Resistance Sensors (RTDs)
What are RTDs?
Resistance Temperature Detectors or RTDs for short, are wire wound and thin film devices that measure temperature because of the physical principle of the positive temperature coefficient of electrical resistance of metals.
The hotter they become, the larger or higher the value of their electrical resistance.
They, in the case of Platinum known variously as PRTs and PRT100s, are the most popular RTD type, nearly linear over a wide range of temperatures and some small enough to have response times of a fraction of a second.
They are among the most precise temperature sensors available with resolution and measurement uncertainties of ±0.1 °C or better possible in special designs.
Usually they are provided encapsulated in probes for temperature sensing and measurement with an external indicator, controller or transmitter, or enclosed inside other devices where they measure temperature as a part of the device’s function, such as a temperature controller or precision thermostat.
General Purpose RTD Probes courtesy RTD Company
The Advantages of RTDs
The advantages of RTDs include stable output for long period of time, ease of recalibration and accurate readings over relatively narrow temperature spans. Their disadvantages, compared to the thermocouples, are: smaller overall temperature range, higher initial cost and less rugged in high vibration environments.
They are active devices requiring an electrical current to produce a voltage drop across the sensor that can be then measured by a calibrated read-out device.
RTD Error Sources
The lead wires used to connect the RTD to a readout can contribute to their measurement error, especially when there are long lead lengths involved, as often happens in remote temperature measurement locations.
Those calculations are straightforward and there exist 3-wire and 4-wire designs to help minimize or limit such errors, when needed.
Often the lead error can be minimized through use of a temperature transmitter mounted close to the RTD. Transmitters convert the resistance measurement to an analog current or serial digital signal that can be sent long distances by wire or rf to a data acquisition or control system and/or indicator.
RTDs, as mentioned above, work in a relatively small temperature domain, compared to thermocouples, typically from about -200 °C to a practical maximum of about 650 to 700 °C.
Some makers claim wider ranges and some construction designs are limited to only a small portion of the usual range.
Insulation resistance is always a function of temperature and at relatively high temperature the shunt resistance of the insulator introduces errors into measurement.
Again, error estimates are straightforward, provided one has a good estimate of the thermal properties of the insulator.
Insulator material such as powdered magnesia (MgO), alumina (Al2O3) and similar compounds are carefully dried and sealed when encapsulated in probes along with an RTD element.
ASTM has standards related to insulation resistance testing to help determine the performance of such sealed probes, specifically E 1652-00.