Don't breathe tanks to zero?

[tplyons]

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.

In all fairness, the DSAT Gas Blender manual and course teaches when you're blending helium mixes, if you didn't fill it, you start over with an empty cylinder.

It reads: "As a blender, if you did not personally mix the gases for the previous dive, you have no way of knowing what combination of gases are in the remaining mix without an oxygen analyzer and a helium analyzer. In this case, you must always start with an empty cylinder." -- DSAT Gas Blender Manual, p. 45.

As I neither dive nor mix helium blends, I've never done this. But as far as Nitrox goes, I never drain unless I need to bleed down to hit a specific mix.

 

[chrpai]

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.

Please. We don't all get to live in the Keys and go diving on our own boat whenever we want. For some of us getting out there and doing a few dives is a real treat and there's no way I'm getting back on the boat (barring a crappy air hog buddy) with 1000 PSI left in a tank when I'm in 20' of water and I know exactly where the boat is. I won't be breathing it down to 200 either though in that situation.

I'm all for safety, but let's get back to the real world instead of the classroom world.

 

[captain]

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.

Back in the day most water got in tanks from poor fill practices and compressors that didn't have the proper driers, not breathing them empty.

 

[boulderjohn]

The concept expressed by the OP is taught to some degree by at least one OW training agency as a part of the OW course. When I get to that point, I modify the instruction to include the points made by a number of people above. The reason I bring this up is that a number of shops will require a very unnecessary visual inspection whenever a tank is taken below a certain PSI. While a few of such shops may be doing it to milk a few dollars out of unsuspecting customers, I believe most of them are just repeating what they have been taught in the past and have not thought through completely.

BTW, one situation in which tanks are intentionally taken to low pressures during a dive is one form of technical diving. In such diving, divers carry double tanks as their main gas supply, but they start the dive breathing from another tank that they carry on the side. When the pressure on that first tank gets low, they switch to the main tanks. They will be sure to keep a very generous reserve in the main tanks for purposes of safety.

 

[The Chairman]

I'm all for safety, but let's get back to the real world instead of the classroom world.

Most dives off of a dive boat here in Key Largo are limited to an hour. I'm sorry, but I hardly ever get to go through half my tank unless I'm on a wreck. Even then, I am constrained by my buddy's supply as well. Unless I'm with Elena, I'm normally not the one to turn the dive.

Back in the day most water got in tanks from poor fill practices and compressors that didn't have the proper driers, not breathing them empty.

Ain't that the truth! However, getting a dive shop to admit to such would take a miracle. They would prefer to blame divers instead! Here's a fun one: http://www.scubaboard.com/forums/ad...does-peggy-work-your-lds.html?highlight=Peggy

 

[chrpai]

Most dives off of a dive boat here in Key Largo are limited to an hour. I'm sorry, but I hardly ever get to go through half my tank unless I'm on a wreck. Even then, I am constrained by my buddy's supply as well. Unless I'm with Elena, I'm normally not the one to turn the dive. Ain't that the truth! However, getting a dive shop to admit to such would take a miracle. They would prefer to blame divers instead!

There's a difference between being limited by a buddy or a time limit set by a boat and being limited by having a rock bottom of 1/3 in a recreational O/W environment. You represented your previous statements as the latter.

 

[The Chairman]

BTW, one situation in which tanks are intentionally taken to low pressures during a dive is one form of technical diving. In such diving, divers carry double tanks as their main gas supply, but they start the dive breathing from another tank that they carry on the side. When the pressure on that first tank gets low, they switch to the main tanks. They will be sure to keep a very generous reserve in the main tanks for purposes of safety.

Since this is in Basic Scuba, I thought we should keep it to OW.

 

[Panama Jones]

Nobody flies with tanks here? (I haven't) Airlines all demand that the be drained and valve open to fly them. Ever seen the moisture on your luggage when it comes off a plane and lands in a hot environment? it's 40 below up there landing in 80 above.

 

[boulderjohn]

Since this is in Basic Scuba, I thought we should keep it to OW.

Yep. I originally intended to make that difference clear in my remarks, but I forgot to include it. My real point for this discussion is that as a result of doing those kinds of dives, I have taken many tanks down to a very low pressure (above ambient) and never had a drop enter them as a result.

 

[The Chairman]

There's a difference between being limited by a buddy or a time limit set by a boat and being limited by having a rock bottom of 1/3 in a recreational O/W environment. You represented your previous statements as the latter.

I was in Bonaire this past week diving. I haven't been there in ten years. I still came up with a thousand or more on most of the dives. Why? We were limited to an hour. You would probably suffer the same fate. I never said it was a "rock bottom" kind of thing. I did post that you should start your ascent with @ ten times your depth or 500 psi, whichever is greater. I get that you want to take exception with everything I write for some reason, and that's OK. Just don't twist my words to something they were never meant to convey. I dive cautiously because I want to see 60 in a few years. I've never been bent or have run out of air since I started to dive SPGs, and that's no accident. Being cautious is good thing.