PVC canister: Am I crazy?

[naimis]

I did a dry fit of the PVC parts I'd bought to make my dive light battery canister, tonight, and am having some misgivings about the feasibility... I'm going to describe what I have and post some pictures I took of the dry fit and ask for comments... I can glue it all up and take it on a dive with me, and if it fails with only the empty pipe, it's obviously not a critical problem, but I wanted to try to get some opinions first..

The canister body is a 1 1/2" PVC pipe, schedule 40. The reason it's schedule 40 is because I can't find anyone in town with sch 80 or 120. Pressure is clearly not an issue, the sch 80 or 120 is just for having the thicker walls for screws for the latches (I'm using the ones that OxyCheq sells). My plan for getting around this issue with sch 40 PVC was to double up the wall width using fittings (4mm + 4mm).

The bottom of the canister is capped. Simple enough.

The top of the canister has a 1 1/2" coupler for the lid to fit into. The latch will be screwed into this (predrilled and tapped 6mm into the 8mm walls).

The bulk of the lid is a 1 1/2" x 1" PVC reducer. The reducer has the same OD as the pipe, so it fits inside the coupler. The reducer has the same problem as the pipe as far as screws, but it also needs something for the O-ring to sit up against, so I cut off part of a coupler and stuck it on the reducer. It didn't cut very straight on my miter, but with the pre-formed side facing the coupler on the pipe, it probably doesn't matter much. I would definitely need to sand both edges smooth, though.

In the reducer, I have a 1" plug. The reducer size and plug were chosen because it was the only one I could find that had the nice flat top that would butt up against the O-ring in the Agro cable gland.

My big concern is whether or not this has any chance of actually staying sealed. The reducer doesn't really fit "tight" into the coupler like a pipe tends to do (I suspect that fittings are tapered so the less depth you get into the fitting, the looser it is).

Anyway, any suggestions would be appreciated.

 

[350xfire]

Not pretty but effective!!! It will work.

 

[DIWdiver]

Naimis -
IMHO this could be made to work well, or to fail reliably. As is often the case, the devil is in the details. I've also heard that God is in the details....

Anyway, there are two things you have to get right for this to seal correctly, at least before you add the gland and switch. All the PVC parts have to be glued well, with no voids in the seal. This is normally very easy, but you have an unusual situation where you cut a coupler in half and slid a pipe all the way through it. It still shouldn't be hard to get a good seal, but it's also possible to be careless and muck it up.

The reason that seal is important may or may not be obvious. There are two places you could think to put an o-ring (you are planning on using an o-ring, right? Without one this is GUARANTEED to leak). One is inside the whole coupling, against the top end of the pipe that is the body of the cannister. This would be a bad place to put it, because the water pressure would tend to push the o-ring into the cannister. If a portion of the ring were to move far enough, you'd get a massive leak. You could put a groove in one of the pipe pieces to prevent this, but this would require tools I suspect you don't have, and it would probably be contaminated by glue while assembling the unit.

The only decent place to put the o-ring is around the pipe portion of the lid, so it gets squeezed between the full and half couplings. Now most o-rings have a bit of mold flash at the ID and OD points. This isn't a problem in most applications which squeeze the ring either axially or radially, and the flash is insignificant. In your application, with nothing to squeeze the ring against the pipe, the flash will prevent a reliable seal against the pipe. However the ring WILL seal well against the faces of the full and half couplings. Which brings us to the second thing you have to do right. The faces of the couplings have to be flat and smooth and parallel enough to seal well.

This shouldn't be hard. They can easily be sanded by hand. Lay sandpaper on a flat surface (a piece of glass works well - I got some scraps of 1/4" plate glass free from the local glass shop, and they even sanded the edges smooth for me), and rub the coupling over it by hand. Progress from 120 or 180 grit to 400. You could go to 600 grit, but I think that is probably overkill. I know that on aluminum, 400 grit makes a satisfactory surface for 70 durometer o-rings.

The only remaining problem will be to glue the half coupling onto the pipe without ruining the sealing surface. Again, that shouldn't be hard.

I imagine that all this is exactly what you were planning to do. If so, congratulations on a good design and good luck on your project. On the off chance that I actually helped, all the better.

D

 

[naimis]

Thanks for the replies. Yeah, the plan was to put the o-ring on the pipe portion of the lid, as you said, and sand smooth the edges that would squeeze the o-ring. Here's another picture of that (sans sanded faces) that I hadn't taken earlier.

 

[kayakdiver]

Here's a simple way to seal a tube: Make a small sleeve that slides over the tube...you can cut a piece from a coupler / repair sleeve, or whatever works. glue this in place 1" or so from the top. *caution, if you use pvc glue, make a cut all the way through the sleeve (so the ring pulls apart)...otherwise the pvc conditioner and glue may make it too tight to slide on. for lid fasteners, you put stainless bolts in the sleeve...the latch is made from a type of flat bungee (Home Depot) ...has holes for adjustments...can be stretched over the top to reattach on other side...or if the tube is large enough, can attach from the sleeve, to other bolt attached on the edge of the cap. If you can't get an oring at your hardware store, you can find them at smallparts.com *Moderate pressure makes an adequate seal...once you get to depth, the pressure does the work....other topic: check out my video on Youtube, search: "underwater gps"

 

[FL Dive]

I cooked a section of sch40 pvc in the oven @400 degrees for 5 mins and formed two small blocks to the inside of my canister to reinforce the wall where the latches are screwed. Once it cools off, seperate the now formed piece, clean and glue it where you need it.

(cut pipe long ways before baking, it will open right up after 5 mins)