tank size and pressure

[CuzzA]

I'm amazed that people will recommend overfilling LP steels to recreational divers who are asking general questions about cylinders.

There are so many reasons why it's a bad thing to recommend.

1. As a practical matter, overfills are simply not available to most recreational divers. Dive shops outside cave country don't offer them.
2. HP steels have been in production since 1987 are are readily available new or used. People who want HP fills can get HP cylinders.
   
3. Burst disc failures can cause injury or property damage. Many if not most divers getting "cave fills" use an over-spec burst disc or blanking plug. Rec divers who don't realize that a burst disc change is part of the package are at greater risk from a failure. I believe @CuzzA had this happen. On the other hand, over-specing or blanking out a burst disc poses its own risks.
   
4. While there are not believed to have been any cylinder ruptures caused by cave fills, that doesn't mean the practice is safe enough for general recreational diving. a) There simply haven't been enough cave fills performed to draw a conclusion, especially to the higher pressures now considered acceptable. b) The field experience with cave fills is limited to those cylinders typically used for cave diving and should not be generalized to other types of LP steels, especially older, smaller cylinders such as LP72s and some LP50/LP52s.
   
5. While the jurisdictional questions are complex and enforcement spotty, it is unlawful under some circumstances.

Yes, it did happen to me. Tank damn near did a flip. I don't know what the exact pressure it let loose at, but it was unlikely to be no more than around 3200 psi. I don't recall the burst plug that was on the tank. I think it was rated at 4k, so probably just failed from old age, not to mention it was simply too close to the burst rating.

I've since put 5250 plugs on my LP steel tanks and have made it a practice to swap them at hydro. As far as overfilling LP tanks, I think you laid out some fair reasons a diver shopping tanks should consider before purchasing one. Mainly the availability of getting the fill they want. As for the safety of filling these tanks, that should be up to the individual. Tens of thousands, if not hundreds of thousands of LP tanks have been overfilled over the course of several decades and not one well maintained, in hydro and current vip LP steel tank has let go. Compare that to aluminum tanks and I think it's good enough evidence that an overfilled LP tank is probably safer than an aluminum tank with the risk of either tank exploding being very, very low.

I generally fill my tanks for what I need. Usually either around 3k or 3.5k. Some people always fill them to 4k. I don't dive in caves and have yet to need that much gas for a dive trip. The biggest utility is for being able to carry enough gas to make multiple drops offshore, yet carry fewer tanks.

 

[BRT]

I'm amazed that people will recommend overfilling LP steels to recreational divers who are asking general questions about cylinders.

There are so many reasons why it's a bad thing to recommend.

1. As a practical matter, overfills are simply not available to most recreational divers. Dive shops outside cave country don't offer them.
2. HP steels have been in production since 1987 are are readily available new or used. People who want HP fills can get HP cylinders.
   
3. Burst disc failures can cause injury or property damage. Many if not most divers getting "cave fills" use an over-spec burst disc or blanking plug. Rec divers who don't realize that a burst disc change is part of the package are at greater risk from a failure. I believe @CuzzA had this happen. On the other hand, over-specing or blanking out a burst disc poses its own risks.
   
4. While there are not believed to have been any cylinder ruptures caused by cave fills, that doesn't mean the practice is safe enough for general recreational diving. a) There simply haven't been enough cave fills performed to draw a conclusion, especially to the higher pressures now considered acceptable. b) The field experience with cave fills is limited to those cylinders typically used for cave diving and should not be generalized to other types of LP steels, especially older, smaller cylinders such as LP72s and some LP50/LP52s.
   
5. While the jurisdictional questions are complex and enforcement spotty, it is unlawful under some circumstances.

I sold all my old 72's. When the guy got them he emailed me and told me how well they worked as 100's. Some of these were almost as old as I am, and I'm old. Don't ever think that people are not filling steel 72's regularily to high pressures. Still no incidents.

