Thermocouples are used t o detect the temperatures in the process of PWHT. If the ends of the dissimilar wires are in contact with each other and also with a hot and a cold surface, a voltage difference between the two junctions is created. These voltage differences are proportional to the differences in temperature. Therefore a properly calibrated instrument to measure the voltage difference can translate voltages in to temperature readings at the hot junction.
Each combination of wires required a separate calibration and an instrument designated for that combination.
The three commonly used thermocouple combinations are (Data from AWS D10.10)
Each combination of wires required a separate calibration and an instrument designated for that combination.
The three commonly used thermocouple combinations are (Data from AWS D10.10)
The letter designation applies only to the temperature voltage relationship and not to the material. For ease of understanding it is to be noted that the negative side in all cases is of red colour.
Thermocouples are available in various sizes. Faster response is obtained from thinner wires but there are chances that the thinner wires are easily get damaged.
The commonly used thermocouple is having a diameter of 0.81mm (#20AWG).
The thermocouple wires are to be in touch at the point where the heat is to be detected. Balance all portions of the thermocouples shall be protected from radiant heat and also from touching each other. Touching each other can be prevented by the sheath provided, usually by the manufacturer. Radiant energy can be prevented by proper protection by ceramic wool.
Thermocouple extension wires: Usually the job will be farther from the instrument for recording and controlling the temperature. Thermocouple extension wires are used to connect the thermocouple at the job and the instrument. The thermocouple extension wires will be of same composition of thermocouples or designated to match the thermocouple system. For a J type thermocouple the thermocouple extension wire is designated as JX, for E it will be EX and for K it will be KX.
The positive side of the thermocouple to be connected to the positive side of thermocouple extension wire and positive side of thermocouple extension wire to be connected to the positive lead of the temperature controlling instrument.Change in leads will cause error in the detected voltage and there by the recorded temperature.
The thermocouples are usually connected to the job/material for PWHT by capacitor discharge method.
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