In principle, any material could be used to measure temperature if its electrical
resistance changes in a significant and repeatable manner when the
surrounding temperature changes. In practice, however, only certain metals
and semiconductors are used in process control for temperature measurement.
This general type of instrument is called a resistance temperature
detector or RTD. RTDs are the second most widely used temperature measurement
device because of their inherent simplicity, accuracy, and stability.

History of the RTD

In 1821, the same year in which Seebeck made his discovery about thermo-
electricity, Sir Humphrey Davy announced that the resistivity of metals
showed a marked dependence on temperature. Fifty years later, Sir William
Siemens recommended platinum as the element to be used in a resistance
thermometer. His choice proved most correct, since platinum is used
to this day as the primary element in all high-accuracy resistance thermometers.
In fact, the platinum resistance temperature detector, or PRTD,
is used today as an interpolation standard from the oxygen point
(-182.96°C) to the antimony point (630.74°C). Platinum is especially suited
to this purpose because it can withstand high temperatures while maintaining
excellent stability and good linearity.

C. Meyers proposed the classical RTD construction using platinum in
1932. He wound a helical coil of platinum on a crossed mica web and
mounted the assembly inside a glass tube. This construction minimized
strain on the wire while maximizing resistance. Although this construction
produced a very stable element, the thermal contact between the platinum
and the measured point was quite poor. This resulted in a slow
thermal response time. The fragility of the structure limits its use today
primarily to that of a laboratory standard.

In a more rugged construction technique, the platinum wire is wound on a
glass or ceramic bobbin, as illustrated in Figure 7-14. The winding reduces
the effective enclosed area of the coil, which minimizes a magnetic pickup
and its related noise. Once the wire is wound onto the bobbin, the assembly
is then sealed with a coating of molten glass. The sealing process
ensures that the RTD will maintain its integrity under extreme vibration,
but it also limits the expansion of the platinum metal at high temperatures.
Unless the coefficients of expansion of the platinum and the bobbin
match perfectly, stress will be placed on the wire as the temperature
changes. This will result in a strain-induced resistance change, which may
cause permanent change in the resistance capacity of the wire.