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 uncertanties or ±0.1 °C or better possible in special desions.
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 RTDsThe 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 lenvironments.
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 straight forward 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 aquisition 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 straight forward, provided one has a good estimate of the thermal properties of the insulator.
Insulator material such as powdered magnesia (MgO), alumina (Al2O3) and similar coumpounds 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.
RTDs Other Than PlatinumRTDs can be made cheaply in Copper and Nickel, but the latter have restricted ranges because of non-linearities and wire oxidation problems in the case of Copper.
Platinum is the preferred material for precision measurement because in its pure form the Temperature Coefficient of Resistance is nearly linear; enough so that temperature measurements with precision of ±0.1 °C can be readily acheived with moderately priced devices. Better resolution is possible, but equipment costs escalate rapidly at smaller error levels. | 
