Abstract
The theoretical and experimental studies described in this paper have shown that a VECO type 32A8 thermistor operating with an internal temperature of 0.578°C above ambient in still water can measure temperature changes of the surrounding water at least as small as 0.021°C. Since the increase in sensitivity of the thermistor is proportional to the increase in bridge voltage, the 4-V bridge voltage required to attain the 0.578°C internal temperature represents an increase in measurement sensitivity by a factor of at least 10. The maximum usable bridge voltage was not determined. Measurement of the time constant of the 32A8 thermistor indicates that if this thermistor is used to measure small temperature variations, there is probably an optimum bridge voltage for the greatest frequency response to be achieved and the frequency response seems to increase as the temperature fluctuation of the surrounding medium decreases in amplitude. A 10% velocity turbulence field was assumed for a typical self-heated bead type thermistor probe operating in water with an average relative velocity of 51.5 cm/sec (1 knot). The noise level at the output of the thermistor bridge circuit due to the turbulence was calculated from a measured curve and shown to represent a temperature variation of approximately 0.002°C; this value compares closely to the theoretical value of 0.0012°C. The self-heated 32A8 thermistor is very sensitive to water velocity changes below 10.3 cm/sec (0.2 knots).
Published Version
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