Moisture Induced Thermistor Failure

Of all the reasons why a thermistor might fail, moisture is amongst the most common. Moisture induced failure is typically tricky to pinpoint in an application. This problem can occur in any epoxy-coated components or in probe configuration that utilizes epoxy-coated elements- except in probes that use hermetic seals. Moisture induced failure is harmful because:

Shifts that happen as a result of moisture will always shift downward in resistance. Changes will continue as electrode materials continue to plate ceramic units. This occurs until the part completely short-circuits. At times, movements and physical shock to parts can break conductive paths, so it is possible for shorted elements to appear recovered. Being aware of symptoms of moisture failure is necessary for users to be able to troubleshoot failures.

There are design tricks that are used to minimize the occurrence of moisture failure when using epoxy-coated parts, but these tricks are not the best solution. Switching to glass-encapsulated thermistors is the best solution for dealing with moisture induced failure. We highly recommend that any underwater probe be glass-coated. The initial cost of these probes is more expensive than other thermistors, but that is a small tradeoff when compared to the long-term benefits of knowing there will be no moisture failure.


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