Even if you pump air in from the top?
oh yeah
We used to build spud guns and use hairspray and a coleman striker assembly to lob potatos like a cannon…gotta build instantaneous pressure VERY high…after all it is a combustion chamber, so a screw on end cap w/ a tire inflator nozzle screwed in should be able to handle a few hundred psi no problem.
I have seen those big blue 55 gallon plastic barrels hold over 40 psi before (used as a mobile smoker trailer’s water supply, fill about 3/4 w/ water, then pressurize, then using a water hose spigot tie into the sink…viola, instant water w/o needing a pump (well other than an air compressor)
The plastic pipe should be able to take the pressure. Plastic plumbing is typically pressure tested for leaks at 120 to 200
psi before commissioning.
And you could pressurize it with a slide trombone. 
Yes, although fitting a double check valve (one way non return valve) in the line between your pump and the Schrader valve may well allow your method to work
Pressure increases by 1 atmosphere every 10m or~33’ so at 10m is 2 atm(1+1), 20m is 3 ATM, 30m is 4 atm, and 40m is 5 atm.
A pressure seal that can withstand a high static pressure might also fail under a sudden change such as that caused by giant stride entry from the side or bow of the boat where the drop to water might be 2m. Dive watches are often rated to 200m since anything under 100m might fail just taking a shower. 200m is 20 atm greater than sea level or (20 + 1) x 14.7 psi or roughly 310 psi. You don’t want your test rig to fail and one end will need to be remove able.
The no-deco sport limit is 135’. Below that bottom time gets very brief and decompression stops on the way back up become mandatory necessitating extra tanks or houka lines to supply the air for these safety stops.
Just stand the pipe upright, make it the desired test length, fill it with water and drop the light in. No compressor needed. 
I can’t wrap my head around this.
There is water in most of the chamber. I would leave maybe 1/2” air at top. Then pump air into vessel…but after each pump, you are saying water will rise upward (float), defying gravity, through the pressurized air and into the bike pump tube?
If this is the case, my kind is blown… :Sp
Edit: Or, does some water vaporize as pressure is reduced (at the end of each pump), then vapor rises to the tube?
I was thinking of using 4” schd. 40 PVC, 1-2 ft long, with a glued cap on one end, and something like a cleanout adaptor/connection glued to the other end. That way, I could unscrew the cleanout cap to load/unload whatever I would test. It would probably need an additional 4” seal for the cap to hold pressure.
One thing I have noticed is that cleanout connections are advertised as not appropriate for pressue applications. However, 80 or so PSI doesn’t seem too high to me.
Not sure if 80psi would be adequate to ensure reliable service but if you were careful entering the water might be ok. What about using a neumatic tool quick connect instead of a schrader valve? Larger diameter threads and higher pressure rating. Common pipe thread size as well. Hit 120-130 psi no problem. Also, a plug has the threads on the outside where pressure will expand the pipe opening the threads. Better to have a cap with the threads on the inside where pressure will actually tighten the fitting.
Looks like you should go with schedule 80 PVC then
http://www.harvel.com/technical-support-center/product-specifications/pvc-pipe-schedule-80
3” chunk of sched 80 pipe (Max WP: 370 PSI)
3” end cap
3” threaded end cap
3” threaded plug
Now all you need to do is devise a way to inject compressed air into the tube w/ a pressure gauge and voila a pressure chamber
I’d still be concerned with the valve blowing out if it’s just tapped into the plastic but maybe adding to the wall thickness where it’s tapped would be sufficient.
It’s not defying gravity, but it will want to rise up the tube after each pump stroke, you would need a pump with say a 8 foot stroke so you could pressurise your tank in one stroke lol , obviously that’s not practical, that’s why you need a reserve of compressed air so you can pressurize your tank in one go as that way the constant pressurized air going in will stop the water coming back up the tube
To try and explain it a bit better, You can’t compress water , but obviously you can compress air, so as you pump the air in to your tank with your piston type pump, your pressuring the air in the tank, which is displacing the water, as the air pressure increases in your tank that water can’t compress and wants to escape, now all the while you are on the pressure stroke with your pump the pressure from your pump is greater than the air pressure in the tank and the water stays in the tank, as soon as you draw your pump back to take the next pressure stroke you have negative pressure on the pump side and the water in the tank under press, rushes back up the pipe to fill the void
That’s why you need either a constant supply of high presure air, or some sort of non return valve on your hand pump
Hope that helps, probably wasn’t the best description but hopefully it makes sense
As long as the air inlet is above the water level this won’t be a problem. Also, the inlet valve should close as soon as the pump stroke ends anyway.
That’s what we thought, and all the while we had no water in the equation it worked fine (we’d built pretty much what the op described, just using black iron pipe and fittings instead of plastic) we had it over 100 psi No problem, but as soon as we tried it with water in the water would come back up the pipe and in to the pump once we got to around 40 psi iirc
We tried a couple of different pumps and a foot pump, happened with them all?
Worked fine if we used a dive bottle to pressurise it, so the op’s build idea is sound
In the end though we built one out of an old air compressor as it was bigger and more practical and looked a bit less Heath Robinson than our first attemps
There are a number of ideas for testing watches and their water resistance. One that I’ve played with uses a 1 liter nalgene bottle but one has to be very careful about putting too much pressure in it. I must say that I am not that comfortable beyond 50 psi and I keep it out of my direct line of sight in case it blows.
Here is one using a water filter chamber:
http://www.christopherwardforum.com/viewtopic.php?f=33&t=10816
Here is a more comprehensive model:
http://www.flynwill.com/Watches/Waterproof/
Not sure if these would be large enough but it may give you some ideas.
John.
Tap it at a coupling you’ll get double wall thickness. For pressure, a propane cylinder with any amount of liquid propane in it will give you up to 190 psi. Just don’t smoke on depressurization.
Worse comes to worse, tap thru the threaded end cap on a flat end…then use a washer and nut and some RTV to seal it
Well…I went out and purchased most of the items for this sucker:
This pic has:
1 x 4” tube by approx. 16” flared piece of schedule 40 PVC pipe.
1 x 4” end cap.
1 x 4” internal fitting with threads on other end.
1 x 4” threaded screw plug.
1 x 4” rubber seal.
1 x Oatey PVC cement (I have some primer somewhere around here).
1 x Fox HP pump (already had that for shock on my MTB).
This is what it will look like open and closed, sans valves.
I have two of these Schrader valves coming which I will mount somewhere…probably on the threaded plug. One for an Ashcroft gauge I have laying around here somewhere, and the other for the HP pump.
I have lots of questions, but the most important one is…am I crazy?
That is exactly what I plan to do with the Shrader valves.