For industrial applications, factors such as extreme pressures, high process temperatures and vibration require a temperature sensor with repeatable accuracy, good threshold sensitivity, predictable drift, fast response time and stability.
Many types of temperature sensors exist, including thermistors and infrared pyrometers amongst others, but the two most common sensors for the process measurement industry are resistance temperature detectors and thermocouples. They both have specific strengths that apply to industrial applications; if those strengths are matched up with the appropriate application, that should be the optimal sensor to use.
Overall, resistance temperature detectors (also called RTDs) offer a wider range of strengths than thermocouples and thus would match more industrial applications well. This has to do with RTDs having better threshold sensitivity (detecting minimal change in temperature), repeatability, and drift than the thermocouple. The accuracy of a temperature sensor consists of a combination of threshold sensitivity, repeatability and resolution. Resolution, unlike the other two, is set by the transmitter. Drift is critical for extending the time between calibrations and the temperature loop running at the right setpoint.
Advantages of Resistance Temperature Detectors (RTDs)
- RTDs have better accuracy and repeatability
- RTD signals have a higher signal-to-noise ratio
- Cold junction compensation and related errors are not associated with RTDs
- RTD drift is predictable. Thermocouple drift errors can be large because of element poisoning and oxidation at high temperatures
- RTDs have better linearity over temperature ranges
- An in-line RTD calibration is available to address output issues from mechanical shock, poisoning, and temperature cycling. These changes can be eliminated by, an option not available for a thermocouple
- RTDs do not need special extension wire
Advantages of Thermocouples
- Thermocouples function better at higher temperatures (above 1100 °F or 593 °C) than RTDs
- Thermocouples sensors are less expensive than RTDs in the common temperature range
- Total installed cost of thermocouples is less for integral mounted transmitters
- Thermocouples are generally more rugged
- Thermocouples have a faster response time than RTDs but this is applicable only in very fast temperature processes (process time lag < 10 seconds)
Temperature sensors are extremely useful in industrial applications but picking the best option for a specific application will lead to better results and the avoidance of wasted time and cost. As we’ve shown, RTDs match a more diverse range of industrial applications than the other most common sensor used in these environments, thermocouples. That versatility derives from better threshold sensitivity, repeatable accuracy, stability, predictable drift, linearity over temperature ranges and not being susceptible to cold junction compensation and other errors that must be factored in when using thermocouples.
Thermocouples do function better at higher temperatures which will fit certain industrial applications better, they are less costly than RTDs both for the sensor and installation, are generally more rugged and can provide faster response times in certain situations. However, some the cost savings between a thermocouple and RTD is mitigated since, with a thermocouple, one needs to purchase a special extension wire and work around using a less accurate and reliable sensor for most industrial applications.
Please contact Probes Unlimited for temperature sensors, including thermistors, thermocouples and RTDs.