An interesting failure mode ...

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Does your pool have a heater? Do you think the expansion of the base metal, with a plastic shell that is molded around it play into the failure rate. I know my pool lives at 60 degrees right now, but I do take the spa up to 100 degrees about once a week. I'm not sure what the expansion rate of the metal from the probe, in relation to the expansion rate of the plastic is, but I think that might play into it.
 
I do have a heater and an attached spa but we don't really use it that often (maybe just a few times in the early season. As the pictures show above, the outer plastic case is relatively thin compared to the black potting compound plug that sits in it. Perhaps there's enough difference in the epoxy and plastic thermal expansion to cause stress cracking over time ... Either way, the thermistor still seems to be working so at the end of the day it was the rat to blame!
 
Maybe the moral of the story here is to just buy a bunch of Pentair temperature sensors (they're like $15 a pop) and just replace them regularly (like every 5 years or so). It wasn't more than a 30min job and it gave me reason to pull my SWG cell and clean that out as well (it was plugged up with a bit of calcium scale).
 
JamesW said:
It doesn't look like the iron came from the inside, but it's hard to tell for sure.
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I have some natural stone work submerged in the pool and Tucson water does have trace amounts of iron in it. My pool ran for years on raw municipal water so over time the plastic has developed some stains. I don't consider it to be any worse that any other plastic part that has gotten some stains on it. My water has never turned tea brown or shown signs of iron issues so it's probably nothing more than years of trace build up. Even my pump strainer baskets yellow with age.
 
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JamesW said:
All pools have some iron.

It's basically impossible to have zero iron.
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All terrestrial (and Martian) water sources will have iron, calcium, magnesium and silicates in them. Water is one of the most powerful solvents in the universe....just ask the Grand Canyon.
 
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