, the sequence of points will change in the 

 same direction, from hot to cold as the depth increases and in reverse as it de- 

 creases. Thus, the recording head has to travel a small distance between each 

 printing, enabling it to run through a full cycle rapidly. 



The instrument was designed to measure temperatures in the range from 28°F 

 to 90°F. The strip chart employed is 11 inches in width, divided into 150 units. 

 Hence, for the accuracy required (at least 0.1 °F), we divided the full range into 

 10 subranges, in which each 11-inch width of tape covers 7° to 7.5°F. The sub- 

 ranges overlap each other by at least half a degree. This enables reading of the 

 chart to better than 0.02 °F. 



A Brown Recording Potentiometer (Brown Instrument Co.) was built into a 

 single unit with the necessary bridge and switching elements, the whole weighing 

 approximately 200 lb. This unit may be transported and mounted on shipboard 

 (fig. Al). 



On the face of the unit are mounted sixteen jacks to accommodate leads of the 

 sixteen sensing elements, sixteen range switches, a voltmeter, and a rheostat for 

 maintaining constant current during operation. 



The sensing elements and thermistor beads are mounted at the ends of cables 

 of appropriate lengths, so that they may be lowered to whatever depth is desired. 

 These cables are plugged into the jacks on the recorder, placing their resistances 

 across one arm of the bridge circuit for the temperature range under measurement. 



Design of Temperature Measuring Unit 



The thermistor bead was chosen as the sensing element for use with this re- 

 corder because of its very great change in resistance with temperature change. 

 Specifically, the Western Electric 14B and Veco 32A-1 thermistors change from 

 approximately 2300 to 3500 ohms over the temperature range of 70° to 52°F. The 

 great disadvantage of thermistor beads is their nonlinearity of response, but this 

 was obviated to a great extent (1) by designing the associated circuits so that 

 each temperature range measured was small and, hence, the resistance change over 

 that range nearly linear, and (2) by calibrating for each range at closely spaced 

 points. 



The beads are enclosed in glass rods averaging 1/16 inch in diameter and about 

 2 inches in length. The bead, itself, is at one end, and the leads pass through the 

 length of the glass and emerge at the other end. For underwater use, the beads 

 must be protected electrically from the water and mechanically from physical 



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