How Far to Open Valves on Rebreather Tanks

[shoredivr]

The attached video provides good historical perspective on where the "open fully and turn back 1/4 turn" advice for opening scuba tank valves came from and why it is not necessary today (starting at ~ 5:06)

Ah, Alec Peirce, Scuba 2000, Toronto, a local dive shop to me. Quite the knowledgable guy too, and a character. Retired from the shop last year, IIRC.

When I was getting into tech diving ten years ago, Alec couldn't see the need for it at that time. It's kind of amusing that this video is posted in the tech section of SB.

*edit*
Here Alec shows a home made 1960s oxygen rebreather. He never mentioned that in the store lol.

 

[BEVANJ]

1.5 turns = a higher chance of roll off on an overhead environment. Dil would be easily noticed and not potentially fatal assuming the diver is competent. O2 not so much, but should be registered if monitoring hud/handset. However, if task loaded, it's an increased liability. It's still not definitively noticeable like on open circuit.

I guess it depends on which scenario you think has a higher chance of happening, roll off followed by hypoxia, or boom followed by catastrophic gas loss.

Personally, I'd rather take the risk of boom followed by catastrophic gas loss, simply due to the fact that I'm always carrying ample offboard bailout. That and the fact that I'd rather not take the risk of having a subtle failure scenario which was partially self-induced by having the valve only 1.5 turns from closed! I'd rather take the obvious failure scenario followed by a full blown bailout, as opposed to a potential subtle failure which could go unnoticed.

Another thought would be that even if you do get a boom followed by gas loss, are you really going to be able to do anything that will allow you to stay on the loop anyway?

-If it's a solenoid failure, you'll PROBABLY have to come off, unless you've got a shutoff like the inspo, but that's another liability as you can accidentally leave it closed, resulting in hypoxia. Unless you feel like feathering your O2 valve, but that's not going to be fun. I think I'd rather just shut the O2 off and bail.
-If it's a hose, you've got to shut down the O2 or dil. If you're lucky enough to be carrying offboard O2 or dil, you might be able to plug it in to stay on the loop. Either way, you've still made the gas in the cylinder that had a hose off it fail unusable.
-If it's a MAV free flowing, you can unplug it (depends on unit) - the one scenario I can think of where a boom scenario might result in being able to be fixed and stay on the loop and easily retain the usable gas from the system where the "boom" occurred.

Bevan

 

[sea_ledford]

1.5 turns = a higher chance of roll off on an overhead environment. Dil would be easily noticed and not potentially fatal assuming the diver is competent. O2 not so much, but should be registered if monitoring hud/handset. However, if task loaded, it's an increased liability. It's still not definitively noticeable like on open circuit.

If you are able to get a roll off on the o2 side of a CCR, you are doing something wrong, have a wonky set up, or are the most powerful back kicker in the world. A roll off on the dill side would still be a trick, unless you have tanks de-inverted it would be pretty hard to accidentally shut down a valve that is tucked in like they are on most CCRs.

-If it's a solenoid failure, you'll PROBABLY have to come off, unless you've got a shutoff like the inspo, but that's another liability as you can accidentally leave it closed, resulting in hypoxia. Unless you feel like feathering your O2 valve, but that's not going to be fun. I think I'd rather just shut the O2 off and bail.

A solenoid failure is about the easiest one to deal with. If it fails open, feathering an O2 valve should mostly be a non-issue, you give a quarter turn every few minutes. If it fails closed you, manually add. One or both should probably happen on every one of your training dives.

-If it's a hose, you've got to shut down the O2 or dil. If you're lucky enough to be carrying offboard O2 or dil, you might be able to plug it in to stay on the loop. Either way, you've still made the gas in the cylinder that had a hose off it fail unusable.

Lucky enough? Having bail out gas is pretty much the bare minimum of gas planning. Having an off board plug in is pretty standard too.

Choosing between roll off or catastrophic gas loss on CCR? I never even considered that as a question. Roll-off is a minor annoyance on OC and a non-issue on CCR.

-Chris