|Diagram of resistance thermometers.|
Explain resistance thermometers:
The fact that the electrical resistance of the metals increases with temperature is made use of in resistance thermometers which are purely electrical in nature. A resistance thermometer is used for precision measurements below 150°C.
A simple resistance thermometer consists of a resistance element or bulb, electrical loads and a resistance measuring or recording instrument. The resistance element (temperature sensitive element) is usually supplied by the manufacturers with its protecting tube and is ready for electrical connections.
The resistance of the metal used as resistance element should be reproducible at any given temperature. The resistance is reproducible if the composition or physical properties of the metal do not change with temperature. For this purpose platinum is preferred.
A platinum resistance thermometer can measure temperatures to within ± 0.01°C. However, because of high cost of platinum, nickel and copper are used as resistance elements for industrial purposes for low temperatures.
The fine resistance wire is wound in a spiral form on a mica frame. The delicate coil is then enclosed in a porcelain or quartz tube. The change of resistance of this unit can be measured by instruments such as Wheatstone bridge, potentiometer or galvanometer.
- The resistance thermometers possess the following advantages over other devices :
- A resistance thermometer is very accurate for low ranges below 150°C.
- It requires no reference junction like thermocouples and as such is more effective at room temperature.
- The distance between the resistance element and the recording element can be made much larger than is possible with pressure thermometers.
- It resists corrosion and is physically stable.
- The resistance thermometers cost more.
- They suffer from time lag.