AN IN SITU TEMPERATURE SENSOR 



F. G. GEIL and J. H. THOMPSON 



Westinghouse Electric Corporation 



Research and Development Center 



Pittsburgh, Pennsylvania 



ABSTRACT 



Discussed is a unit for temperature measure- 

 ment at a remote point (accurate to io.02°C) 

 with a high Q mechanical resonator whose fre- 

 quency is temperature dependent . The resonator 

 is novel in that a mechanical Q near 80,000 is 

 attainahle at 30 Kcps with an aluminum resonator 

 whose support posts may he one-third the size of 

 the resonator itself. The posts allow for rapid 

 thermal stabilization. 



INTRODUCTION 



of any length connects the sensor to the counter, 

 the DC power and temperature signal being super- 

 imposed on the same line. This sensor is 

 especially suited to unattended operation for 

 long periods of time without losing calibration 

 and the frequency can be easily used to modulate 

 a transmitter if needed. The frequency for 25°C 

 is about 36 Kcps for the resonator shown, and 

 the frequency change is approximately 10 cps for 

 each degree Centigrade. The resonator housing 

 is evacuated to eliminate any mechanical loading 

 on the resonator because the accuracy of tempera- 

 ture measurement is related to the mechanical Q 

 of the resonator. 



An in situ temperature sensor has been devel- 

 oped for use in oceanographic measurements and 

 is capable of measuring the temperature of the 

 ocean accurately to iO.02°C. This instrument 

 would also be used in conjunction with an induc- 

 tion conductivity meter which, it is hoped, will 

 determine salinity to an accuracy of 0.02 parts 

 per thousand. The temperature sensor-conductivity 

 meter package will ultimately be capable of opera- 

 tion to depths of ^,000 fathoms. 



PRESENT METHODS 



At present the most popular method for mea- 

 suring temperatures to this accuracy employs a 

 thermistor in a Wheatstone bridge circuit, the 

 thermistor being connected to the bridge by long 

 wires . While the thermistor is capable of this 

 accuracy, there are some shortcomings to its con- 

 venient use, the most significant being the 

 heating of the thermistor due to the current in 

 the bridge circuit. A newer method is to use a 

 thermistor in an RC network in a phase shift 

 oscillator, but this requires an extremely stable 

 circuit or one that is calibrated often. 



DESCRIPTION OF DEVICE 



A more satisfactory way to measure tempera- 

 ture employs the device shown in Fig. 1. Inside 

 the brass housing is an aluminum resonator which 

 acts as a high-Q resonant filter whose frequency 

 is temperature dependent. The resonator and 

 amplifier shown compromise an oscillator circuit, 

 and the temperature is converted to a frequency 

 which is transmitted through a pair of conductors 

 and registered on a counter. A two wire cable 



A more detailed view of the temperature sensi- 

 tive resonator is shown in Fig. 2. The disc 

 between the two posts vibrates in a flexural mode 

 with two nodal diameters, two piezoceramic ele- 

 ments being fastened at the pickup and drive 

 points. The motion is best depicted in the lower 

 left view; in the top view it may be imagined as 

 occurring in and out of the paper, the top and 

 bottom quadrants moving opposite to the left and 

 right. The nodal diameters are stationary. 



The two piezoceramic elements are bonded to 

 the disc with conducting epoxy. The elements 

 are made of a lead zirconate titanate and the 

 disc's resonant frequency is far below their 

 natural resonance. Wires are attached to the 

 elements with conducting epoxy using small phos- 

 phor bronze springs for compliance as shown in 

 Fig. 2. It is important to recognize that the 

 disc responds to the longitudinal or left-right 

 motion of the element and not to the thickness 

 motion. The aluminum is connected to the ground 

 potential of the oscillator and the circuit is 

 thus completed to the bottom plates of the 

 elements . 



Heat is conducted to or from the disc by way 

 of one of the two posts in the resonator of 

 Fig. 1. Ultimately, both posts will be exposed 

 in order to reduce the temperature time constant. 

 Large posts may be used without affecting the Q 

 because the load presented to the disc by the post 

 is reactive. The equal and opposite forces on 

 the post cancel, leaving the post out of the 

 resonant system. The response of this disc to a 

 temperature step function results in a time con- 

 stant of about one minute, or one minute to com- 

 plete 63$ of the frequency change. This time 

 constant can be reduced by decreasing the ratio 



35 



