Don't breathe tanks to zero?

[billt4sf]

At dinner some divers were saying that you if a tank gets breathed down to near zero pressure, bad things can happen TO THE TANK. Something about humidity and rust, refilling the tank and then breathing head down.

What's the story here?

Bill

 

[Jon_R]

Not an expert just parroting what I was told in class but theory is with valve open and no pressure moisture is more likely to enter the tank.

If on land and in a humid environment leaving valve open can also lead to humid air getting in tank.

I was told if you bring the shop an empty tank they won't fill it without a visual inspection for those reasons.

Odd part is not on a nitrox tank for that shop. They have to come into the shop empty so people vent them in the parking lot then close the valve.


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[Tortuga68]

Don't breathe your tank down, no problem

 

[mdax]

I don't think you'll find many experienced, regular divers breathing their tanks that low. There are many good reasons. Most important is the concept of keeping reserves in case of trouble....you know, for you AND your buddy.

Folks who try and extend dives by draining tanks near empty are usually totally unskilled and you can easily see why they're hoovering through gas by watching them swim.

 

[The Chairman]

There are at least two aspects to look at here. As for the tank, you should not take it below 15 psi (one atmosphere) unless you want to open it up to inspect it. That's really more than what's needed to keep out moisture and other crap from getting into the tank.

That being said, why would anyone want to breathe their tank down so low? There are very few circumstances where this would be appropriate. In fact, I can only think of one: a pool. For all other situations, you want to keep a sufficient reserve to accommodate any contingencies, including your buddy running out of air. For most divers, doing a no-deco dive, the goal to come up with 500 psi is a good one. I suggest to my students that they should start their ascent when their tank pressure equals 10 times their depth or 500 psi, whichever is greater. That's enough to surface with, and you might even be able to do a safety stop.

However, it's also important not to blow off important things like safety stops just to get out with 500 psi in your tank. Yes, I've seen this happen and it's kind of scary. After all, those dive masters can be quite intimidating on the boat when they demand that you surface with no less than 500 psi. They want you to plan your dive in such a way that you come up with some reserve. However, if you have to go below that to do accomplish a safety stop, there should be no question: DO THE SAFETY STOP. Don't run out of air during the stop, but at this point, I would be willing to surface with only 50 psi or so. If your buddy is close and has plenty of gas, go on their octo for the duration. Just don't run them out of air.

 

[emoreira]

At our school, we do an exercise in the pool where we breath a tank until the second stage reg cannot give air. This, however, does not mean that the tank is empty (0 ATM relative or 1 ATM absolute).
This excersise is intended to show to the students how does it feel an empty tank and how a second stage behaves when low on air.
A tank in this condition, still needs some minutes to completelly vent the air inside until zero relative pressure.

 

[vladimir]

I have breathed my tank down to ambient. Actually a little bit over, but not so much that the crew couldn't remove my regulator without purging it first. That maneuver comes with an implicit tsk-tsk.

But if I have a long surface swim, as in that case, I'll do it three or four meters deep instead, and if my tank runs dry I am comfortable doing a CESA from that depth.

As has been mentioned, my understanding is that you can no longer assume moisture has been largely excluded once the tank is depressurized, thus necessitating a visual inspection.

 

[Tortuga68]

As for the tank, you should not take it below 15 psi (one atmosphere)

I guess you mean gauge pressure

 

[chrpai]

Odd part is not on a nitrox tank for that shop. They have to come into the shop empty so people vent them in the parking lot then close the valve.

Me thinks that's a lazy gas blender. He doesn't want to take the time to analyze the starting FO2 and starting pressure and do the math of how much O2 to add. He just wants to start at zero every time and follow the same math each fill.

 

[bl6394]

You might consider too that analogue gauges are subject to reading inaccurately at the extreme ends (i.e. low) of the scales. You might think that you have 100 - 200 psi of air in your tanks - when you take that last breath...

Cheers,

Bjorn