 

[2airishuman]

You were saying overfilling cylinders is bad and dangerous, I doubt the tank you're overfilling cares if you're a rec or cave/tech diver.

No, but cave divers have a different level of risk acceptance than open water divers.

There's no evidence to support the claim that overfilling steel cylinders in good shape is dangerous.

Neither is there sufficient evidence to support the claim that it's perfectly safe. How many cave fills have there been in the history of cave diving? There are thought to be about 15,000,000 recreational dives made in North America per year. There are millions more non-diving related cylinder fills (mainly welding, soft drink, and medical gases) every month. There's been maybe one rupture per billion fills. How many more cave fills will it take to demonstrate a comparable safety record?

You also said that you don't count overfilling by 10% as overfilling, which is BS because every single psi you fill above the working pressure of the cylinder is essentially an overfill.

Well, the thing is that the industrial gas industry has been doing that since 1944, and scuba divers have been doing it since the 1950s, and there have been billions of fills to show that it's safe. Billions, with a B.

I'll continue to overfill my 232 bar steel cylinders and so will every other diver that I know, because there's no evidence at all to support the idea that it's bad.
I won't fill them to 150% of working pressure, but I won't hesitate for a second to have them filled to 120-125% if possible.

You do what you want.

I just wish we wouldn't encourage people who don't even ask about overfills to go out and get an LP cylinder, and overfill it, when there are better alternatives available.

 

[CuzzA]

FWIW, it's not just limited to cave divers. Plenty of Eastern seaboard wreck divers, commercial and recreational spearfisherman and probably quite a few normal recreational divers who appreciate the extra gas that can't be acquired in any other way other than multiple tanks or a CCR.

We don't know how many fills, a million? Lets say a million... with zero failures. Pretty damn good. The actual dive you do probably has better odds of an accident.

 

[NAM001]

K...

With compliance being a good thing...

W...

absolutely it does. at the same time there are relative time proven safe risks to take. that still does not make it right, only statistically safe to date, and that could change in a heart beat.

so goes the battle with the word intent and logic.

 

[2airishuman]

FWIW, it's not just limited to cave divers. Plenty of Eastern seaboard wreck divers, commercial and recreational spearfisherman

If there had been an accident or incident in the cave community we would know. Other more individualistic groups less so.

and probably quite a few normal recreational divers who appreciate the extra gas that can't be acquired in any other way other than multiple tanks or a CCR

I'm going to push back there and say that:

• There is such a thing as an HP120. I have 7 of them. My average purchase price was under $200 each. Or you can buy new ones, DGX has them in stock for $399 which includes a VIP and a nice valve.
  
• If you don't like HP120s because they are too tall, well, there are HP117s; or if 120 cf isn't enough you can get an HP130.
• There is such a thing as an HP149, DRIS' web site says they have new ones, used ones come up for sale regularly

Now there are people who say that there favorite pet LPxx trims out just perfectly for them but that the HPyy of similar dimensions just won't but the reality is that if you want more gas, there are options out there that work for nearly all divers that will give you more gas without resorting to an overfill.

We don't know how many fills, a million? Lets say a million... with zero failures. Pretty damn good. The actual dive you do probably has better odds of an accident.

Well, using that line of reasoning, if the odds of a cylinder rupture are one in a million, look at what it does to dive safety.

According to DAN there are an average of 4.7 fatalities per million dives. Most people on SB believe that their own risk is lower because the 4.7 figure includes some number of medicals, perhaps as many as 50%, and includes fatalities in various high-risk groups such as people making their first 20 dives after certification, people making dives far outside accepted norms and practices, and so on. So for you and me and the OP and most other people on SB, the chances of any one dive ending in a fatality are probably closer to one in a million.

Now, if the odds of your cylinder rupturing are also one in a million, you've just doubled your wife's chances of collecting on your life insurance policy.

Which is my point. One in a million is not good enough because it's a still a relatively large avoidable risk in an activity that is largely very safe. One in ten million would be enough that there would be threads raised at A&I and a hue and cry for improvement. But we don't know, because there aren't that many overfills.

