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Technical Reference

Common Temperature Transducers

Thermocouple

RTD

Thermistor

Humidity

References

»	Watlow
»	Omega

RTD ( 'Resistance Temerature Detector' )

All metals produce a positive change in resistance for a positive change in temperature. This is the main function of the RTD. RTD can be constructed from nickel, copper, or nickel/iron.

Each metal has a different alpha coefficient and operating range. An RTD's alpha coefficient must be matched to its instrument or an error of several degrees can occur.

RTD provides a Resistance vs. Temperature output and are passive devices, needing no more than 1.0 mA to run. The most common RTD is a 100 ohm, platinum sensor, with an alpha coefficient of 0.00385 ohms/ohm/deg C. Same precautions that apply to the thermocouple also apply to RTD.

In addition, the following precautions should be observed:

Construction: Due to its construction, the RTD is more fragile than the thermocouple, and the precautions must be taken to protect it.

Self-Heating

Unlike the thermocouple, the RTD is not self-powered. The current that passes through the device causes I2 R (Joule) heating within the RTD, changing its temperature. This self-heating appears as a measurement error. So attention must be given to the magnitude of the current. An RTD immersed in a thermally conductive medium will distribute Joule heat to the medium and the error due to self heating will be less. To reduce self-heating errors, use the minimum current that gives the required resolution. Use of large RTD is preferred. Although the following should be taken into consideration.

Thermal Shunting

It is an act of altering the measurement temperature by inserting a measurement transducer. It is more problem with RTDs than with thermocouples, as the physical bulk of RTD is greater than that of a thermocouple.

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Thermal EMF

The Pt-Cu connection that is made when the RTD is measured can cause a thermal offset voltage. The offset-compensated ohms technique can be used to eliminate this effect.

Note: Above notes have been taken from Temperature Handbook of Omega and request for permission to use has been sought.

Reference Links

Theory

RTD based on applications

RTD tolerance classes

The two most common ways of measuring industrial temperatures are with resistance temperature detectors (RTD) and thermocouples. The selection is based on the following determined criteria: temperature, time, size, and overall accuracy requirements.

Temperature	:	If process temperatures fall from -328 to 932°F (-200 to 500°C), then an industrial RTD is an option. But for extremely high temperatures, a thermocouple may be the only choice.
Response	:	If the process requires a very fast response to temperature changes--fractions of a second as opposed to seconds (i.e. 2.5 to 10 sec)--then a thermocouple is the best choice.
Size	:	A standard RTD sheath is 0.125 to 0.25 inches in diameter, while sheath diameters for thermocouples can be less than 0.062 inches
Accuracy	:	If the process only requires a tolerance of 2°C or greater, then a thermocouple is appropriate. If the process needs less than 2°C tolerance, then RTD is the only choice. Keep in mind, unlike RTD that can maintain stability for many years, thermocouples can drift within the first few hours of use.

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Last Updated April 28, 2009

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