Why choose LP tanks over HP Steel tanks?

[Storker]

I've only seen a couple of 300bar cylinder, not popular here because of weight. I'm presuming twins would be better than a single 300bar for gas volume, maybe 300bar better as a single for weight in cold climate but I know @Storker uses 300bar so perhaps he can shed some light as to teh benefits (apart from the obvious)

Pro 300 bar: Lower water volume, so:

• Lower buoyancy, so less lead on the belt
• Takes up less space in the trunk for the same capacity

Con 300 bar: Compressibility kicks in at 230-240 bar, so:

• Capacity only ~90% of nominal due to compressibility
• A PITA to calculate partial pressure blending
• For a decent fill, they really should be allowed to cool down, and the be topped up. Even if I overfill my tanks by ~10%, if I don't top them up, they're down to ~280 bar when I'm assembling my kit.

About total weight: up to 12L, they're ok. 15L 300 bar tanks are a b!tch to schlep (I had one of those, but hardly ever used it since I didn't need that much gas for a single dive and it didn't hold enough gas for two proper dives). My 10x300s weigh the same as what the 15x200 I had (and sold) weighed.

The 300 bar tanks/twinsets I've see around here have been 2x6, 2x7, single 10 and single 12L. I've standardized on 10L 300 bar since that gives me a couple of kilos less weight to carry compared to 12x232s, and at least 4kg less compared to 15x200s. And I have some 27-2800 surface liters, which is basically the same as I get from a 12x232 and only some 10% less than what I get from a 10x200.

Some people claim that getting 300 bar fills can be difficult, but I've never had that issue where I've dived with my own tanks. I guess there are regional differences.

 

[Storker]

You have 200 and 300, no?

And 232. 12x232, D8.5x232 and D12x232 are quite popular up here. The two former are rec sets, while the latter probably is the most popular tech twinset. It gives you about 5500 surface liters, or some 200 Cu.Ft. You can also find D15x232 and D18x232, but at least the D18s are mostly for really long dives if you don't have a rebreather.

 

[runsongas]

The only HP aluminum I know of are barely HP. The standaard AL 80 is filled to 3,000 PSI, while the high pressure tank is filled to 3300 PSI--barely a difference in pressure.

luxfer has the carbon fiber overwrapped aluminum 106

L6X® Hoop-wrap SCUBA cylinder

 

[BRT]

That's obvious.

So my question to that would be, is there a huge difference in what pressure shops will fill to (to justify LP over HP) in the US?

Every-where I've been Ali's get filled to 200-210bar and those holiday op's offering steels fill them to 240..

If some handed my a tank with 2400 (165) they'd get a Paddington stare....

Why? You accept your AL80s with short fills. An AL80 with a 200 bar fill is 75cf. An lp85 with 2400 psi is 77cf

 

[BRT]

Maybe my question wasn't clear.

There must have been some historical reason for the LP and HP They weren't' invented to get around shops laziness..

East of teh pond, this is just an alien concept. The choices are Steel or Ali, which you choose between for buoyancy and gas volume.

As I understand it (please correct me) you not only have Steel and Ali but LP or HP of both.. very confusing and trying to understand where the concept/requirement first came from

LP tanks are one spec. 3500 psi tanks are that same spec at a higher pressure with DIN valves to meet regulations with the 3500psi pressure. 3442 tanks are done under some sort of an exemption that allows them to have the yoke valves because they are not 3500 psi tanks. All caused by regulations.

 

[runsongas]

the 3442 psi and 3500 psi exemption tanks have a slightly different spec than the LP 3AA spec tanks. their hydro pressure is 3/2 instead of 5/3 with 3AA. the 3442 psi pst tanks have the same special permit as the 3500 psi, just the lower service pressure allows them to qualify for 232 bar valves with standard 3/4 neck instead of being forced to use 300 bar din.

