# How to simulate water pressure at depth

Hi,
I'm building a small underwater tethered / robot / submarine /
doo-hickey and I need a cheap way to pressure test the individual
components to make sure they don't leak. Max diving depth will be 70'
which is around 32psi and each component will be no larger than 6"x6".
I know each component should be tested to 1.5X to 2X its expected
range.
Anyway, my thought was to fill some already pressure resistant device
such as a pressure cooker or autoclave chamber (hello ebay!) with water
and then use shop air to increase the pressure. I don't need a viewing
window as I would basically take it up to pressure, leave it there for
a few hours, relieve the pressure and then check to see if anything
leaked.
Questions:
1) Is this concept of pressure testing sound?
2) Is there a cheaper way to do it?
3) Most importantly, is there an easier way to do this?
Thanks and please resist the urge to point out the 100's of other
failure points that I will be facing in this project such as watertight
bulkhead fittings, ballast systems, propulsion, electronics, stability
control, corrosion, battery systems, faily safe safety systems, etc,
etc... :)
-M
60 psi or better seems a bit much for a pressure cooker or an autoclave. I would think you could make one out of a 6 or 8 inch pipe nipple and a couple caps (should be able to find one at any "industrial" plumbing supply...). That should take at least 100 psi safely. I would be inclined, for safety reasons, to fill it as full as possible with water and keep the volume of air space to a minimum and not to get too cozy with it while it is under pressure...
Jerry
The actual pressure (including the 14.7 psi for surface air) will be ~ 45 psi.
You'll want to do your testing around 100 psi.
You might want to consider building a test vessel from large-diameter pipe and flat plate.
Don't forget that you'll need something that can be sealed against *more* than your test pressure...
Seems so.
I don't *think* so...
Not really.
Wouldn't it be easier and cheaper to dangle the parts on a 70' wire in a deep lake?
LOL! The minimalist approach!
I love it.
(except that it would need to be 100-140' deep)
-- Jeff R.
Corny kegs, formerly used by soda fountains, but now mostly used as surplus items in the home brewing hobby, are cheap (\$10 surplus), have a nice big opening, have a 100 psi working pressure, last forever (all stainless), and are easily resold. A homebrew hobby supplier will also have the fittings to connect your shop air.
6"x6".
device
viewing
watertight
stability
A good, heavy duty paint or sandblaster pot should work.....
stainless), and
I've got one of those in my garage, but I don't think something 6X6 will fit through the opening....
1 yes 2 Only if you already have a boat and live near the water. 3 No
But I would suggest using something more like a dead weight tester instead of air. That is using a piston to pressurize the paint pot or whatever. The reason is stored energy. Lots of energy with compressed air. Very little stored energy using a piston like a master brake cylinder to pressurize the tank. Also if you have a leak you might be able to realize it sooner.
Dan
snipped-for-privacy@hotmail.com wrote:
Start with a bathtub or swimming pool. Then work up to the local Dam or ocean drop-off.
Lake Tahoe would be way cool as it is very deep.
Good luck - sounds like a fun RC.
Hope you will have a camera port!
Martin
You could do worse than to look at the leak testers that the watchmakers use. Mostly they test under a vacuum condition while submerged in water. The leak tester has a clear pane or a bell jar that allows the watch to be seen. The watch is submerged and a vacuum is drawn, and the repairman looks for bubbles escaping. If any are seen, they are duly noted and the watch is removed before it can absorb an appreciable amount of water through the venting gaps. Repairs are then made to the seals, etc. Some testers subject the case of the watch to air pressure first, for a period of time, then submerge them and observe for bubbles.
I like the simplicity of the long rope and a lake idea, myself, though it makes it a bit tougher to determine the actual failure mode or point.
Cheers Trevor Jones
"Gary Brady" wrote: Wouldn't it be easier and cheaper to dangle the parts on a 70' wire in a deep lake? ^^^^^^^^^^^^^ Depends. Not everyone lives on the shore of a deep lake.
wrote: (clip) But I would suggest using something more like a dead weight tester instead of air. (clip) The reason is stored energy. Lots of energy with compressed air. (clip) ^^^^^^^^^^^^^^^^^^ If you have air available, and you know how, you can do it safely. Have a small volume of air in contact with the water, connected to the supply through a small orifice, or a nearly closed valve. If you get a rupture in the test vessel, the pressure will drop, and air will only hiss from the supply. (The danger from high pressure gas or steam exists only if there is a substantial volume which can explode.)
The opening is about a 4 x 3-1/4 inch oval. I thought the OP said the parts were small.
Lake? Whats that?
Gunner, California High Desert
"Pax Americana is a philosophy. Hardly an empire. Making sure other people play nice and dont kill each other (and us) off in job lots is hardly empire building, particularly when you give them self determination under "play nice" rules.
Think of it as having your older brother knock the shit out of you for torturing the cat." Gunner
That's that thing you think you see off in the distance..
That's merely a Mojave mirage. We have a river up here, but it's only 3-6' deep in most parts. That wouldn't work.
P.S: I'm about to make my annual trek to the Bay Area and even after taking out a loan for gasoline, I won't be able to to afford to make the extra leg down to your house to go shopping and fill up my pickup bed, damnit. Maybe next year.
- Better Living Through Denial ------------
Dynamic Websites, PHP Apps, MySQL databases
A bit back a friend of mine used to dive, he explained that to test the air bottles they were filled with water (pressurised)the air bottles were placed in a tank full of water to contain any faliures. If you want to test your enclosures this might be a way as you'd fill the enclosure with water under pressure and see if any came out, rather than setting up a 140' tank in your living room to see if any went in.
Just my 2cents (though, I spend pounds!)
Lee
If you keep the chamber full of water, and then pressurise it with air, the only air in the system will be that in the pipe between the chamber and the compressor. The volume will be minimal, and as the stored energy is porpotional the the product of the pressure and volume of the gaseous part only , very little energy will be stored.
Tom
Normal pressure cookers only go to 15psi, but that is steam so there has to be a good safety margin built in. Just be aware you are overpressurizing the vessel and be prepared for catastrophic rupture.

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