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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:
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:
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) | Wide -320 to 1000°F (-196 to 538°C) | Medium -350 to 600°F (-212 to 316°C) | Short -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:
For more information on RTD Sensors, check out:
