Slowly piecing together doubles?

[beanojones]

DA, I'm not sure he should even think about overfilling old steel tanks like that. That's a 2250 tank. It lost its plus hydro in 1978. Its 64 cu ft filled to rated pressure.

It's not a 2400 tank with a plus hydro to 2640. It's a 2250, with no plus.

Cheetah, sell that steel 72 to somone who wants to dive it as is. There are plenty of people who like them just fine, but for tech diving that tank is kind of out of it. In Hawaii, you could sell it for $75.00 in a heartbeat (if it is in hydro). Old steel tanks last forever.

DiveRite Express ships any order over $50.00 free, including tanks. Other places have better
prices too.

DA's math is much easier to understand if you do the tank calculation in Metric. In metric, you measure tanks by the liquid capacity, and then just multiply by the pressure to get you available air in litres. Much easier to understand that way. Because pressure is measured in atmospheres, where 1 is what is around you, instead of 14.7. Decompression math is much easier that way too. 10 m = 1 atm instead of 33ft=14.7 psi. (it's not as fun in fresh water.)

 

[Rick Inman]

How do you usually handle having different size tanks from your buddy? Just call the dive when diver x runs out of air on his smaller tank? Force your buddy to get a different tank?

Aquamaster, I read the whole post, and I won't lie - you COMPLETELY lost me on the math...

I wouldn't worry about the math so much right now - that will come with the technical classes you should take. The point is to get that agency thinking (back in the boat with 500psi) out of your head, because it does not translate to doubles or technical diving.

The concept of true gas management must change. I would begin my reading this article by our own NWGratefulDiver, Bob Bailey: NWGratefulDiver.com Once you understand the gas management concept, you can then begin to understand the math, sac rates, tank factors and volumes and how to apply that to your diving.

One thing about doubles, it's perfectly ok to end a dive with lots of gas left.

 

[TSandM]

I have dived doubled LP85s and LP95s, and doubled HP100s. For me, those three trim out similarly, but the HP100s give you a bit more gas for a bit less weight. Since you are in SoCal, it's HIGHLY unlikely you will be finding a shop to give you cave fills, but it shouldn't be difficult to get a full HP fill in 100s.

The best thing with doubles is to hook up with some folks who dive them, and try several different sets. Some combinations of tank and body just don't work, and some are far more comfortable than others. (I don't like double 72s, because they put me on my head, but my husband can trim them out fine.) You shouldn't have any problem in Southern California, finding some doubles divers to hang out with.

 

[CuriousRambler]

Thanks for the article, Rick! It explains everything a bit better for sure. I've already had the 72 listed for sale on the boards, but I thought I'd consider the options before anyone popped out with a serious interest. I'll see what I can do to get in a few sets of doubles to see what works, but I figure there will be at least a few dive curve where the entire rig will be so foreign that I won't know what's good or bad haha.

 

[TravisD]

DA, I'm not sure he should even think about overfilling old steel tanks like that. That's a 2250 tank. It lost its plus hydro in 1978. Its 64 cu ft filled to rated pressure.

I thought that + ratings had nothing to do with age, and were simply a factor of the hydrotester confirming that it meets the REE specs at time of testing? There's nothing that would prevent a tank that was not + rated from regaining that rating, unless it can't meet the requirements of the test.

 

[Gombessa]

I thought that + ratings had nothing to do with age, and were simply a factor of the hydrotester confirming that it meets the REE specs at time of testing? There's nothing that would prevent a tank that was not + rated from regaining that rating, unless it can't meet the requirements of the test.

That's correct. I'd venture to say that the vast majority of LP tanks that have "lost" their plus ratings have done so simply because they were not plus-tested during subsequent hydros, not because they failed to pass.

However, the local tech shop here (that doesn't bat an eyelid when I ask for 32-3500psi fills on LP80s) is VERY wary of overfilling our old 2250psi Lp72s past 2475. That's enough for me to not do it.

 

[CuriousRambler]

So aside from the obvious of holding more air, what are the pros and cons of getting HP tanks instead of LP tanks? I'm noticing a lot of places only carry a couple LP tanks, but an entire line of HP or FX tanks.

 

[Gombessa]

So aside from the obvious of holding more air, what are the pros and cons of getting HP tanks instead of LP tanks? I'm noticing a lot of places only carry a couple LP tanks, but an entire line of HP or FX tanks.

