simple tank size question

[Katch]

Please correct me if I'm wrong...
A low pressure 100 and a high pressure 100 will hold the same amount of air, and give you the same dive time. The only difference would be the physical size and weights of the tanks, right?

Steve

 

[captain]

Right, 100 cubic feet is 100 cubic feet no matter how you package it.

 

[mahjong]

The buoyancy characteristics of the tanks might differ as well. That's a minor issue, though it might be a consideration esp given that I don't think there is a true LP100 tank, and so if you go LP then you will be looking around 100 at some tanks that are quite large in physical size and weight.


Another issue is quality of fill. Take an LP95, for example: I can most often get a fill on this tank where I have 100+ cuft of air, maybe even up to 110 cuft, and this is not a dangerous Florida cave diver overfill. Maybe to 3000psi at most, but more often somewhere between 2800 and 3000 (the rating 2640). This is an easy and mild overfill. It is much more difficult to get a full slow fill on a HP100, a full 3442psi, where you actually come away with 100 cuft of air in your tank. This is just to make the point that there is the issue of getting good fills on HP tanks. I dive an LP95 and routinely go down with over 100 cuft of air in my tank. I dive with people who mainly use HP100s and they rarely go down with 100 cuft of air in their tanks, maybe 90 to 95. Not a major issue, but an issue nonetheless.

Please correct me if I'm wrong...
A low pressure 100 and a high pressure 100 will hold the same amount of air, and give you the same dive time. The only difference would be the physical size and weights of the tanks, right?

Steve

 

[TSandM]

Size, weight and cost . . . in general, HP tanks are more expensive. Buoyancy characteristics can also vary, usually only a little.

The big discussions about LP versus HP tanks have to do with two things. One is the availability of HP fills where you live. I don't really empathize well with this, because none of the Seattle area shops have any problems filling to 3442, but apparently there are places, perhaps with older compressors, where the shops or boats are reluctant to provide complete fills for HP tanks.

The other is the availability of fills ABOVE rated pressure. This is common practice in Florida cave country. If somebody will fill your LP tank to HP pressures or more, then you basically have a bigger tank for a lower price. Many people do not consider this good practice, and it is not common anywhere else that I know of.

 

[VooDooGasMan]

Why do you ask about this katch ?

 

[mahjong]

I was trying to describe the middle ground of your scenario. We've all heard the stories of those Florida cave overfills. That's extreme, and extremely dangerous, and requires as a first suicidal step overriding the burst disk that is there for safety purposes. Overfilling an LP tank by a couple hundred psi, without going into the so-called HP or even MP pressure range, can't be advised, technically speaking, though I suspect it's pretty routine. And there might be more at issue than simply saving money by purchasing LP rather than HP tanks. I dive older regs and prefer not to hit them with 3442psi. 3000psi is the max pressure I want to put on the piston/seat/spring mechanism (I always check IP on my regs; right now I am dealing with one MK10 that swings 25psi between 3000 and 500--maybe a tired spring?). I am also short in height and feel most comfortable with tanks that max out at 24" long, which limits the size of an LP tank I can go for. Add to all this the fact that the smallest tank used by the people I dive with is 100 cuft. A couple hundred psi gives me a fill that probably benefits most my psyche going into a dive, as I acquire some comfort knowing that I am not carrying the smallest amount of air on shore dives where long surface swims are best avoided by remaining at depth (when conditions are decent I dive my double 72's--and I cannot understate how nice it feels to carry that much air, relative to others--and the air is there if others might need it in a jam). And the importance of feeling right mentally going into a dive cannot be understated.

But all of this is not to advocate breaking the rules, which we can only emphasize as always being a formula for disaster.

Size, weight and cost . . . in general, HP tanks are more expensive. Buoyancy characteristics can also vary, usually only a little.

The big discussions about LP versus HP tanks have to do with two things. One is the availability of HP fills where you live. I don't really empathize well with this, because none of the Seattle area shops have any problems filling to 3442, but apparently there are places, perhaps with older compressors, where the shops or boats are reluctant to provide complete fills for HP tanks.

The other is the availability of fills ABOVE rated pressure. This is common practice in Florida cave country. If somebody will fill your LP tank to HP pressures or more, then you basically have a bigger tank for a lower price. Many people do not consider this good practice, and it is not common anywhere else that I know of.

 

[Jim Lapenta]

It is common around the Great Lakes and Atlantic Coast where lots of wreck divers hang out. In Florida they call em cave fills. around here they are wreck or just plain "good" fills.

 

[VooDooGasMan]

Overfilling LP has been done for 40+ years, maybe it is a florida ritual, but as a DEEP PENETRATION GREAT LAKES WRECK DIVER, it started with the steel lp 72's, double burst disk, or just plug it.

When the navy divers in the UP of Michigan were adding helium and oxygen then pump it way past fill pressure, It was not long before I was doing it on my own as a kid, as my mom bought the whole set up from them when they went to the other side of the world. I packed LP72's al50's 63's 80's and never had a single problem.

Now in salt water I dive steels, lp 80's lp95's and SP 80's (3000) and HP 100's, and overfill them for more CF. I always come back with a 1000 psi.

 

[elan]

Please correct me if I'm wrong...
A low pressure 100 and a high pressure 100 will hold the same amount of air, and give you the same dive time. The only difference would be the physical size and weights of the tanks, right?

Steve

... And weight distribution which will affect the trim. And this becomes critical when you dive them as doubles especially in fresh water. Some tanks are light on the tail and even if you have enough weight overall you have to add weight for trimming.

 

[Katch]

Why do you ask about this katch ?

I noticed the LP100's were a lot cheaper than the HP100's. The heavier LP tank may actually be more beneficial, in that I can reduce the 35lbs I currently need for ocean diving in the drysuit (with AL80's). I'm not in any hurry to buy any, but if a good deal comes around locally, I may make the plunge.


Steve