 

[Basking Ridge Diver]

Now, if the odds of your cylinder rupturing are also one in a million, you've just doubled your wife's chances of collecting on your life insurance policy.

Your pushing too hard for a cause that does not impact you. Not sure why the push back - I read your posts and mostly your points seem reasonable.

Like all things - if you don't like it - don't do it. But you don't have to go all out to show everyone that disagrees with you - they must be wrong... We get your point and we disagree with it. Others may or may not disagree as well. But this seems like overkill - state your position and let it ride...

Safe diving.

 

[CuzzA]

If there had been an accident or incident in the cave community we would know. Other more individualistic groups less so.



I'm going to push back there and say that:

• There is such a thing as an HP120. I have 7 of them. My average purchase price was under $200 each. Or you can buy new ones, DGX has them in stock for $399 which includes a VIP and a nice valve.
  
• If you don't like HP120s because they are too tall, well, there are HP117s; or if 120 cf isn't enough you can get an HP130.
• There is such a thing as an HP149, DRIS' web site says they have new ones, used ones come up for sale regularly

Now there are people who say that there favorite pet LPxx trims out just perfectly for them but that the HPyy of similar dimensions just won't but the reality is that if you want more gas, there are options out there that work for nearly all divers that will give you more gas without resorting to an overfill.



Well, using that line of reasoning, if the odds of a cylinder rupture are one in a million, look at what it does to dive safety.

According to DAN there are an average of 4.7 fatalities per million dives. Most people on SB believe that their own risk is lower because the 4.7 figure includes some number of medicals, perhaps as many as 50%, and includes fatalities in various high-risk groups such as people making their first 20 dives after certification, people making dives far outside accepted norms and practices, and so on. So for you and me and the OP and most other people on SB, the chances of any one dive ending in a fatality are probably closer to one in a million.

Now, if the odds of your cylinder rupturing are also one in a million, you've just doubled your wife's chances of collecting on your life insurance policy.

Which is my point. One in a million is not good enough because it's a still a relatively large avoidable risk in an activity that is largely very safe. One in ten million would be enough that there would be threads raised at A&I and a hue and cry for improvement. But we don't know, because there aren't that many overfills.

I think you misunderstood. As of right now our made up odds are 0 in a million.

As for gas capacity. A OMS LP108 at 3k is 136 cuft. At 3.5k is 157.5 cuft. And at 4k it is 180 cuft. Now I think 4k is too extreme for my taste, but in cave country it's just another fill. Nevertheless, if I can get a third full dive out of that 3.5k fill, that's 6 dives out of 2 tanks. A great day. Add 3 more guys doing the same, that is 8 tanks. Now if I can only get 2 full dives out of a HP 120 we're hauling 12 tanks. Not terribly more, but you might be wasting gas after every trip. How about an overnight trip. We're talking 16 tanks vs 24. It may not sound like much, but that's another 320+ pounds those engines have to push. So now we're burning more fuel, paying for more fills, more vips, more hydros... you get the point.

I am not advocating anyone overfill a tank. Just like I wouldn't tell anyone they shouldn't given the flawless success rate. I agree with you, we should use caution when giving advice to someone buying a tank. Most tank threads mention the LP and HP overfill or normal issue. But really at the end of the day they should talk to the guy manning the compressor because he controls how much gas you get and is the one who has the highest risk from a tank failure. He will pretty much make the HP vs LP vs AL decision for you. If your shop can't fill past 3k, a LP tank looks pretty attractive compared to an AL80.

 

[CuzzA]

I had not read this thread, until just now, but perhaps I am wrong or someone else is. It's my understand an OMS LP108 is rated 108 cuft. at 2400 and 112 cuft. at 2640. You'll often see both volumes listed which makes it confusing.I believe this is not common as most LP tank volumes are rated at the plus pressure.

 

[CuzzA]

It also appears I am just a broken record here. Should have read the thread first.