 

[Jim Lapenta]

So far, except for a few rare instances, I've never had a problem getting my Lp cylinders filled to 2640. Usually, if it's a quick fill like some places do, they'll actually take it up to 3000 and it cools to 2700 or so. I have seen, on the other hand, many, many divers get shorted on HP fills. A place might take them to 3400 but that cylinder is hot and once in the water it drops to 3000 or even less.
When I fill tanks at the shop, no one gets a fast fill. If you want a fast fill, go somewhere else.
When I'm filling my tanks it's usually 4 -6 at a time. So if the banks are charged I'll slowly fill with the 3500 PSI bank to about 2400, move to next and do the same on down the line. By that time the bank is getting low so run the compressor to bring it back up while I top off slow from the 5000PSI bank. By the end of the evening, I've got 6 cylinders cool filled to 2800 - 3000 PSI or a smidge more. When I go to load them the next day I'll check each one. Sometimes they cool to 2600 or so. That being the case I'll bump them up so when I get in the water at the quarry I have 2800 on the LP's.
If I'm going to the Great Lakes or know I'm doing 100ft plus dives? Then they get "wreck" filled before leaving. Kinda like cave fills.
I never fill al cylinders to more than 3000 cool. I used to dive with a shop that regularly would put 3500-3800 in al 80's. I once got one that had 4000 in it.
That was when I was very new and didn't think anything of it. I mean the instructor did this so it was ok, right?

 

[Jim Lapenta]

As to issues with getting LP fills? I've only heard of one shop in West Virginia that actually would not fill current hydro and vis LP 72's because they were more than 20 years old. That's just dumb. I have 72's from the 50's and the 70's that get used on a regular basis.

 

[boulderjohn]

I've only heard of one shop in West Virginia that actually would not fill current hydro and vis LP 72's because they were more than 20 years old.

In a different context, I created a concept I called the reduction funnel, a process by which a perfectly fine idea is reduced to nonsense. Here is how it works in this case.

1. Someone owning a shop learned that some older alloys of aluminum tanks, all of which had been discontinued over 20 years before, had problems with cracking and were dangerous to fill.
2. Rather than have a policy related to specific tank models with that alloy, that person made a rule saying that they would not fill any aluminum tanks over 20 years old.
3. Others heard that rule, liked it, and applied it themselves. It became a common shop standard.
4. Years and even decades passed. The tanks that had the problem had now been discontinued much, much longer than 20 years before, so that number was now meaningless.
5. The reason for the rule was forgotten, and people figured 20 years on aluminum tanks was the issue--they just couldn't be trusted after that many years.
6. Then people missed the part of it being an aluminum issue, so the rule of thumb of 20 years applied to all tanks--no tanks could be trusted after 20 years.

And so an original rule that made sense (don't fill the couple of specific tank brands that used a bad alloy of aluminum for a couple of years) went through the reduction funnel and became an absurd standard that no one understands but will probably live forever. That is especially true now that enough shops have figured out that there is no law requiring training or certification for tank inspection or filling.

 

[Bob DBF]

There must have been some historical reason for the LP and HP They weren't' invented to get around shops laziness..

It was an evolution here, when diving started here most tanks were repurposed from WWII surplus so there were a variety of sizes and pressures dependant on they came from. I think the first mass produced SCUBA tank was the old steel 72 at 2250#, although it might have been the 1800# tank @Sam Miller III was around then and would know.

Since then the 3AA SCUBA tanks have been made in 2400#, 3000#, 3180# service pressures, and in various sizes. The 3000# and 3180# tanks are made by Faber.

During the production of 2250 72 steel tank, but before the higher pressure 3AA tanks were produced, the aluminum 3AL 3000# tank was produced, and is to this day.

The steel tank standard for DOT, earlier the ICC, is 3AA which has a working pressure, and a 10% overfill is it passes certain standards at hydro. HP tanks are made with a special exemption from 3AA standards. Each exemption has its own specifications and exemption number stamped on the tank and is not allowed the 10% overfill. Most are 3445#, the 3500# has been discontinued but is still in use.

Don't know if this helps.


Bob