Drift of sheathed Chromel/Alumel thermocouples

not registered; not registered

The drift of the output signal from sheathed chromel/alumel thermocouples is believed to result from corrosion of the thermoelectric conductors by diffused air. The extent of the drift depends on the maximum temperature and the period of time that the thermocouple is exposed to the elevated temperature. During use as temperature sensors, it is inevitable that temperature gradients exist along the length of the thermocouple itself. These gradients results in nonuniform thermoelectric properties of the conductors which also affects their output. The drift observed in the sheathed chromel/alumel thermocouple which are used in the JOYO reactor has previously been reported. Additional tests and investigations were conducted to confirm the above phenomenon and to measure their effects. The results of these investigations are summarized as follows: (1)The drift of the thermocouple which is mounted in the reactor outlet pipeline is larger than the drift of the thermocouple which is mounted in the reactor inlet line. (2)Drift of the thermocouple head portion, which is exposed to high temperature, is larger than the drift of the thermocouple end portion which is exposed to room temperature.