Attaching temp sensor in PVC pipe.

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strategy400

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Jun 6, 2013
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Southern California
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I just bought one of these because I don't like the idea of a metal sensor that is connected to a 110v module touching my pool water. The only thing is it's not really made for a pressurized application. I'll try it and report back.
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strategy400 said:
I just bought one of these because I don't like the idea of a metal sensor that is connected to a 110v module touching my pool water. The only thing is it's not really made for a pressurized application. I'll try it and report back.
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I'm not sure which sensor you're referring to, but the one I used (DS18B20) takes a 3v-5v input and is connected to a module that runs off of a 5v USB power source. So yes, the USB power source runs off 110v, but there's a lot of stuff in between that and the metal sensor that touches the water.
 
I think we need an isolation power supply. I need to isolate my 12VDC temperature sensor circuity from its 120VAC power supple (wall wart). I'm going to replace the one that came with it (because it doesn't have isolation protection) with this one:

You'd need to find one for the voltage your setup requires.

Edit: That part is NOT pool rated, so not the right thing for my or your application. The search continues.
 
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howardtopher said:
I'm not sure which sensor you're referring to, but the one I used (DS18B20) takes a 3v-5v input and is connected to a module that runs off of a 5v USB power source. So yes, the USB power source runs off 110v, but there's a lot of stuff in between that and the metal sensor that touches the water.
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I'm using a sonoff module that run directly off 110v. I know the sensor "shouldn't" receive that voltage, but I like to put safety first. I've seen some of those modules literally melt down and who knows where voltage/amperage is going then. Even the 110v is from the load side of a GFCI, but I strive for layers of safety. My concern is that the thermowell won't stand up to pressure because it is designed to be inserted in a spa wall. We'll see.
 
Dirk said:
I think we need an isolation power supply. I need to isolate my 12VDC temperature sensor circuity from its 120VAC power supple (wall wart). I'm going to replace the one that came with it (because it doesn't have isolation protection) with this one:

You'd need to find one for the voltage your setup requires.
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Replying to my own post! That is NOT the correct solution. I'll post when I find one.
 
Find a place to install a reducing tee with 1/2" threads and install one of these:

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Then put the 10K thermister inside the compression top.

I hope this helps.

Chris
 
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Plastic thermowell came today. Every part of it seems robust. I suspect it will hold up. My plan is to install it in a cap on a tee. The ds18b20 fits in it with a little wiggle room. We'll see.20240615_093152.jpg
 
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I think what you have might work, but this would be a better choice, because it's flat on both sides. You could even sand off the raised printing. They have them at Home Depot, and probably most other hardware stores.
www.homedepot.com

NDS PVC S&D Cap, 3 in. 3P06 - The Home Depot

Seal the end of 3 in. Sewer and Drain (S&D) pipe with the 3 in. PVC S&D Cap. This cap is used to stop the flow of water at the end of the pipeline. The cap is made of heavy-duty polyvinyl chloride (PVC)
www.homedepot.com
Screen Shot 2024-06-17 at 9.04.23 PM.png
You've hit upon the challenge of the curved cap you found, because the hole should be dead center to achieve the most even seal. With the flat cap, the location of the hole would not matter.

EDIT: My bad. The link is not only to a 3" cap, but it's not rated for a pressure application (it's for non-pressurized drainage systems only). Got ahead of myself there. The trick would be to find this shape in Schedule 40 that is rated for pressure. And I'm not sure that's a thing. Not at a Home Depot, anyway.
 
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Well, it does exist, if you can get it shipped to you and have it show up matching the picture.
www.247garden.com

1-1/2 in. Schedule 40 PVC Cap/Socket End Plug Pipe Fitting NSF SCH40 ASTM D2466 1.5"

247Garden SCH40 PVC 1-1/2" End Cap/End Plug NSF Schedule-40 Pipe Fitting
www.247garden.com www.247garden.com
25cap_2.jpg

Alternative: there are threaded end caps that might also work. This would be useful to later get at the sensor without having to cut out a glued-on cap.
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If their on-line spec's are to be trusted, both of these are rated for 150 PSI.

The nice thing about the threads is you could later replace a faulty sensor, or change it altogether to some other brand/model that wouldn't have to be of the same diameter. You'd just replace the threaded cap and drill whatever hole you needed. You really should have a way to get at the sensor's nut, which is going to be on the inside of your plumbing.
 
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why not use a liquid-tight cord grip? super easy. 1/2" or 1/4 npt. pick your size. $4 at hardware store.
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Why not just use a thermowell with a 10K thermister with a 1/2" tee you install anywhere convenient? No complicated leak source to deal with. Even SS thermowells are cheap. @Katodude showed me this at his pool... temp diff across the well wall is insignificant and you can trim that out with calibration.

Chris
 
setsailsoon said:
Why not just use a thermowell with a 10K thermister with a 1/2" tee you install anywhere convenient? No complicated leak source to deal with. Even SS thermowells are cheap. @Katodude showed me this at his pool... temp diff across the well wall is insignificant and you can trim that out with calibration.

Chris
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I think he's worried about the metal to water contact. Whether that's a real issue or not, he was specifically looking for a non-conductive solution.
 
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Dirk said:
I think he's worried about the metal to water contact. Whether that's a real issue or not, he was specifically looking for a non-conductive solution.
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Thanks, I didn't have time to review the total thread. I wouldn't worry about a high quality SS thermowell. I've seen them used for industrial boiler feed water making very high temp super-heated 1500 psi steam. Contaminants are controlled to virtually zero with anionic and cationic beds that are regenerated with strong acid and caustic. If you get any contamination it can easily cause an emergency shutdown that almost always costs $1MM to fix. But everybody has to make their own choices and I get that. Plastic is not very conductive so "calibrating out" the temp difference could be an issue.

Chris
 
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