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 or ±0.1 °C or better possible in special decisions. 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.
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 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 SourcesThe 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 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 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 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.
|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 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.
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 achieved with moderately priced devices. Better resolution is possible, but equipment costs escalate rapidly at smaller error levels.
All RTDs used in precise temperature measurements are made of Platinum and they are sometimes called PRTs to distinguish them.
Standard Platinum RTDs(SPRTs)The ITS-90 (International Temperature Scale of 1990- used as a worldwide practical temperature scale in national metrology labs like NIST, NPL et al) is made up of a number of fixed reference points with various interpolating devices used to define the scale between points. A special set of PRTs, called SPRTs, are used to perform the interpolation in such labs over the ranges 13.8033 K (Triple point of Equilibrium Hydrogen) to the Freezing point of Silver, 971.78 °C. The Hart Scientific website provides a glimpse into the realm of precision SPRTs and readout equipment used in calibration labs. They operate one of the very few labs in the USA with accreditation under NVLAP to the ISO/IEC 17025 standard.
Platinum RTD Output EquationASTM Standards E 1137 for Industrial Platinum Resistance Thermometers specifies that the resistance-temperature relationship for such devices for the range 0 °C to 650°C, to within the tolerances given below, will be described by the equation:
R(t) = R(0)[1 + At +Bt^2]
Where: t = temperature (to ITS-90), °C, R(t) = resistance at temperature t, R(0) = resistance at 0°C A = 3.9083 * 10^-3(°C), and, B = -5.775 * 10^-7(°C^2).More details and the equation for -200 °C to 0°C as well as the inverse, temperature as a function of resistance are provided in the standard. The standard is a copyright product of the ASTM and may be purchased at their website, www.astm.org. In Europe, the former German DIN standard, DIN 43760, had been the major, recognised source for RTD properties. Not withstanding this fact, the British have long had standard BS 1904:1964. Both recognize the 0.003850 temperature coefficient for platinum RTDs. Now the IEC administers the “DIN” standard as IEC Standard 60751. The Callendar-Van Dusen equation and others are used to correct for the non-linearity of the resistance-temperature relationship for very high accuracy measurements, such as those performed in a metrology or calibration laboratory