Construction and Working of Electrical Resistance Thermometer

Construction and Working of Electrical Resistance Thermometer Points : Construction and Working of Electrical Resistance Thermometer The most commonly used platinum Resistance Thermometer. These resistance thermometer elements are used over a wide range to establish the international temperatures sale. Resistance type temperature bulbs use sensing elements in the form of wires or foil. The films deposited on insulating surfaces are also used for temperature sensing in the wire type the arrangement is commonly a helical coil wound as a double wire to avoid inductive effects. The laboratory type resistance thermometers have the temperature-sensing element wound on a cross mica former and enclosed in a pyrex tube.The tube may be evacuated or filled with inert gas to protect the platinum. The element is normally sealed in glass when used for temperature up to 150°C and ceramic for use in temperature up to 850°C. This sealing provides mechanical strength and protection from chemical attack.

The measurement of change of resistance of thermometer due to temperature change is measured by wheatstone bridge. Ordinary wheatstone bridge is not used for measurement of changes in resistance due to disadvantage of error due to contact resistances and lead resistance error.

Slight modification of the wheatstone bridge, such as a double slide wire bridge, eliminate most of these problems. Some times the ratio meters (ohmmeter) are also used instead of wheatsstone bridge, but suitable for large resistance measurement.