what works well for a waterproof seal ? - Page 1
Forum section: electronics / projects

author: JBeale
author_rank: Frequent Contributor
timestamp: March 23, 2016, 11:45:33 pm
content: For an outdoor sensor installation, I want to make a waterproof joint between a 316 stainless tube and a direct-burial type CAT5 cable. There will be some high-precision 10k impedance circuit nodes I want to keep water away from, and it will be in a damp environment. I want to keep water out as long as possible, hopefully indefinitely. I was thinking about some sort of epoxy or even hot-melt glue but what I have read is that almost every kind of adhesive material is water-permeable to some degree. Any recommendations?The tube is something like this:https://www.brewershardware.com/4-Stainless-Steel-Temperature-Probe-End-PE4-316.html

author: crazyjim30
author_rank: Contributor
timestamp: March 23, 2016, 11:49:26 pm
content: Run the wiring in liquid - tight conduit. There should be various connectors that make the joint between cables liquid - tight as well.Sent from my SM-G900V using Tapatalk

author: JBeale
author_rank: Frequent Contributor
timestamp: March 24, 2016, 12:16:29 am
content: Thanks for the reply. The direct-burial type CAT5e is supposed to be waterproof already. Using conduit might not hurt, but I still need some way to join it to the 1/4" OD SS tube. Not sure if there is an available conduit connector for that. If there is, I suppose it would rely on a silicone or rubber gasket under compression for the seal. Is that more long-term waterproof than any kind of thermoset adhesive solution, for example?

author: CatalinaWOW
author_rank: Super Contributor
timestamp: March 24, 2016, 12:24:07 am
content: It all depends on how dry you need it and for how long. It is virtually impossible to seal something forever. Even a glass envelope has a leak rate that can be a problem in extremely long life (many decades) equipment. Commercial telephone equipment here in the states that needs to be dry often is continually fed with dry nitrogen. You will see the bottles at random locations next to poles or junction boxes.I would recommend gaskets of a rubbery plastic class material suited for your other environmental requirements (resistance to oils, acids,fuels or whatever). Things like butyl rubber or nitrile rubber. Many of the adhesives give off water or even worse effluents as they cure which end up trapped on the wrong side of your seal so generally are avoided when trying to maintain a dry environment.You can also include some moisture grabbing material like silica gel in your cavity to postpone the day of reckoning, although getting the most out of these materials is a major project by itself.

author: Monkeh
author_rank: Super Contributor
timestamp: March 24, 2016, 12:34:05 am
content: Silicone sealant. Fill tube, insert sensor and wire, remove excess silicone pressed out.

author: JBeale
author_rank: Frequent Contributor
timestamp: March 24, 2016, 12:36:31 am
content: Thanks for the info, it confirms what I thought (that it is a hard problem.) Didn't realize even solid glass has a leak rate. No chemical resistance requirements, other than rainwater. I was hoping for years worth of no-maintenance service life, but that may not be possible.I have read about concerns with RTV silicone curing releasing acetic acid which would cause trouble to electronics.

author: Monkeh
author_rank: Super Contributor
timestamp: March 24, 2016, 12:45:55 am
content: Quote from: JBeale on March 24, 2016, 12:36:31 amNeutral cure silicone is easily available.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: March 24, 2016, 12:49:08 am
content: Quote from: JBeale on March 24, 2016, 12:36:31 amMany people are successful, but many types of RTV do release acetic acid which then can cause problems. Your mileage may vary.One approach which can work if gasket part is hard. Drill and tap a hole in the side of your tube. Use RTV or other sealant to seal your wires (make sure you get full fill). Once the RTV is cured (preferably baked out), seal the hole with a screw with an o-ring under the head. Other similar approaches will work. The idea is to split the seal into two parts, one which deals with complex geometries and uses less than ideal seal materials, and another part which simplifies the seal geometry allowing use of better materials.

author: tautech
author_rank: Super Contributor
timestamp: March 24, 2016, 12:51:10 am
content: Quote from: JBeale on March 23, 2016, 11:45:33 pmAny permanent installation outdoors should be in Conduit and for the junction to your small diameter SS tube, I'd terminate the conduit with a threaded connector to adapt to a bulkhead gland fitting, maybe with threaded bushes to match sizes. Easy.

