Working of Thermo-electric Pyrometer

Working of Thermo-electric Pyrometer Points : Working of Thermo-electric Pyrometer If two wire, of different metals are joined together at each end so as to form a complete electrical circuit, it is found that a current flows in the circuit and which is a function of the temperature of the “hot junction” if the other — the “cold junction” maintained at the constant temperature.

If the two ends of the wires at cold end are joined with the indicating instrument, the thermo-electric effect is the same, provided the ends are maintained at the same b4cold temperature.

In the thermo-electric pyrometers the emf set up is either measured by a potentiometer or is allowed to send a current through a galvanometer connected being proportional to the thermo-electric einf. if the resistance of the circuit is constant. Since the thermo-electric emf is really dependent upon the difference of temperature of the two junctions, it is necessary to keep the temperature of the “cold” junction constant if the temperature of the “hot” junction is to be accurately measured.

The complete circuit of the thermo-electric pyrometer. If the high-resistance galvanometer is used, the ballast resistance may be unnecessary. This method is most suitable when the temperature to he measured is very high. The absolute constancy of the temperature of the cold junction is not then of such importance as when the temperature to be measured is low. For measurements of lower temperatures the cold junction maybe contained within a vacuum flask, and maintained at a known low temperature.

It is customary to use copper leads to the indicating instrument, and if neither of the thermocouple wires is copper both of the junction between the thermocouple wires and the copper leads must be in the vacuum flask. Crushed ice may be used in the cold junction flask. Alternatively, the reference junction may be maintained at a steady, through higher, temperature in a small electrically-heated chamber.