LP or HP tank?

[NorthWoodsDiver]

To answer the OP's question we need to know how much air you need and what fills you can get.

as example, a low pressure 104 filled to 2640psi is about 104-106 cubic feet however... Technically speaking if you don't get the + at hydro time then your LP tank can only be filled to 2400psi making no longer a 104 so your not getting the full potential of the tank.

In Florida and other places where cave divers frequent there is something called a "cave fill" which unscientifically speaking means jamming as much air as one can into a LP tank up to the hydro pressure or beyond (ie 5/3's the fill rating).  Filling that same LP 104 to 3500psi or so will get you in the ball park of 130 cubic ft (I dont have a calculator handy to get the exact numbers)

Then filling a HP 130 to 2640psi or the rating of that LP tank should yield about 104 cubic feet.

So what does that mean? size wise a LP 104 and a HP 130 are pretty much the same tank. The only major difference is the fill rating. Your going to be carrying around a tank of about the same size and weight.

In the end having a HP tank will allow you to fill you tanks to a higher pressure LEGALLY yet if a good compressor isn't available you will get a good amount of gas even at a lower fill pressure.

IMHO I wouldn't recommend a yoke connection to anyone buying a new tank/reg. though a yoke will work fine on a higher pressure fill it is not designed for "technical" diving and most instructors wont accept yoke regs anymore for tech classes. The yoke handle is a great snag point also. If your on the fence about yoke or DIN go 200bar din, you can buy a threaded insert (or the valves usually come with them) to allow you to use a yoke reg. Generally speaking a DIN valve has a better resale value also. I own almost 20 tanks and a dozen regs and converted everything to DIN a long time ago. its way more expensive to convert them then to buy them already DIN and have a couple adapters laying around...

With all that said I wouldn't recommend anyone go buy a huge cylinder for single tank diving. your more likely to push NDL with a bigger gas supply and having a big cylinder on your back can be unbalanced feeling. getting twin tanks of a smaller capacity will afford you redundancy and they are more comfortable on your back. If you have aluminum 80's laying around use them for doubles, they work well if you have a V-weight to offset the buoyancy of the tanks. besides that 2 aluminum 80's gives you 160 cubic feet of gas  meaning you could use the full potential of what the largest steel single cylinder could provide and not come back empty. Most places can fill to 3000psi too so depending on what fills you can get this should be a factor.

A steel tank will have a better resale value but an aluminum 80 is a handy tank to have around. their light weight design lends them well to staging or decompression use and rebreather divers like them for bailout too.

If your hell bent on getting a big capacity steel cylinder I would argue for a HP tank simply because you can get them legally filled to a higher pressure yet at the lower pressure they are the same capacity as the LP tanks anyway.

Hopefully that helps...

T