Electrical Engineering ⇒ Topic : Resistance Thermometers

Resistance Thermometers

It depends on the basic property that the resistance of most materials varies with the temperature.The property of variation of resistance with temperature is induced by temperature coefficient of resistance α and as a sensing element, α should be as high as possible so that the resistance change is sufficient even for small temperature changes. Another desirable quantity is that the variation should be linear, i.e., ΔR/ΔT = m (slope) is linear. The metals satisfying the above requirements are iron, nickel, copper, silver, and platinum. Of these, platinum has the best linearity, copper and silver come next, iron and nickel have the least linearity and can be used between certain temperature  only. Resistance thermometers operate in the region 250 to 1000°C.

The fundamental equation governing a resistance thermometer is:

                                R_t = R_ref ( 1 + α ΔT)                                               .............. (1)

where R_t is the resistance at temperature t°C, R_ref is the reference temperature resistance, normally 0°C, α is the temperature coefficient of resistance, and ΔT is the difference in temperature between operating and reference temperatures.

Resistance element is usually enclosed in a tube or well and immersed in the medium whose temperature is to be measured. The element forms one arm of the bridge circuit and the null galvanometer is calibrated in terms of temperature to read the resistance change directly. Most often a bridge amplifier is used.