Operation of Thermocouple Instruments

Operation of Thermocouple Instruments Points : Operation of Thermocouple Instruments, Material, Applications In these instruments use of seebeck effect is made according to that, if the function of two wires of dissimilar metals such as iron and a copper-nickel alloy is heated and the free or the cold ends are connected to a mull-voltmeter, an emf is caused. The emf generated at the junction is nearly proportional to the temperature. This junction is called a thermo-couple and the resulting emf is called thermoelectric emf. This is present at the output terminals of the device, which is measured with the help of PMMC instrument. The scale of this instrument can be calibrated to read the current through the heater. Thermo-couple instrument has a square-law response i.e. the angular deflection of the instrument is a measure of 12 in the heater and the rms value of current or voltage is indicated by the scale reading. As the heater has either positive or negative temperature coefficient, so the reading is affected. Fig.No.3.4. shows the circuit of thermo-couple instrument.

Material The thermocouple is the junction of two dissimilar metals such as iron and the copper-nickel alloy. The heater material is a very fine wire of non-magnetic material having high resistively. Applications The thermocouple instruments are used for measurement of current from power frequencies up to the 100MHz, the upper limits are determined by the skin effect and stray capacitance errors and depends on whether the instrument is an ammeter or a voltmeter, also its current rating. Because these instruments can be used for high frequencies, so these are called RF instruments. Thermocouples are also employed in pyrometers used for measurement of high temperature.