Thermocouple Protection

About the thermocouple protection

Thermocouples are delicate instruments which must be treated carefully if their specified operating characteristics are to be reproduced. One major source of error is induced strain in the hot junction which reduces the e. m. f. output, and precautions are normally taken to minimize this by mounting the thermocouple horizontally rather than vertically. In some operating environments, no further protection is necessary to obtain satisfactory performance from the instrument. However, thermocouples are prone to contamination by various metals and protection is often necessary to minimize this. Such contamination alters the thermoelectric behavior of the device, such that its characteristic varies from that published in standard tables. It also becomes brittle and its life is therefore shortened.

Protection takes the form of enclosing the thermocouple in a sheath. Whilst the thermocouple is an instrument with a naturally first-order type of step response characteristic, the time constant is usually so small as to be negligible when the thermocouple is used unprotected. When enclosed in a sheath, however, the time constant of the combination of thermocouple and sheath is significant. The size of the thermocouple and hence the diameter required for the sheath has a large effect on the importance of this. The time constant of a thermocouple in a 1 mm diameter sheath is only 0.15 s and this has little practical effect in most measurement situations, whereas a larger sheath of 6 mm diameter gives a time constant of 3.9 s which cannot be ignored so easily.

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