Pros: No need to find a shop willing to "overfill" to get you to 100cf. Even if you do get short-filled, you'd probably still have ~ 80cf, comparable to an AL80, with the buoyancy benefits of steel tanks. Potentially less hassle every five years over finding a hydro facility capable of plus-rating the tanks.

Cons: Some shops (and particularly boats) may only have compressors that can fill to ~3000psi, leaving you with a short fill in your HP tanks. Also, some shops may mistake the tank for an LP and short-fill you. Higher pressures may increase the strain on your regulator's high pressure seat, requiring more frequent servicing (though I doubt more than the 1-2 years recommended by manufacturers anyways). 300bar DIN-only (only an issue if you're running singles and have yoke regs).

 

[DA Aquamaster]

A steel 72 is still a 3AA steel tank made to the same engineering standard as a 2400 psi service pressure LP tank. People either forget that or do not know it.

I know shops get all sweaty filling steel 72's and many will not even fill them past the 2250 psi service pressure - but will fill a 2400 psi tank that may be almost as old to 3600 psi. The thing is that if a 3AA steel tank is well maintained it will last for a century or so with no real fatigue issues. I have seen welding tanks from the WW1 era pass hydros and do it with a plus ratings.

So in short, I don't sweat filling my steel 72 doubles to 2600 or 2800 psi. You'd have to push it to 3200 psi to get the same overfill you get when pushing a 2400 psi tank to 3600 psi.

I would not try to take a single steel 72 and turn it into doubles, as it is a pain getting two of them to match and to find 6.9" or 7" bands, bt I woudl not pass on a wel matched pair or an existing set of double 72's. They make great tanks for offshore diving with nitrox in situations where the bottom times are in the 20 minute range at 150' or so.

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I agree that it is easy to do the math in liters, but only if you are working with an SPG in bar and know your sac in liters per minute.

If I know my 100 cu ft tank is a 12 liter tank (holds 12 liters at 1 bar, 14.5 psi absolute) I can multiply the number of bars I used to get the volume used (for example 50 bar x 12l = 600 liters)

But that is the point of a tank factor as well. If I know that my tanks hold 5.8 cu ft per 100 psi it gets real easy to figure out how many cubic feet I used or have left. If I used 700 psi I just need to be able to multiply 5.8 times 7 (take the 7 times the 5 to get 35, then take 7 times the 8 to get 5.6 and add them together to get 40.6. Or if you are math impaired, just round the tank factor upt o 6 and do 6 x 7 = 42. Close enough for government work in most cases.

On the other hand mixing the two systems is a problem.

To use both liters and PSI I have to know several things.

100 cu ft X7-100 = 12 liters internal volume

3442 psi = 237 bar

.6 cfm sac rate = 17 lpm sac rate

1 bar = 14.5 psi (round it to 15 to make the math easier)

But then the mental math gets really complex. So use one sytem or the other but don't mix them.

 

[Padipro]

Pros: No need to find a shop willing to "overfill" to get you to 100cf.

If you're diving a set of LP 95's or 108's you wouldn't need to "overfill" them to have damn near 100cuft as these tanks are 95 or 108 cuft at the rated pressure.

Cons: Some shops (and particularly boats) may only have compressors that can fill to ~3000psi, leaving you with a short fill in your HP tanks.

I dived the old style, 3500 psi, HP 100's for years and I never had a problem getting them filled to 3500. In the US anyway, you may find a problem with this other places though.

Also, some shops may mistake the tank for an LP and short-fill you.

I find it hard to believe that anyone could mistake an HP tank for an LP and short fill it. That would be like mistaking an AL tank for an LP steel. They are completely different in size, a 100 cuft HP steel is about the same size as an AL 80 where a LP 95 or 108 is the size of a small school bus.

300bar DIN-only (only an issue if you're running singles and have yoke regs).

The new HP tanks are only 3442 psi and have a 200 bar valve with a removable insert, the exact same valve you find on many newer AL tanks. You can use both a yoke or a DIN reg with these newer tank valves. The older style HP tanks, like the ones I have, are 3500 psi and have a 300 bar valve which will not work with a yoke reg. In fact the thread size on these tanks is different and it's extremely difficult to even find a valve that will fit them any more. I installed a 300 bar H halve on mine and had to special order them because only a few manufactures made them and almost no shop carried them. You can't even buy these older tanks at a dive shop any more.