Sucking a steel tank dry

[Clammy]

So what constitutes sucking a steel tank dry (or any tank)? I know it's really bad because of the rust situation. I've sucked one or 2 of my hp100 tanks pretty damn low. One low enough that my transmitter read no pressure (but my spg still read 100+ pounds). I'm just wondering how low you have to go before you have to worry about getting a vis on it. When I came top side and closed and cracked the valve, air still came out.

 

[ean3x]

I wouldn't worry about it. As long as you have some pressure in the tank there is no way for outside air (14 psi) to get in. Even if you let all of the air out of the tank and closed the valve, there is no way air could get in. Think about it this way, the seals are able to hold 3500 psi in, they can certainly hold 14 psi of air out. The only time you have to worry about it is when you let the air out and leave the valve open for an extended period. Even then there really not that much to worry about.

 

[Clammy]

Well I should have mentioned, I was still underwater when I dropped one of my tanks to under 200. Although.. I was able to breath from it all the way to the surface so I suppose there shouldnt' have been a problem.

 

[Clammy]

This is me being stupidly overly worried only because on my last vis on one tank I had some rust spots... damn boat fills!!!

 

[Buccaneer]

This is me being stupidly overly worried only because on my last vis on one tank I had some rust spots... damn boat fills!!!

You know I never really thought about boat fills! I'll bet that is what happened to my tank. Get this- I went to my LDS for a VIP (they did the previous one the year before) and they found rust to the point that it needed a tumble. The worst part was my LDS brags about their hyper-filtered air, so when the manager saw my tank, he started accusing me of getting my fills somewhere else- and he was serious, and kinda bent about it! I couldn't believe his attitude, especially since I had been divemastering their for over 2 years. Oh well-

 

[DA Aquamaster]

Yes, it seems no shop ever wants to admit to themselves that their compressor may have a faulty coalescor or filter stack so the default response to accuse the rusty tank owner of cheating on them with another compressor.

Bad fill techniques are the other way water gets into tanks. If a drop or two of water is present in the tank valve or in the fill whip, it will get blown into the tank during the fill. A fill operator should ALWAYS crack the valve on the tank to blow out any water that may be inside the valve downstream of the seat. And if there is any water anywhere close to the fill whip, he should do the same with the whip prior to connecting it to the valve.

With regard to boat fills, I suspect filling tanks that have just been in the water and are exposed, along with the fill whip, to spray on the boat accounts for far more water in tanks than faulty moisture separation and filtering.

 

[dannobee]

With regard to boat fills, I suspect filling tanks that have just been in the water and are exposed, along with the fill whip, to spray on the boat accounts for far more water in tanks than faulty moisture separation and filtering.

You haven't been around some of the SoCal dive boats.

I always VIS my tanks when I come back from a Channel Islands trip, just to be sure. Most boats are good, at least one had moisture problems for a while.

 

[WD8CDH]

As long as air still comes out when you crack the valve you haven't sucked it dry.

 

[Teamcasa]

This is me being stupidly overly worried only because on my last vis on one tank I had some rust spots... damn boat fills!!!

Don't worry, around here rust spots are pretty common and I suspect you are right about the boat fills.
ean3x is also right about the pressure. Any pressure of 15 psi will keep the outside air out.

 

[drbill]

I've had two HP120 tanks fail... one on its third hydro and the other on a VIP a year after its second hydro. The first was attributed to hot fills (failure due to over-expansion) by the manager of the shop that filled it 95% of the time. The second was undoubtedly a result of LOTS of diving (300-350/year) and wet boat fills. California dive boats are notorious for this. Of course I won't stop diving them... I just switched to my neutral buoyancy aluminum 3300 psi tank and haven't had a problem with it (first hydro was in 1976).

However, Brian, I'd be careful about draining your tanks down. You showed not only great diving skills but also air consumption on our dives together. What happened this time?