Corrosion resitant K Type Thermocouple  - Page 1
Forum section: electronics / projects

author: PeterFW
author_rank: Frequent Contributor
timestamp: March 22, 2016, 12:01:23 am
content: Hello!Ages ago i bought a chinese PID controller and SSR that i finally want to put to good use. Back when i bought it, it was intendet for a sous vide cooker.My interest has shifted and now id like to head my PCB etchant bath and other chemicals that are quite corrosive with it.It needs to be corrosion proof and water tight, but i can not find something readily made that is not too expensive.I thought about a sand filled glas tube with the element in it, a hollow PTFE bar or (if i can get hold of one) a Titanium tube.What are your thoughts?Greetings,Peter

author: Marco
author_rank: Super Contributor
timestamp: March 22, 2016, 02:32:15 am
content: can't you just brush on some two component epoxy?

author: qwaarjet
author_rank: Regular Contributor
timestamp: March 22, 2016, 03:38:23 am
content: I've used stainless tube that has the end caped to make custom rtd probes just slide the probe in and cap with epoxy to hold it in place. Then i can be swagged locked into just about anything. though I've done the dip in epoxy trick for less robust or sacrificial setups though.

author: Ian.M
author_rank: Super Contributor
timestamp: March 22, 2016, 05:30:00 am
content: Short of fusing a glass bead encapsulated thermocouple into the end of a glass tube, I don't think you are going to get corrosion resistance on the cheap.

author: ywara
author_rank: Contributor
timestamp: March 22, 2016, 10:55:24 am
content: Cheapest (<$20) option is probably to get some PTFE rod and run a drill most of the way through. This will have a fairly slow response time, but should prevent any corrosion due to immersion.But you really want a 316 or 321 stainless thermowell plus head. You can get this from Omega including PFA coating for less than $300.

author: m98
author_rank: Frequent Contributor
timestamp: March 22, 2016, 12:32:24 pm
content: I'd use some PTFE shrink tubing with glue on the inside and a small stainless steel bead to seal the open end

author: Kleinstein
author_rank: Super Contributor
timestamp: March 22, 2016, 01:06:26 pm
content: The usual chemicals to etch PCBs (Cu) will also attack stainless steel or titanium - essentially any metal except maybe gold and similar. So it's more like using plastics or glass. So a plastics tube with a glass bead at the bottom could work.Most plastics are resistant to acids - so no need for PTFE. One could use simple PE and just close the end by melting it.

author: PeterFW
author_rank: Frequent Contributor
timestamp: March 22, 2016, 10:01:25 pm
content: Hello and thaks for your replys!Stainless steel is indeed not suitable for what i am dooing, last time i looked it up i thought titanium should have been able to do.The idea with the Epoxy coating was interesting, i will look into that, thank you!The main reason i thought about PTFE was, because it will hold up to higher temperatures.HDPE would do as well but gets soft too quickly, i think.I might want to try some stuff that might get heated to >100°C at some point.

author: Marco
author_rank: Super Contributor
timestamp: March 22, 2016, 10:27:24 pm
content: There's an alumina protection tube on Aliexpress, bit expensive at 38 Euro but I imagine the Titanium tube isn't cheap either.If the thermo couple doesn't have to be put into the middle of the tub couldn't you just (JB-)weld a cheap square bit of alumina into the side of the tub and glue the thermocouple to it and cover it with insulation?

author: PeterFW
author_rank: Frequent Contributor
timestamp: March 24, 2016, 12:50:40 am
content: Quote from: Marco on March 22, 2016, 10:27:24 pmI thought about that but the thermal transfer would be bad, i am using a PE tank.But... the more i think about it, it could work.Everything i can put the thermocouple into would insulate it as well, i think i will just try that. I will clamp a small piece of aluminum on the side and screw the thermocouple into it.Great idea, that might just work, thanks!

author: Marco
author_rank: Super Contributor
timestamp: March 24, 2016, 03:03:16 am
content: Quote from: PeterFW on March 24, 2016, 12:50:40 amI meant opening a small hole in the PE tank and gluingan alumina MOSFET insulatorinto the hole. You'd have multiple mm of epoxy covering the gaps, unlikely for anything to get through that in a hurry, you could even heat up the alumina and press it into the PE first before applying the epoxy.This would give you decent thermal resistance to the liquid.

author: AF6LJ
author_rank: Unknown
timestamp: March 24, 2016, 03:19:41 am
content: The epoxy sounds most sensible, just don't allow it to bend.Use a thick coat everywhere except for the junction, use a thin coat there to limit the thermal hysteresis. The only problem you might have is some thermacouple wire are insulated with Teflon.

author: XOIIO
author_rank: Super Contributor
timestamp: March 24, 2016, 04:42:54 am
content: Another vote for epoxy, I used heat shrink and hot snot to make sure the leads on my temp sensor thingy weren't exposed to water or whatever else you stuck it in but it won't be tolerant to stuff at boiling temperatures very long. A nice two part resin would work good, also acting as strain relief.I'd probably make a little tube out of tape or paper to "mold" it into shape, and you should be able to get it to be pretty thin, reducing it's thermal mass and increasing it's response time.

author: PeterFW
author_rank: Frequent Contributor
timestamp: March 24, 2016, 04:59:00 pm
content: Quote from: Marco onYesterdayat 02:03:16 PM>Quote from: PeterFW onYesterdayat 11:50:40 AMI meant opening a small hole in the PE tank and gluingan alumina MOSFET insulatorinto the hole.Sorry, i knew what you ment! Those things are a bit too much on the expensive side though, so i "modified" your idea.I really do not want to tank to have anny chance to leak, when there is heated sulphuric acid in there.Quote from: AF6LJ onYesterdayat 02:19:41 PMQuote from: XOIIO onYesterdayat 03:42:54 PMGuys, i remembered something... let me check. Yes, it is still there, a tin of PU resin:Still a liquid, i bought this years ago to seal up a lamp, only used 1/10th of it, this might just work, it must be out of date by now but i will mix a small batch and see what happens.