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Near Miss from Failing Thermocouple

The Near Miss

A Chemist was heating a pressure reaction when he suddenly observed a large temperature jump to above 200°C.  Fortunately the pressure vessel held at the higher than intended temperature and no damage occurred.

The Investigation

It was found that the thermocouple leads were intermittently shorting out in the thermocouple plug causing an erroneous temperature measurement.  The short circuit was due to bare thermocouple wires which had become twisted together to make an additional thermocouple junction.  The wires were bare because they had been left like that at manufacture or because the tension in the lead had pulled on the insulation.  No cord grip was fitted to the plug.

The Actions

This plug and similar ones have now been fitted with cord grips. This should prevent tension in the lead pulling the insulation away from the terminals and it should also stop leads from twisting within the plug.

Comments

These types of thermocouple wires do have a tendency to self strip with tension, besides the obvious prevention of this act, one must also decide if the loss of temperature control would be very critical.  This would depend on the heating medium employed and the possible maximum temperature.  In such a case it might be decided that this was still an unacceptable risk, a better choice might be to change to Pt100 sensors, which will indicate a short (or open) circuit as an error.  Such a change however, is not a straightforward swap, but would need new electronics to drive the sensor.

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