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I need to insert a temperature probe into my pool piping. I am on the pressure side of the filter, so around 10-20 psi typically. My initial idea was to use a screw in plug in the existing 1/2" NPT side inlet tee, drill a hole and epoxy the probe in place. Assuming that worked, it would make it difficult to replace or inspect the probe. Also, the probe is 5mm diameter teflon, so gluing is challenging, requiring specialty adhesives that have no data about full-time submersion or resistance to swimming pool chemistry.

I know sticking probes into pipes is a pretty common requirement, so there must be existing solutions, but I don't know the name of the correct fitting. The closest thing I can find is a cable gland with NPT threads, and while I can find them rated IP68 (dustproof and waterproof to 3+ m) and 10 bar (145psi), I don't know if this is their intended use, i.e. permanent submersion and preventing water intrusion from what would normally be the "dry" side (the inside of an electrical enclosure). I'm fairly certain this would probably work, but I'd rather have a bit more confidence, and, if possible, use the correct fitting for the job.

My electrics are all in a NEMA rated outdoor enclosure with proper waterproof glands located several feet above the temperature probe, so I'm not concerned from that aspect. I just don't want a slow leak that gradually drains the pool.

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  • I usually purchase a T and a threaded plug, since you have a metric thread you might be able to tap a plug to the correct size. – Ed Beal May 26 at 14:07
  • @EdBeal I may not have been clear. My probe is a smooth sheath 5mm in diameter, no threads. The side inlet tee has a 1/2" NPT threaded socket on in. – Colin Young May 27 at 18:08
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The usual way is to use a T with the middle outlet being vertical:

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With a proper fitting, you screw this hollow adapter in. Ecnerwal says in a comment it is called a "thermowell".

enter image description here

Fill this with mineral oil and dip your temperature probe in.

The oil transmits temperature, but the probe is not submerged in water, nor under pressure, so it can be inspected and changed.

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    "Thermowell" is the usual English-language term. Puts all the leak sealing on the fitting, probe insertion/removal/replacement does not require shutting the system down. For swimming pool use be careful of getting something that won't be destroyed by the pool water chemistry (i.e. you might need to find titanium, rather than brass, if I recall correctly - or 316L stainless.) A thermal grease (rather than oil) can be used if the fitting is not vertical. – Ecnerwal May 26 at 14:22
  • "thermowell" that makes sense, thanks. – bobflux May 26 at 21:47
  • Excellent! Thermowell has resulted in many, many search results. I think titanium is the right metal. I know many of the stainless steels do not do well in pool environments, although I'll need to research the 316L (that and 304L are IIRC generally used for saltwater) – Colin Young May 27 at 18:11
  • It appears as though 304, 316 and the L versions of each are both good for submerged applications in swimming pools. – Colin Young May 27 at 18:24
  • My swimming pool filter casing is stainless, and it's spotless, so there's probably one stainless variant that fits the bill... – bobflux May 27 at 21:55

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