GeneralInformation

Application Hints

Since you are actually interested in the temperature of the surrounding medium, accuracy depends on the ability of the sensor to conduct heat from its outer sheath to the element wire.Several factors come into play. Themost commonly noted is “time constant”(thermal response time).Time constant, or thermal responsetime, is an expression of how quickly a sensor respondsto temperaturechanges. As expressedhere, timeresponse is defined as how long it takes a sensor to reach 63.2 percent of a step temperature change (see graph to the right).Response is a function of the mass ofthe sensor and its efficiency in trans-ferring heat from its outer surfaces to the wire sensing element. A rapidtimeresponse is essential foraccuracy in a system with sharptemperature changes.Time responsevaries with the probe’s physical sizeand design.The response times indicated arerepresentative of standard industrial probes.
General Information

What is the response time of mineral insulated thermocouples?

Application Hints The smaller the diameter, the fasterthe thermocouple will respond.Grounding the junction will alsoimprove the response time by approximately 50 percent. This isbased on the sensor achieving 63.2 percent of the final reading, or to the first time constant. It will take about five time constants to obtain steady state readings.
Time Constant(Thermal Response Time) 63%Time Step Temp. Change Time Constant Thermocouple Time Response
Average Response TimeSheathStill Water (seconds)*DiameterGrounded JunctionUngrounded Junction 0.010 in.<0.02<0.020.020 in.<0.020.030.032 in.0.020.070.040 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 mm0.040.13 1.5 mm<0.150.352.0 mm0.250.553.0 mm0.400.904.5 mm0.952.006.0 mm2.003.508.0 mm5.007.00 *Readings are to 63 percent of measured temperatures. 23