Re: NTC thermistor failures

On 10/07/2025 4:20 pm, Don Y wrote:

I'm looking to purchase some thermistor-based probes to
monitor the performance of the HVAC system (return and
supply duct temperatures along with liquid and suction
lines.  Maybe the temperature of the compressor, etc.)
 As part of my research, I look at the problems people
(consumers) report with the units used in their systems.
 I would have thought NONE of these would fail:  they are
out of the way so not likely to be physically disturbed;
often encased in a protective sheath (e.g., probes);
shouldn't be operating beyond their design constraints
(unless poorly designed); etc.
 Even the pigtails from such devices should be reasonably
safe from disturbance!
 Yet, I see folks replacing them and proud that they
did so without calling in the HVAC contractor!
 Are there other "wear" factors that come into play?
What sort of service life (in a conservative design)
should I expect?

About the only failure mode that I'm aware of is drift - thermistors are sintered metal oxides, so there are gaps in the material and stuff could diffuse in.
The good ones are enclosed in glass beads - Yellow Springs were the first to introduce them - and those drift a lot less than their predecessors. I had my nose rubbed in this back when I was working at Kent Instruments in England in the early 1970's, and I've used them from time to time since then.
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Bill Sloman, Sydney