What PSI can you breath a tank down to?

[maniago]

WRT research, there are a few old threads that discuss this, but don't quite answer the question:
http://www.scubaboard.com/forums/regulators/205303-regulator-behavior-low-pressure.html
http://www.scubaboard.com/forums/regulators/26644-does-regulator-deliver-gas-below-its-ip.html

WRT the second thread, I can breath through my unbalanced first&second stage stage (XS Highland & Aqualung ABS Octo) when its not attached to a tank - hard, but doable. That tells me that I can breath my pony tank (to which they are attached) to ambient pressure - maybe a tad less since I can draw a slight vacuum.

But with my balanced rig (Aqualung Titan LX cold 1st&2nd) I can't breath it off a tank. That tells me that there is some min pressure above ambient that allows it to work. What is that? (Its not the IP, ie 125-140psi.) I can call Aqualung to ask, but I thought I would check here first to see what the consensus is at the extreme ends of the reg performance curve, if there is any. Thanks for any input.

 

[Superlyte27]

Why?
LOL, what practical purpose does this information hold? My tanks never get below 1200psi.

 

[maniago]

Why?
LOL, what practical purpose does this information hold? My tanks never get below 1200psi.

To understand how something performs at its edge limits is to appreciate its design...

 

[rsingler]

On an unrelated thread I tested this (partially) with a Dacor Pacer (standard downstream valve) and with a SP D-400 (one of the most sensitive/lightest spring center balanced regs around).
The Dacor saw its cracking effort rise above 5" water as the IP dropped from 127 to 60. It was still breathable, but with tremendous effort. The D-400 still had a cracking effort of only 1.4" as the IP dropped to 60, so it was breathable even lower than that.
So the question is, what tank pressure gets your IP down to 60? Depending upon internal friction in the first stage, it might be a low as 80psi, though I've never tested it.
Anecdotally, a few divers have commented on the "last extra breath" they get at 15 feet during an out-of-air emergency ascent. That says that a reg is breathable maybe right down to 2-3 atmospheres of pressure, or 45 psi, as they get a little air from what's left in the hoses, with a completely empty tank. Now, what completely empty meant during those episodes isn't known.
I'd guess you can breath a tank right down to 50 psi, though with an unbalanced second stage you won't get much with that last breath.
Try it on the surface at the end of a dive sometime. Of course, your LDS may require a VIS if you bring in a tank that low to get refilled.

 

[Akimbo]

I often use Progressive Equalization, so several times a dive the online cylinder drops to or to or below the IP. My favorite regulator is an early production Oceanic Omega, which can drop the online cylinder to 90 PSI (ambient/over bottom pressure) before I notice resistance and equalize from the off-line cylinder (through the isolation valve on a doubles manifold). As I recall, my Atomic Z2 is about the same.

I usually notice mild inhalation resistance between 400 and 500 PSI on an unbalanced first stage. I start sucking pretty hard below 300.

 

[Zung]

I don't have any quantitative data, but I did it for real 3 times during one of my very best dive:



We were dropped outside of the lagoon, upper right of the pic, went down to 54 meters, swam across the channel to the left, turned right, and swam back to the dive shop near the pier, lower center of the pic.

By the time we got there, I sucked dry my tank, my wife's, and the dive guide's. The whole thing lasted 62 min..

 

[antattack]

That was an exceptional dive.

 

[herman]

If your first stage has an ACD (the devise that is intended to keep water out of the first stage with the dust cap off) there is the chance it will block the flow, otherwise there is no difference in the way a balanced or unbalanced first stage be it piston or diaphagrm, reacts to tank pressure lower than IP....they all open fully and stay that way so any of them will allow you to pull a tank to ambient and slightly below ambient..if you try. Some will be more difficult than others due to the quality/condition/size of the filter.
How hard it is to open and pull air though the second stage is a function of spring tension, diaphragm stiffness and friction in the assembly. I have never seen one that you could not draw air through even with no tank connected, some are harder than others but none that you can't pull air through,

 

[Zung]

... I can see the difficulty one might have with an out of air emergency, and 62 minutes would have meant significant nitrogen loading!...

It's by far not as bad as it may appear.

After we crossed the channel, we started swimming up slowly and "easing" out the deco requirements. My tank went dry when we were in maybe 5-10 meters. We could surface then, but the setting was so beautiful we decided to finish all our tanks there. No emergency situation whatsoever.

Btw this is my preferred profile: instead of hanging in mid water and doing nothing, I'd rather do my deco's on a reef where there's plenty to see.

 

[maniago]

If your first stage has an ACD (the devise that is intended to keep water out of the first stage with the dust cap off) there is the chance it will block the flow........I have never seen one that you could not draw air through even with no tank connected, some are harder than others but none that you can't pull air through,

Well there is no doubt that I can't suck anything through the Titan reg un-attached - its worse than trying to suck a Wendy's frosty through a straw in a Minot snowstorm. I'll check on the model to see if its ACD equipped. Does seem odd that that item would prevent draw vice just be a porous filter though....