HELP! Authoritative answers only, please!

[Crush]

In October 2009 I began a thread http://www.scubaboard.com/forums/basic-scuba-discussions/309644-can-you-swap-tank-underwater.html because I had read of someone swapping cylinders (i.e., removing their regs from one set of tanks and putting them on another) underwater and breathing on them. This was contrary to what I had been taught - after all, isn't a plug of water, accelerated by 3000 psi, pretty much like a bullet? I appreciate all those who responded, many of which claimed to have swapped tanks underwater in the good 'ol days, and while not recommended, it was likely to be OK.

Fast-forward to January 2010. I began a post, http://www.scubaboard.com/forums/computers-gauges-watches-analyzers/319853-quick-disconnect-spg.html , asking essentially if it was possible to switch gauges underwater (I was really asking about quick-disconnect, but let's assume that I am the Hulk and can twist the damn thing off). Most responded with something like "a plug of water, accelerated by 3000 psi, pretty much like a bullet."

What gives? What am I missing? Why can I swap regs underwater, but swapping an SPG will cause it to shatter?

Please?

Postscript: I know that second-stage regs are stepped-down to about 120 psi while gauges operate at full pressure. Here I am assuming that all those who swapped regs underwater also had SPGs on those regs that didn't explode.

 

[ptyx]

First, if you're seeking authoritative answers on the net... Well, good luck to you

Second, I've never submitted my regs to that treatment, so caveat emptor. However...

The main difference between a regulator and a SPG is that the first one has flow, while the other hasn't. While sea water in a regulator will damage it, clog the filter and probably help ruin it long term, if you're breathing on it it will get dried out eventually through the second stage.

In case of emergency, swapping a regulator underwater can be done, and a good soak in distilled water followed by letting air through it (all ports open) will clean it - in theory. Early servicing later on will probably be a good idea as well.

On the SPG... unless your SPG leaks, any water in the hose will stay in there. Any water in the bourdon tube will ruin the SPG. Since the tube is sealed at the end there is just no way to clean it. If you're using a quick disconnect close to the SPG it's even worse as any drop of water on the SPG side WILL get pushed to the SPG by the pressure.

I guess that when swapping a regulator underwater, you just hope that no water makes it to the HP hose - or not enough to reach the bourdon tube (which seems reasonable given the pinhole it needs to go through and that - once again - it's sealed at the other end).

 

[Kern]

The regulator hose is called a low pressure for a reason, it has a few bar in it. The SPG on the other hand is attached to a high pressure hose, it has 200-300 bar in it.

 

[ptyx]

Btw, I haven't seen many answer in that previous thread about SPGs shattering.
Corrosion is an issue. Water in the bourdon tube will probably cause inaccurate readings (not even sure about that). I guess if you really work at it you could shatter one...

But the bullet theory doesn't work. Especially when swapping a regulator.
Even if any water gets in the HP hose will compress air remaining in the hose.
As this air compress, the pressures will balance and the bullet will be stopped.
Hopefully in the middle of the hose and not in the SPG (not so sure with a QD).

Now, would I try this with my gear... no!

 

[Crush]

The regulator hose is called a low pressure for a reason, it has a few bar in it. The SPG on the other hand is attached to a high pressure hose, it has 200-300 bar in it.

Brilliant!

 

[Wookie]

My authoritative answer is that if you were physically strong enough there is no reason you could not swap a QD on a SPG underwater. Additionally, you would destroy an electronic SPG immediately not from some "bullet effect" but because salt water and electronic SPG's don't play well together. Salt water in the bordon tube will cause it to corrode, and they are hard to rinse out, so it will become inaccurate very quickly. Rebreathers are often equipped with a quick disconnect for "off-board bailout gas" so it s done with rebreathers often in an emergency.

 

[Garrobo]

AUTHORITATIVE ANSWER: Hell, I don't know.

 

[DCBC]

...those who responded, many of which claimed to have swapped tanks underwater in the good 'ol days, and while not recommended, it was likely to be OK.

...but let's assume that I am the Hulk and can twist the damn thing off). Most responded with something like "a plug of water, accelerated by 3000 psi, pretty much like a bullet."

There are training considerations that have disappeared over the years. Things such as harassment, is no longer done. While untaking diver training with the Navy in 1972, we were subjected to harassment. This included having our masks ripped off, air shut-off, blackout procedures and while buddy breathing, having your my regulator handed to you at depth. The pack-drill was to remove your tank, replaced the regulator and carry on with the exercise.

The problem can occur when the air pressure is exhausted from the 1st stage and water floods into it. Although this is cleared out when pressure is reapplied, it's hardly preferable to allow moisture into the first stage. What is preferable however may not be what is required. No doubt many new divers when rinsing their regulator get water into the first-stage.

Taking-off or replacing a quick disconnect hose underwater shouldn't be a problem. Commercial divers do this with air tools often. I suppose you have to evaluate if it's worth any additional expense of maintenance and replacement of gear.

 

[Blackwood]

Why would you be swapping SPGs on a pressurized cylinder anyway?

 

[Crush]

Why would you be swapping SPGs on a pressurized cylinder anyway?

In link http://www.scubaboard.com/forums/computers-gauges-watches-analyzers/319853-quick-disconnect-spg.html you will see that I was musing about the possibility of detaching my AI computer if it were to fail. My SOP would be to abort the dive. However, I dive a backup computer, so in theory if I can still monitor my tank pressure I can continue the dive.