author: Niklas
author_rank: Frequent Contributor
timestamp: March 24, 2016, 09:02:55 am
content: Some experiences from a sensor application that is submerged into a water based liquid in a -40'C to +85'C application:- The pressure inside a sealed off tube will change with temperature, Pressure*Volume=constant*Temperature- Glue might not bond to the insulating jacket of a cable, especially not with cyclic thermal expansion and mismatch in expansion coefficients- Tubes are not always perfectly circular and that will change the compression rate of an o-ring sealing. Also surface roughness and scratches can cause leakage.- Mismatch in thermal expansion between cable and tube can put strain on sealing and damage wires/connectors.- Plastic tubes can shrink, both in diameter and length, as the plastic is annealed. 1% might not sound that bad, but 1% on 1 meter is 1 cm.- Once inside the cable, the fluid can wick for long distances and also from a low to high position.

author: XOIIO
author_rank: Super Contributor
timestamp: March 24, 2016, 09:44:07 am
content: You could rough up the surfaces and use JB weld, I'm not clear on how you are connecting them though, but it is a very durable and usually pretty leak proof connection, I've only had a couple issues with leaks on smooth surfaces at 300 psi, and even then they are very small.Stick some desiccant packs in there too with whatever solution you go with.

author: Fungus
author_rank: Super Contributor
timestamp: March 24, 2016, 10:01:35 am
content: Conformal coat everything. Seal it as best you can, put heat shrink around the joint and bind it. Leave some space inside the box for packets of desiccant.The problem will be the metal tube. Metal expands/contracts with temperature, whatever you use to seal it needs to be flexible.

author: roffvald
author_rank: Unknown
timestamp: March 24, 2016, 11:41:34 am
content: On a veteran ship I volunteer on we use a type of silicone sealant called Tec 7. It's marine grade, and has held up to Norwegian storms and saltwater just fine, haven't seen any signs of water penetration anywhere I've used it.

author: peter.mitchell
author_rank: Super Contributor
timestamp: March 24, 2016, 12:59:44 pm
content: a gasket that can undergo plastic deformation to match the surface along with some grease to fill in the micro gaps is a pretty common approach if it just needs to be watertight, rather than sealed.

author: RobertBG
author_rank: Regular Contributor
timestamp: March 24, 2016, 02:15:22 pm
content: Quote from: roffvald on March 24, 2016, 11:41:34 amI was going to recommend 3M 5200.I am assuming that it is probably close to Tec 7 as some of the guys from overseas have compared them to each other.5200 is one of the toughest,longest lasting marine sealant/adhesives I've come across and it is used by most of the better boat manufacturers to seal out moisture and help bond things.I once sealed the deck on a 28ft speed boat with 5200 on Friday at the end of the day and called it quits before bolting the 2 halves together.Fast forward to Mon morning when I came in I hoisted the boat by the top deck with our gantry crane and moved it to our finishing department without the deck screwsLuckily for me 5200 held it together.I always knew the stuff was strong but I never would've expected it to hold a completed boat together with twin 502 Mercruisers lolAs for how long it lasts,it takes years to deteriorate even in the harsh marine environment.But be warned once it cures it is going to be there for a whileHere is a quick video on the stuff and if you're worried about sealing out moisture this is one of the few things used to seal through hull fittings below the water line so it should handle what you need

author: Pirate
author_rank: Newbie
timestamp: March 24, 2016, 02:39:40 pm
content: Ultra Ever Dry.... works on virtually everything.

author: MickM
author_rank: Regular Contributor
timestamp: March 24, 2016, 09:03:09 pm
content: Subaru guys use this:http://www.amazon.com/Permatex-29132-MotoSeal-Ultimate-Gasket/dp/B000HBGHKE?ie=UTF8&psc=1&redirect=true&ref_=oh_aui_detailpage_o05_s00

author: CatalinaWOW
author_rank: Super Contributor
timestamp: March 24, 2016, 10:19:11 pm
content: There is a difference between waterproof and sealed. Waterproof basically means no visible water gets through under some set of pressure, temperature and duration conditions. Almost all of the approaches above will achieve waterproof status.Sealed means a water vapor content below some specified level. Often means keeping the dew point below -40C or some similar temperature. This does require knowing initial water content, internal volume, water diffusion rates and moisture sources and sinks inside the cavity. Much tougher. You have to judge if you need this level of dryness. Unfortunately, unless you have lots of time and a good lab the only way to find out if your approach is good enough is to try it. Experiments last as long as you hope this thing will live.You can minimize the chances of needing super dryness by taking several steps on your circuit card and assembly. The first step is board layout, providing as much space as possible between critical components. This often is opposite what you want to do for noise and space reasons. Then choose a high quality substrate for the board. Inferior materials will have residual salts and other stuff in the laminate along with voids that can capture etchants. Then make sure the board is as clean as possible after assembly. Again any residual fluxes and other material is your enemy. Then baking the boards to make sure they are as dry as possible on installation. Then conformal coating. Finally, controlling the time from board completion until installation in its final environment.All of these things can become so much trouble that regular maintenance becomes an attractive option.

author: SL4P
author_rank: Super Contributor
timestamp: March 25, 2016, 04:07:01 am
content: How about approaching the problem on two levels - literally.An inner containment to seal against low-risk, passive ingress, and an outer ruggedised containment to protect the inner vessels...?

author: radar_macgyver
author_rank: Frequent Contributor
timestamp: March 25, 2016, 06:09:39 am
content: How about a threaded cord grip, like these:http://www.heyco.com/Liquid_Tight_Cordgrips/(the stainless kind)If you don't need to service the sensors, fill the tube with potting compound. A cord grip would still be good to strain relieve the CAT5.

author: Paul Moir
author_rank: Frequent Contributor
timestamp: March 25, 2016, 06:21:10 am
content: I had one application where the moisture migrated through the wire's insulation and into the sensor head. We ended up drilling a tiny hole in the bottom to relieve the vapour. Fortunately the sensor ran quite warm and could drive off the moisture once it had a place to go.I would presume direct bury cat-5 would have a decent WVTR? I guess I'd want to know.

author: ez24
author_rank: Super Contributor
timestamp: March 25, 2016, 06:30:48 am
content: Russia packs their guns (AK-47 and ammo) in grease. Grease is waterproof. Heavy sticky grease

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: March 25, 2016, 07:43:05 am
content: potted if it is meant to be permanent.http://uk.rs-online.com/web/p/potting-compounds/1608481/orhttp://www.bulgin.com/products/circular-data-connectors/standard-ethernet.htmlto make it removable.You would have to weld a small box to the pipe to fit the chassis mount end of the connector though

author: SeanB
author_rank: Super Contributor
timestamp: March 25, 2016, 11:20:13 am
content: As it is only an outdoor temperature sensor just get a stainless steel encased Pt100 or Pt1000 thin filmsensor, and use a stainless steel hydraulic compression fitting to hold it, and a small plastic ( probably a glass filled weatherproof nylon box) box for the 2, 3 or 4 wire connections to the probe. Join the wires in the box, and fill the box with silicone grease and put the lid on. That probe will last at least 3 decades in the open without issue, and the sensor will be usable at least for that long.

author: stmdude
author_rank: Frequent Contributor
timestamp: March 25, 2016, 06:24:58 pm
content: Lots of good tips in this thread.However, don't forget, if you're hermetically sealing a box, there's more than your stuff in it. There's air, with moisture in it, which will corrode things over time.The high-end, "i'm a professional" way of tackling this is to fill the box with an inert gas before sealing it completely.However, that's very impractical for a hobbyist, so might I suggest something like 3Ms "Flourinert" oils? It's non-conductive, thermally and chemically stable fluid that you can submerge your electronics in, with no ill effects. This prevents the air-moisture from corroding the electronics, even if you still have a bit of air in the box. As long as the fluid is covering the electronics, it's shielded from the air.As a bonus, it's also a great heat-conductor, if that's an issue in your design.