What is an RTD?

In the same year that Seebeck discovered thermoelectricity, Humphrey Davy (1778-1829) claimed that the resistivity of metals showed a marked dependence on temperature. Fifty years later, William Siemens used platinum in a resistance thermometer. The propitiousness of this choice has been proved by the use of platinum as the main element in high precision resistance thermometers. In fact, the Platinum Resistance Temperature Detector or PRTD is used today from the triple point of hydrogen (-259.34 °C) to the freezing point of silver (961.78°C). Platinum is particularly suitable as it can withstand a wide range of temperatures while remaining highly stable and suffering almost no deterioration.

In 1932, C.H. Meyers proposed building a resistance temperature detector (RTD) comprising a platinum wire wound in a helix round a cross-shaped mica support, all encased in a glass tube. This system minimises the tension on the wire while maximising the resistance. Although this construction produces a highly stable element, the quality of the thermal contact between the platinum and the point to be measured is poor and consequently the thermal response time is slow. The fragility of the structure restricts its use today to that of a laboratory instrument.

A different laboratory instrument has replaced Meyer’s design. This is the “birdcage structure proposed by Evans and Burns. The constraints on resistance caused by the time and temperature are thus minimised and the “birdcage” became the norm in the laboratory. Its fragile structure and sensitivity to vibrations mean that it is not suitable for industrial environments.

More robust construction techniques are shown in the figures opposite: a bifilar platinum wire is wound round a glass or ceramic coil. The bifilar winding reduces the magnetic induction and associated noise. Once the wire has been wound round the coil, the structure is sealed with molten glass. Unless the coefficients of expansion of the platinum and the coil are perfectly matched, expansion of the wire will result in a change in resistance with the consequence of a possible permanent change in the resistance of the wire.

There are RTD versions which offer a compromise between the birdcage and the sealed spiral. This approach uses a helicoidal platinum spiral wound round a ceramic cylinder and held in place by sintered glass.

Modern manufacturing techniques use a platinum or metal film on a flat, etched ceramic substrate, guided by laser and sealed. The film RTD offers a significant reduction in assembly time and has the advantage of increased resistance for a given size. The manufacturing technology means that the devices are small with low thermal inertia; they can respond rapidly to changes in temperature.

Film RTDs are slightly less stable than wire RTDs but the benefits in terms of size, production cost and ease of use mean that they are very widely used.