Why is the 3000 PSI Fill Standard at Most Dive Destinations?

[Doc]

My intuitive assumption is that the dive operators in "vacation" locations prefer to restrict the dive times and try to do this by short-filling to only 3000 PSI. Am I off base here?

To your original question, then.

-They do not wish to strain their compressor systems, an extra 500 psi really "costs" a whole lot versus what you get in return.

Some ops supply 100 cu ft tanks and only run them up to 2500psi, saving their compressors and simultaneously confusing the guests and having them engage in breakfast mathematics. There is also major fussing with weights, and you really get to see who the real divers are as the tanks cam straps need to be adjusted.

-Many resorts do overfill, but this is to compensate for leaky threads (or valves) on any given tank in a resort environment.

You can see this on the "turnover day", usually Sunday. The tanks all are sitting there, some as long as from Friday night. By Sunday morning they have leaked down- some as low as 2300psi. Guest complaints are heard and after the first dive they're thrilled with newly filled tanks at 3200psi.

 

[BuoyantC]

Did I see someone just post there is less buoyancy shift in steel than aluminum tanks? I thought we educated those folks already.

For what it's worth, I love my steel HP diving dry in the cold Great Lakes, but here in the warm Marianas water, I prefer an AL80 or I'm overweighted and my trim is off. And don't ask me why, but I never notice any buoyancy shift anyway.

Al80's are just so standard around the world, function well, are corrosion resistant and durable, why would anyone want to change it?

 

[DA Aquamaster]

It is common knowledge that air and nitrox both weigh less in a steel tank than in an aluminum tank of the same volume. My personal theory is that the lighter aluminum molecules use up all the lightness in the immediate vicinity causing the gas to weigh more in the AL tank.

Kidding aside, I suspect the poster meant to say something more along the lines of most steel tanks are neutral to slightly negatively bouyant when empty rather than 4.4 lbs positive like an AL 80 so there is less of the "swing" for a similar volume of gas ending up on the positive side of neutral buoyancy for the tank.

 

[shoredivr]

Kidding aside, I suspect the poster meant to say something more along the lines of most steel tanks are neutral to slightly negatively bouyant when empty rather than 4.4 lbs positive like an AL 80 so there is less of the "swing" for a similar volume of gas ending up on the positive side of neutral buoyancy for the tank.

My bad, that's indeed what I was getting at in a poorly phrased way.

 

[BuoyantC]

I believe ya'. It just grinds me when I hear even dive instructors condemning AL tank use for students because it causes them to POP up when near empty!!!! stooopid.

However if you are weighted properly, It really shouldn't make any difference (steel or aluminum), other than the initial amount of weight needed to achieve proper buoyancy.

 

[String]

And you believe watercooling any tank during filling will benefit from this.

Sorry to disappoint you,the heat is generated inside the tank,and it takes hours to cool a realy hot tank to cool down with some water on the outside

You fill it in a cold water bath. Its standard and works very nicely. Done it myself many times as to all the reputable shops. Its far more efficient than waiting for a tank to cool once hot as you're keeping it cool whilst filling.
For the same gas flow rate you get less heating for a tank in a proper cooling water bath then you do with one just air cooled sat in a room.

 

[String]

I doubt the dive operators short fill in an effort to restrict dive times.

Having worked in busy resorts ive not come across one that does this because it causes more problems than it solves.

Firstly people bitch if they get below 190 bar (some will bitch if < 220 but 190 EVERYONE does).

Secondly the massive variations in SAC for the customers mean its non sensical. You may well have a group where 1 person can get there 232 bar in 20 minutes and another that makes 190 last all day. You have no way of knowing who gets that short fill and if they all get that low fill you're looking at maybe a 10 min dive before turning the group thereby pissing EVERYONE off. You also get people then pushing their reserves more and more likely to run out.

There's no benefit at all short-filling tanks for guided dives.

....its FAR better to make them swim against a current or take them a few metres deeper than normal or ideally if they're in shorties just drop below a steep thermocline instead. That way they dont moan and you still get to control the dive time somewhat but if done properly they dont notice so dont complain as they just blame their own air consumption.

 

[String]

An AL tank tolerates wet compressor fills better (on an on-going basis) than steel.

Actually they don't. That's been disproven many times. Al tanks even need extra testing regimes in some areas now as their mode of failure is far more spectacular and sudden.

 

[Eric Sedletzky]

Man I feel lucky.
I love those california dive boats like the Peace boat that let you bring your own steel tank. They bank 32% and fill to 3500 psi hot, which cools to about 3200 - 3300. I'm fine with this because I'm using one of those long thin 7 1/4" steel 3442 120's.
Life couldn't be better.

You can take those aluminum 80's and stick them you know where. If you want to start with the square end that's your thing.

Unless of course I happened to be in a great tropical dive location and all they had was aluminum 80's, then yeah I would use them. But I would also bring an extra cam band and string a 5 lb. weight on it and snap it down around the base of the AL 80 to off set it.
But until my kid's out of college I don't see that happening for a loooong time.

 

[String]

and there's not a buoyancy shift the way there is with aluminum tanks.

The buoyancy shift is simply not true. A tank of a certain capacity holds a specific amount of gas. That gas weighs exactly the same regardless of whether you put it in a steel, aluminium, carbon fibre, plastic, glass or any container.

A 12l tank contains roughly 3.4kg of gas. That gas weighs 3.4kg regardless of what tank its in.

The difference is when empty Aluminium tanks are near neutral or worse, float. That means you need to wear extra lead to keep that tank down. For example a 12l steel maybe 1kg negative and an equivalent aluminium maybe 1.5kg positive. That means 2.5kg more lead is needed on the belt to dive the aluminium tank. The buoyancy shift empty to full is identical though just the overall setup requires more lead.