Resistance Temperature Detectors (RTD's)
Time Response Characteristics
The response of an ARi Resistance Temperature Detector is defined by 2 noticable characteristics when exposed to an instantaneous (step) change in enviroment temperature. These are:
A.) Time Constant (tau). The time to reach 63.2% of the complete step change in temperature.
B.) Response Time. Time to reach within 0.5% of the final temperature in a step change. This is approximately equal to 5 times the time constant
The response of a temperature sensor to a step change in enviroment temperature tends to follow a second order differential equation. However, this is approximate, since if the mass of the sensor is small in relation to the mass of the fluid passing over it (such as in the case of a liquid), the response may approach a first order differential equation. A typical response is as follows:
Time constant has application for more common experiences in process control, ie, ramp change or sinusoidal changes in enviromental temperature. The response of a sensor under these conditions are:
Time constant is related to the enviromental conditions by the following approximate relation (Ref NASA TN 2599)
Knowing the time constant for a given sensor at one given set of conditions, it can be computed for another set of conditions.
Time constant data for ARi ARiDET Resistance Temperature Detectors are as follows:
SELECTION/MATCHING
Completed probes may be matched in pairs to the nearest 0.01 ohm at 0 °C for differential temperature measurements to 0.05 °C
Completed probes may be supplied with selected ice point resistances within a narrow band to eliminate the need for compensation in indicating circuits.