What size of steel tank for beginner doubles?

[TSandM]

Well, I have to say that the information in this thread has been interesting, and that I had never actually sat down and done the weight and volume comparisons that were done here. I'd certainly be more tempted to buy HP tanks now, despite their starting expense, if the dimensions and balance were the same. So thank you to the people who took the time to do the math for us!

 

[rjack321]

Relying on the statistics kept by PSI, and knowledge gained from divers who were diving in that time. Not to mention we have some tanks from the early 70s, that I still dive and that still carry a plus rating. Also information gleaned from writings of cave divers during that era.

Amazingly, there are guidelines available for overpressured cylinders. They were discussed during my PSI course. The ~100 year service life when overpressuring seemed to be more than adequate to me.

This is total bunk. PSI has never and will never advocate filling a plus rated LP tank to even 2641psi at 70F. Nevermind 3600 to 3900psi. Will never happen, they CYA better than anyone else in the entire scuba industry.

Regarding tanks from the early 1970s, most of those should not be getting plus ratings today. This is because the hydro shops cannot technically, in a non-destructive way, calculate wall stress. They essentially rely on the REE numbers and nobody can figure out what the allowable expansion on a Lp72 should be for a plus rating. So sometimes you'll find 1960s and 70s Lp72s with current plus ratings. But if you dig into the paperwork behind that plus you'll almost never find the proper documentation.

 

[SparticleBrane]

Leadking (Lee from XS Scuba/Seapearls) posted some good stuff awhile back.

A few things:
This post says:

U.S.tensile strength (105,000-125,000 psi for 3AA cylinders) is not the same as European tensile strength (135,000-155,000 psi as required by EN 1964 part 1 and ISO 9809 part 1) and I have test data that shows Faber cylinders delivered to us show a tensile strength of 115,000-123,000 psi for their 3AA cylinders (as required by DOT)

As an example, all of the E cylinders, Exempt or soon to be SP, Special Permit cylinders are made from a harder steel and cannot exceed 159,000 psi yield which exceeds 3AA tensile yield by as much as 50%. As you can see the E/SP cylinders being produced more closely follow the European metallurgy which allows for higher pressures.

A little later from the same thread:

I spoke with our engineers today and found that DOT 3AA cylinders have a typical tensile strength of 105,000-120,000 psi and Worthington's tensile strength on our X-Series (E-14157) is 135,000-155,000 psi with nominal at 145,000 psi which they tell me they hit "pretty well on the money" every time.

So it would appear that Faber LP tanks are in the 115,000psi - 123,000psi tensile strength range, and Worthington X-series tanks are usually hovering around 145,000psi tensile strength.

...is that really such a big difference? Personally I'm not concerned about pumping LP tanks (the newer ones, not the older 72s...) to 3600.

 

[Jimmer]

Leadking (Lee from XS Scuba/Seapearls) posted some good stuff awhile back.

A few things:
This post says:


A little later from the same thread:



So it would appear that Faber LP tanks are in the 115,000psi - 123,000psi tensile strength range, and Worthington X-series tanks are usually hovering around 145,000psi tensile strength.

...is that really such a big difference? Personally I'm not concerned about pumping LP tanks (the newer ones, not the older 72s...) to 3600.

Is it really such a big difference...no I suppose not, 115 000, is not HUGELY lower than 145 000. BUT the 3442 PSI tanks are designed to have a certain factor of safety at 3442 with 145 000 PSI material, whereas the Faber's also have (I assume) the same factor of safety at 2400 but with a lower tensile strength. You ARE stressing the LP fabers MUCH MUCH more when you overfill them to 3500 than when you are filling an X series to 3500.

 

[SparticleBrane]

And yet they just seem to keep passing hydro time after time (if you keep them well maintained -- obviously for tanks that are rusty or corroded this doesn't apply).
They do way more fills down in cave country to these pressures than anywhere else I can think of. If a tank explodes from overpressurizing, you'd hear about it. I haven't heard of one yet...

 

[Jimmer]

I'm not arguing that they pass hydro and don't explode, but that doesn't make it a great idea. If you want 130's buy 130's, don't buy 95's and then pump the hell out of them to make them 130's. I have 2 sets of LP Fabers, and they are kick ass reliable tanks, but that doesn't change the fact that they are being pressed well beyond design specification when being grossly overfilled.

 

[SparticleBrane]

Meh you take all the fun out of it.
Luxfer says the minimum burst pressure on their tanks is 2.5x service pressure. That's 7,500 psi for an Al80.
I'm definitely not worried about steel. Crank those suckers up.

 

[BeckyKagan]

Am I the only one here who thinks that 3900 psi in a 2400 psi tank is a little insane?

Saying it is "ok" because "everyone else does it" is still a logical fallacy and does not make it any less insane.

I agree Aquamaster. Unfortunatly a lot of N.Fla shops fill to 3800-4000psi and there have been accidents. If your having to overfill tanks that much get some larger doubles or a rebreather. That kind of pressure isnt good for the tank, your regulators or my safety.

WHoever said they'd send their dive buddy back to get their tanks filled if they were at 2640 id tell you find another buddy because im not diving wtih someone who has to overfill their tanks that much for another COUPLE hundred psi turn pressure.

Just my 2 cents =)

 

[WD8CDH]

If the LP95 was the most common tank and it was usually overfilled to 4000psi, then I would absolutly use an HP119 instead. Exactly the same capacity at the same pressure, same carrying weight, same buoyancy, same size but greater safety due to the stronger steel. I would NOT substitute a LP95 with a HP100 because I would then have reduced capacity at the same fill pressure.

Is the trim the same?

Yes, The trim would be the same and the capacity of either one would be the same at 4000 psi.

 

[Scared Silly]

This is total bunk. PSI has never and will never advocate filling a plus rated LP tank to even 2641psi at 70F.

I would have to do some digging but IIRC they do have some guidelines for filling and compensating for temps above 70 degrees. But irregardless, once the temp is 70 degrees it the final pressure must not be over the rated pressure. These are the guidelines and as has been discussed what is done on practical matter is often different.


One thing not mentioned in the HP vs LP cylinder differences are the buoyancy differences. LP cylinders tend to be more buoyant than HP. Take the Faber LP85 versus HP100 and put the exact same amount of gas in them. I do not care if it is 100cuft or 10cuft. The HP cylinder will be around 3 pounds more negative. that becomes 6 pounds when doubled. Coincidentally, the HP100 is also 3 pounds heavier dry (6 pounds when doubled). This may not be much but for me it was. I do not need the extra 6 pounds even when diving dry. And I certainly do not want to be schlepping more weight than I need to. So I went with the LPs. So outside of the HP is really a LP but just put what you need in it argument I think there is a need for both.