Resistance Thermometers

About resistance thermometers

Resistance thermometers, which are alternatively known as resistance-temperature devices (or RTDs), rely on the principle that the resistance of a metal varies with temperature according to the relationship
 

R = R₀(1+a₁T+a₂T²+a₃T³+ . . . +a_nTⁿ)

This equation is non-linear and so is inconvenient for measurement purposes. The equation becomes linear if all the terms in a₂T² and higher powers of T are negligible. This is approximately true for some metals over a limited temperature range and, in such cases, the resistance and temperature are related according to:

R ≈ R₀(1+a₁T)

Platinum is one such metal where the resistance-temperature relationship is linear within ±0.4% over the temperature range between -200°C and +40°C. Even at +1000°C, the quoted accuracy is ±1.2%.

Its good linearity and chemical inertness make platinum the first choice for resistance thermometers in many applications. Platinum is very expensive, however, and consequently the cheaper but less accurate alternatives of nickel and copper are sometimes used. These two metals are very susceptible to oxidation and corrosion and so the range of applications where they can be used is strictly limited even if their reduced accuracy is acceptable. Another metal, tungsten, is also used in some circumstances, particularly for high-temperature measurements. The working ranges of each of these four types of resistance thermometer are as shown below:

In the case of non-corrosive and non-conducting environments, resistance thermometers are used without protection. In all other applications, they are protected inside a sheath. As in the case of thermocouples, such protection affects the speed of response of the system to rapid changes in temperature. A typical time constant for a sheathed platinum resistance thermometer is 0.4 seconds.

The resistance thermometer exists physically as a coil of resistance wire of the chosen metal. Different instruments available have resistance elements ranging from 10Ω right up to 25Ω. The devices with high resistance have several operational advantages. The first of these is that any connection resistances within the circuit become negligible in their effect. The second advantage is that the relatively high voltage output produced by these instruments makes any induced e. m. f. s produced by thermoelectric behavior at the junction with connection leads negligible in magnitude.

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