Resistance Sensors (RTDs)
RTD Construction
Until about 25 years ago there were only two basic types of Platinum RTD constructions: Wire-wound and Special Construction Wire-wound with careful design to minimize thermally induced stresses on the platinum wire. Both type were fabricated from thin platinum wire wound carefully on electrical insulating bobbins of ceramic.
The former models were the commercial varieties and were generally useful up to about 650 °C without encountering any serious errors. Above that there were errors that would be caused by the stresses induced into the wires due to the differences in thermal expansion between the wire and the bobbin material plus some changes in electrical resistance.
Typical sensor sizes were a few 10s of millimeters.
The latter models were even more carefully designed and fabricated and were often quite fragile and had elongated sensing regions measure in inches. Despite these limitations, they were often free from thermally induced errors and could be used to temperature approaching 1000 °C. They were often designs similar to or actually used as Standard Platinum Resistance Thermometers (SPRTs).
About 25 or 30 years ago, thin-film platinum RTDs began to be fabricated using techniques similar to integrated Circuit manufacturing. The results were a wide range of new, lower cost RTDs, with smaller thermal mass, thus quicker time response to temperature change.
They provide significant improvements over IC Thermometers and thermistors by having wider temperature ranges and smaller measurement uncertainties at a competitive prices. However, they cannot not provide reliable measurements much above 400 or 500 °C.
RTDs Other Than Platinum
RTDs can be made cheaply in Copper and Nickel, but the latter have restricted ranges because of non-linearity 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.
| Featured Link Burns Engineering, Inc. (USA), a global supplier of high performance temperature measurement solutions. |
Didn’t Find What you Wanted Here?
Try A Check for Temperature Sensor Suppliers on GlobalSpec
Popularity: 12% [?]
