Resistance Temperature Detectors(RTDs): Application limitations, Comparison of types and Failure mode ~ Learning Instrumentation And Control Engineering Learning Instrumentation And Control Engineering

Resistance Temperature Detectors(RTDs): Application limitations, Comparison of types and Failure mode

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Application Limitations of RTDs:
RTDs can be quite bulky, which can inhibit their use in applications.Self heating can be a problem with RTDs. In order to measure the resistance of an RTD device, we must pass an electric current through it. Unfortunately, this results in the generation of heat at the resistance according to Joule’s Law:
 P = I2 R 
This dissipated power causes the RTD to increase in temperature beyond its surrounding environment, introducing a positive measurement error. The effect may be minimized by

limiting excitation current to a bare minimum, but this results in less voltage dropped across the device. The smaller the developed voltage, the more sensitive the voltage-measuring instrument must be to accurately sense the condition of the resistive element. The magnitude of the errors generated by self heating effects vary, but are dependent on the size and the resistance of the RTD. These errors can be reduced by heat transfer and by minimising the excitation current. The response time of RTDs is typically anywhere from 0.5 sec to 5 seconds. The slowness of response is due primarily to the slowness of the thermal conductivity in bringing the device to the same temperature as that of its environment. The response time increases for increased sensor size, also the use of thermowells can double the response time.

Comparison of RTD Types:

Evaluation criteria Platinum RTD
100Ω wire wound
and thin film
Platinum RTD
1000Ω thin film
Nickel RTD 1000Ω
wire wound
Balco RTD 2000Ω
wire wound
Cost High Low Medium Medium
Temperature range Wide
-400 to 1200°F
(-240 to 649°C)
-320 to 1000°F
(-196 to 538°C)
-350 to 600°F
(-212 to 316°C)
-100 to 400°F
(-73 to 204°C)
Interchangeability Excellent Excellent Fair Fair
Long term stability Good Good Fair Fair
Acurracy High High Medium Low
Repeatability Excellent Excellent Good Fair
Sensitivity (output) Medium High High Very high
Response Medium Medium to fast Medium Medium
Linearity Good Good Fair Fair
Self-heating Very low to low Medium Medium Medium
Point (end) sensitivity Fair Good Poor Poor
Lead effect Medium low low low
Physical size(packaging) Small to medium Small to large Large Large

Failure Modes of RTD Sensors:
An open circuit in the RTD or in the wiring between the RTD and the electronic control circuit will cause a high temperature reading. Loss of power or a short within the RTD will cause a low temperature reading.

For more information on RTD Sensors, check out: