[PSUBS-MAILIST] Compass Video

Fri Jun 14 09:46:16 EDT 2019

 Alan,
I like the idea of epoxying the transition between bare wire and insulation.  I think that's a good basic rule to follow.  I forgot about it this time but will follow it next time.  As far as potting the entire volume of the PVC housing, I have enough confidence in the housing that I just didn't think I needed to.  Even in the worst case possible, failure and water entering through the wire insulation, it's not like that water is going to immediately seep the ten feet of wire that will be involved...and even if it managed to do that, it wouldn't be a flooding threat that would prevent me from surfacing.  However, point taken regarding flooding and damage to electronics and wiring.  It's easy to say "it's only $30" but that does add up eventually into real money.  I have already learned that.  I kept adding sensors to my SCM project over a period of weeks and months.  Each sensor by itself was pretty cheap and I thought, gee why isn't everyone doing this.  It wasn't until I added everything up that I realized I had about $1000 in parts alone invested in it.
Regarding overall pressure failure at the wire entrance sealed with epoxy, I think taking into account practical application is reasonable.  At maximum depth, I will be around 300 psi, however the "area" of epoxy exposure is less than .049 inches (1.2mm) which equates to withstanding about 14.7 psi.  It's obvious that my method certainly would not survive thousands of PSI, but it doesn't have to.  You mentioned Doug Jackson having many failures with his DIY penetrators and I've seen those videos also.  My impression is that many of his failures were self inflicted, for example using the 5-minute epoxy product with the needle like dispenser that mixes automatically.  I've used that same product (once) and found that it didn't mix the two parts nearly enough for a strong bond.  That particular product is meant for the home owner who wants to repair a tea cup.  Ultimately, even with that he was at least once able to get a good enough seal for 2000 psi.  So I'm not surprised that Hank has had better luck with his penetrators using better epoxy and mixing practices. 
Jon

    On Thursday, June 13, 2019, 06:02:59 PM EDT, Alan via Personal_Submersibles <personal_submersibles at psubs.org> wrote:  

 Jon,I like the choice of PVC & think it would be quite strong, & if youpotted inside, even stronger.I am paranoid as I have destroyed a lot of electronics with salt water.And had water sucked up inside the wire jackets for a metre due toheating & cooling.Maybe on the compass project strip a section of the wire going throughthe epoxy & use one of the techniques in the link to improve the PVCbond X 3 times. And go over the board with the epoxy. Was doing this on my LED driver last night.Alan

On 14/06/2019, at 9:32 AM, Jon Wallace via Personal_Submersibles <personal_submersibles at psubs.org> wrote:

 Alan, all great points and I wouldn't argue against them.  You may be right about the epoxy/pvc adhesion, and as usual the internet isn't much help.  I can find as many "don't do it" as "it works fine" opinions out there.  I should have scuffed it up a bit though, that would have helped with adhesion.  In the past I have fabricated my own "blue globe" fittings out of brass compression fittings but I wanted all plastic so I wouldn't have to worry about decay of the brass by salt-water.  I mentioned a couple of times in the video that this was a non-critical component but maybe should have impressed that a bit more on the viewer.  PVC certainly would not be my choice for a critical application.
Jon

    On Thursday, June 13, 2019, 08:22:08 AM EDT, Alan via Personal_Submersibles <personal_submersibles at psubs.org> wrote:  

 Jon,
that was one of the most comprehensive "How to" videos I've seen.
Have one of those compasses so will emulate that.
I have reservations about epoxy sealing the wires though.
I did a lot of experiments with glues & PVC wire jackets & found a lot of
the glues disappointing. I found one product specifically for PVC boats
that stood out.
I have mentioned before that I potted a couple of wires for 1" in to a WD40
filled housing & the WD40 just ran out between the epoxy & wire jacket
without pressure. 
Under pressure I would guess that the PVC jacket would be more compressible
than the  epoxy resin around it & would tear away from it, letting water in.
Suggest epoxying the whole board both sides & the wires where they are 
soldered on, in case of water egress.
I have strong reservations about the epoxy sticking to the PVC pipe also.
I would be interested in others opinions on this as potting wires is 
something we are doing a lot of.
Alan

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