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W A T L O W Application Guide

The Art of TemperatureSensing

Thermocouple Response Time constant” (thermal response time).Time constant, or thermal responsetime, is an expression of how quickly asensor responds to temperature changes. As expressed here, timeresponse is defined as how long it takes a sensor to reach 63.2 percent of a step temperature change (see Ref. 30).Response is a function of the mass ofthe sensor and its efficiency in trans-ferring heat from its outer surfaces tothe sensing element. A rapid time response is essential for accuracy in a system with sharp temperature changes. Time response varies with the probe’s physical size and design. The response times indicated arerepresentativeof standard industrialprobes. Since you’re actually interested in thetemperature of the surrounding medium, accuracy depends on the ability of the sensor to conduct heat from its outersheath to the element wire.Several factors come into play.Themost commonly noted is “time Ref. 29Ref. 30

Temperature Sensors Average Response TimeSheathStill Water (seconds)*DiameterGrounded JunctionUngrounded Junction

Time Constant(Thermal Response Time)

0.010 in.<0.02<0.020.020 in.<0.020.030.032 in.0.020.07 0.040 in.0.040.130.063 in.0.220.400.090 in.0.330.680.125 in.0.501.100.188 in.1.002.300.250 in.2.204.100.313 in.5.007.000.375 in.8.0011.000.500 in.15.0020.000.5 mm<0.020.031.0 mm0.040.131.5 mm<0.150.352.0 mm0.250.553.0 mm0.400.904.5 mm0.952.006.0 mm2.003.508.0 mm5.007.00

63 PercentTimeTime Constant

Step Temp. Change *Readings are to 63 percent of measured temperatures.

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