Can you swap a tank underwater?

[drdaddy]

I want to thank the OP for the question and thanks to you all for sharing the possibility of doing this. Amazing!!!

In an emergency, I now know it is possible (though I really think some of you guys are crazy ). However, I think that if I ever planned to be down long enough to have to do this (and didn't have twin tanks each with own dedicated reg), I would sling a spare tank with an attached reg set. Also, it would probably need to be Nitrox that I'd be breathing for that long underwater, in order to stay within the NDL.

So MPETRYK, don't try this at home or underwater!

 

[elmer fudd]

My basic OW instructor told us that, if water enters the 1st stage, attaching this reg to a fresh bottle and opening it up will accelerate the plug of water to an incredibly high speed, leading to the likely rupture of your gauges. I have thus far not felt the need to try to prove him wrong with my own gear (although perhaps I could rent his...).

I've built a few precharged airguns in the past and I'd find this extremely unlikely. First off, with an absolutely ideal setup configured for maximum velocity, about the fastest you can accelerate anything with air is around 1600 fps and the setup inside a first stage reg is about as far from ideal for that purpose as you can get. Typical precharged airgun velocities are in the neighborhood of 900 fps. Since your valve doesn't open up instantly, you'd get a very slow acceleration. Second, since the barrel, (your HP hose), is plugged, as the projectile approached the end of it the trapped air inside would be compressed and would tend to act like a spring slowing down the projectile.

Now that's making the assumption that the high pressure air didn't just vaporize the water in the first place, which is what I suspect would happen.

Still, let's assume that I'm totally wrong. That the water plug holds together, gets accelerated, doesn't get slowed down by the trapped air at the end of the hose and somehow impacts the inside of your gauge at say 500 fps, (I think I'm being generous with that velocity). I believe the inside hose diameter of an HP hose is 3/16" or pretty darn close to .177". Water is about 1/11 as dense as lead and a good size plug of .177 lead might weigh 33 grains. That would make your water plug about 3 grains, so using the kinetic energy formula, 500x500x3/450240=1.66 ft lbs. That's not enough energy to break your skin much less shoot through a metal pressure gauge.

 

[Tigerman]

It's not good for your regs or cylinders, but it can be done.

I've taken off my first stage and replaced it underwater, to stop a leak.

The regulator needed a good cleaning afterwards.

The cylinder -- don't know, it was a rental!!!!!!!

The cylinder shouldnt have any problem unless the ambient pressure is higher than the tank pressure?
The tank will be blowing air out till you shut it down and then blow air out when you open it again so there shouldnt be any water getting in.
The fact that the air comes out of the tank is also whats going to blow air through your reg (which should be serviced afterwards).

I actually considered doing this on a dive earlier this year, but decided not to simply because the problem didnt appear fixable and turned out when I got to the surface it wasnt because the o-ring was blown to pieces (details in my signature)

 

[AfterDark]

Your basic OW instructor misinformed you.

... Bob (Grateful Diver)

He probably just didn't want any student trying it with his rental gear.

 

[Thalassamania]

Yes it it possible.

Yes I have done it.

It gets the inside of the reg wet.

As long as the tank has more pressure in it than the water outside there's not problem with the tank.

What other fairy tales did your instructor share with you?

 

[Deefstes]

I've built a few precharged airguns in the past and I'd find this extremely unlikely...
... and some ..
...That's not enough energy to break your skin much less shoot through a metal pressure gauge.

I had to give thanks to this post elmer fudd, not because it is useful by any stretch of the imagination but just because it was beautiful

No one seems to have addressed the matter of water entering your gauges and being trapped there. Sure, water entering the breathing loop will eventually be flushed through the DV and give off slightly wet breathing for a while but what happens to the water that enters the HP hose towards the gauges? Is it possible to get that moisture out again? I've been told, and I think it was here on SB, that water entering the HP hose is an efficient method to mess up your SPG if ever you were looking for one.

 

[Thalassamania]

It is possible for water to enter the pressure guage and, in time, rot it out. But the hole at the regulator end is very, very small and so is the shaft that couples the hose to the gauge, so I think it unlikely, but not impossible. Disassembly followed by fresh water and then alcohol rinses should do the trick.

 

[elmer fudd]

Perhaps I overdid it a bit with the details?

Sometimes though I think it's best to give a detailed explanation of why something is bull. That way people reading it know that you didn't just give them some opinion you made up out of thin air.

 

[Deefstes]

No, don't get me wrong, I really did like your reply. I wasn't being sarcastic. I'm a bit of a glutton for knowledge and it was a beautiful piece of math, physics and rebuttal of BS.

 

[miketsp]

I don't know if it's still standard practice but during CMAS 2* "circuit training" in the pool you had to keep swimming from one corner of a pool to the next, with a blacked mask & without equipment, and then use whatever you found there.
Sometimes you'd find just a cylinder to breathe off and others you'd find a cylinder plus an unmounted reg set. You'd take a couple of breaths off the cylinder, mount the reg set on the cylinder and then use it. This was done using standard Scubapro regs and no special maintenance was done afterwards - although this was pool water and not salt water.
The instructor said the small amount of water would clear after a few purges. The super-dry air from the cylinder would take up all the water very quickly.