A RAPID RESPONSE HIGH ACCURACY THERM A.L PROBE 



H. M. HOOVER 

 Airpax Electronics, Inc. 

 Fort Lauderdale, Florida 



INTRODUCTION 



The temperature measuring system described 

 here was evolved in an effort to furnish the 

 oceanographer with an accurate, rapid response 

 measuring system, approaching the precision of 

 the reversing thermometer but being a continuous 

 reading device and one not requiring manipulation 

 by highly skilled personnel. Realized is an 

 arrangement (Fig. l) with a raw accuracy of 

 ±0.05°C over a 25°C span which may be corrected 

 to io.02°C. The system will maintain this accu- 

 racy, when properly used, for periods of months 

 and it is not affected by normal handling. The 

 time constant is better than one second in flowing 

 water . 



PROBE SELECTION 



In approaching such a device, the basic 

 sensor choice was almost immediately limited to 

 resistance type elements by considerations of 

 accuracy. Thermocouples and filled system ther- 

 mometers are not sufficiently precise. The same 

 is now true of relatively novel methods such as 

 paramagnetic susceptibility or capacitance 

 change. In the area of resistance thermometers, 

 the thermistor is outstanding for its high out- 

 put and rapid response. However, our concern 

 over long term thermistor stability, even after 

 aging, made us abandon the device and employ 

 metallic sensors for the very high accuracies 

 required. Here the stability and ease of repro- 

 duction, inherent in platinum, dictated its 

 choice in spite of its low output. Fortunately 

 the linearity of platinum is quite good. Using 

 platinum, stability and reproducibility of resis- 

 tance change is several orders better than the 

 accuracy required, and in fact, a thermometer not 

 dissimilar in construction defines temperature 

 between -182°C and +603 o C. 1 ' 2 



The probe output follows the Callendar rela- 

 tion of temperature to resistance 



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T 

 100 



\100/ 



(1) 



Fig. 1. Probe assembly. 



Superior numbers refer to similarly numbered references at the end of this paper. 



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