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RTD Accuracy

RTD Accuracy – Time Constant (Thermal Response)
An RTD sensing element like any other thermometer, senses its own temperature. Since you’re actually interested in the temperature of the surrounding medium, accuracy depends on the ability of the RTD to conduct heat from its outer sheath to the element wire.

Several factors come into play. The most commonly noted is time constant (Thermal Response Time). What is Time Constant?

Time Constant

Time Constant or Thermal Response Time, is an expression of how quickly a sensor, in this case an RTD, responds to temperature changes. Time response is defined as how long it takes a sensor to reach 63.2% of a step temperature change.

Response is a function of the mass of the sensor and its efficiency in transferring heat from its outer surfaces to the wire 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 actual element mass.

Response characteristics can be altered by changes in element sizes, sheath diameter, wall thickness, and special packaging techniques. One common alteration is to manufacture a probe with copper tip to improve thermal sensitivity.

The bottom line is that you can often improve performance with careful design and material selection, but more often than not, the standard probe provides response characteristics to meet the vast majority of industry applications.