Don't breathe tanks to zero?

[Slamfire]

Two weeks ago I had a dive shop attendant tell me, rather condescendently, that it was recommended that I drain my tanks before starting to mix a new batch of trimix. I told him I NEVER do that. I wanted to reply that in school I was taught about PV=nRT and that he should try to gather a little more knowledge to alleviate the ignorance that is forcing him to drain tanks before mixing. I behaved and didn't.

I did tell him that I was not about to vent out 85 cu ft of 12/58 just because I wanted to mix in some 21/35 into the tanks. It is a lot of helium to throw out. And I was not asking him to blend anything for me. I just told him to top it up to 2600 psi with air. He then started asking questions about what was inside, and then I had to explain to him that it was pre-mix done on top of left overs from another dive. That's when he felt compelled to unsolicitedly share his mixing wisdom.

 

[The Chairman]

I guess you mean gauge pressure

Absolutely!

 

[Griffo]

It's amazing how much crud gets continually repeated by the dive industry. I too have been told not to drain my tanks as "they risk water ingress". The same guy the next week told me to please drain my nitrox tanks out back so he could PP fill them from empty. *sigh*. I've also been told that water will somehow magically go against the force of pressure and end up dripping into the tanks if I drain them too quickly, that salt-water will somehow end up going backwards through my reg and into the tank if I breath it dry etc etc. I also guess someone needs to tell Faber that empty tanks are bad because all my factory fresh tanks turned up without even valves in them! My gosh they must be full of humid air eating away at the steel before I even got them.

 

[Mustard Dave]

As long as the cylinder remains above ambient pressure, nothing will get in. If you allow the cylinder to completely drain, there is a very good chance moisture will get in if you leave it wide open. I have only drained a tank completely on one occasion, when I split my twinset into two singles. To do this, you need to drain them so you can unscrew the manifold, and then fit blanking plugs. I bought the plugs from my LDS and fitted them. I took the completely empty cylinders to an inland dive site, dropped them for a fill and they didn't bat an eyelid.

Breathing tanks down to zero is obviously never a great idea underwater, for reasons that are obvious. A particular concern, which is often overlooked, is crack-bottle DSMBs. For those not familiar with them, they are a normal looking DSMB with the addition of a fitting for a cylinder valve. A small cylinder is attached and when you are ready to deploy it, you open the valve. The problem with these is they can easily be filled by screwing them into your main cylinder pre-dive and opening the valves. For this reason, I suspect very few ever go for a test. Ideally, when deploying a crack bottle DSMB, you should open it, allow enough gas in for it to reach the surface fully inflated, and close it. Unfortunately, many people just crack them and let them go, allowing salt water in. There are many horror stories of crack bottle failure doing the rounds.

 

[chrpai]

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.

Lake Travis is down 54' right now. I have many dive sites available to me that for whatever reason I can be cruising around in 10'-15' of water with no deco or safty stops or exit points or any other considerations to consider. A lot of these dives are search and recovery dives looking for goodies left by the drunks. I'll start the dive down around 50' sweeping back and forth up to 40' 30' 20' 10'.... If I'm in 15' of water with 500 PSI in an LP95 I'll breath it down to 200 psi without concern. That extra 300 psi is 11cf of gas. That's enough gas for another 13 minutes of diving.

I've done this countless times and never gone out of gas.

 

[MrChen]

I was asked to leave the tank outside by the door and the valve open on my last tank rental when I was returning it. This had a nitrox fill on it.

 

[The Chairman]

In the dark ages, before SPGs were the norm, almost every dive ended with us being OOA... at least until we pulled our reserve rod. Tanks were quite often brought into the shop completely drained and sometimes with some water intrusion. Give a tiny bit of salt water a year in a tank with elevated pressures and the corrosion can render it unsafe. It's why the industry went to the yearly inspection program and also why keeping air in the tank became important.

In reality, it doesn't matter why you try to finish your dives with a reserve as long as you do it. Dive shops are going to try anything to get you to dive safely. Obviously, they think you'll care more about the tank than yourselves.

 

[redacted]

As long as there is pressure in the tank, nothing will enter that tank except through a fill whip. So shops usually check pressure before a fill so they7 know the only thing in that tank is what they or some other fill op has added. Of course, that does not keep a fill op from screwing up and getting water in your tank but tehy seem to gain some comfort (or charge for another VIS) by thinking they know the user did not screw it up.

Now, is it really so bad to drain YOUR tank. Not if you exercise just a little care not to let anything harmful enter. And ambient air that would enter through an open valve or a removed valve is not harmful unless you also allow something else in. But the stories are interesting.

I like to inspect my own tanks periodically - before a VIP and after any suspect fill.

Be good or Santa will leave you nothing but coal.

 

[DivemasterDennis]

As top the poster's question, no one has answered yet. Breathing a tank down to "no air available" will not necessarily damage the tank. But it's not a good idea. Some pressure in the tank keeps things out of it that we do not want to enter. Do not store tanks empty. Store at about 200 psi. As to impact on the tank from breathing it to zero, it will depend upon tank age, condition, and how long it is left at that level. Some dive concessionaires will get bent out of shape if you breathe a tank down because they may have a policy of inspection and drying when that happens. If it's you tank, you will want to take good care of it, but don't necessarily has to have an inspection if you breathe it down occasionally. (but do get the regular recommended inspections- visual and hydro, as prescribed.)
DivemasterDennis

 

[The Chairman]

There are very few circumstances where this would be appropriate.

I've done this countless times and never gone out of gas.

Actually, I'm sure you could count them if you wanted to, and like I said: few! I have done the same on a Venice Beach, Fl dive looking for sharks teeth. I've done it in a pool like spring (Alexander). I would have issues with a diver doing the same diving off of a boat in key Largo. Heck, I would have issues with myself for doing that. To be sure, I rarely get on the boat with less than a third of my tank left. But like I said, I'm cautious and don't like the needless drama of running out accidentally running out of air.