HP100 and HP120 - how much more air than AL80 at 3000psi?

[GaryBDavis]

My regular dive buddy has the lungs of a squirrel and is always coming up with lots of air left. We are both diving AL80 and when I’m at 1300, he’s at 1600, but when I come up with 500, he has 1200 left.

I’m moving from a BCD to a BP/W and going to a steel tank for the weight (I’m trying to get rig to the point where I don’t need weight pockets or a belt). So, I have a fresh choice of what tank to get. I’ve narrowed it down to HP100 and HP120. Both are 7.25 diameter tanks so it will be compatible with tank racks on the boats. Most of the time, I’ll be getting my fills from the on-board boat compressor or the on-site compressor where we dive localy so I expect they will be filling me to the same pressure as the AL80 my buddy has. From what I’ve read, at 3000psi, the AL80 has 77cu ft, HP100 has 89cu ft and the HP120 has 107cu ft. So the HP 100 and HP 120 would give me 15% and 30% more air than my buddy, respectively.

Given the pressure differences above would the HP 100 put be close to my AL80 buddy in terms of running out of air at the same time or do I need to go to the HP120 to keep up?

Seems the rate at which the pressure drops is non-linear, so it may not be that simple of math. I need someone more knowledgeable than I to chime in.

Thanks,
Gary

 

[aquaregia]

I think you'd need more datapoints to establish the relation. It's quite possible to have wildly different SACs for different parts of the dive (ascent, descent, etc). I'd try to dive the tanks first. I only did one dive in a HP120 but I found that I didn't care for it much, whereas the HP100 is quite nice. If I recall correctly, the price difference between similar tanks isn't huge, so I'd get the biggest one you like to dive that you can stand to carry.

If your goal is to have no ditchable weight, make sure that's really what you want. I had an incident where a friend descended with her valve off and could not reach it to turn it back on. There was also a recent fatality in Florida wherein a victim was unable to ditch weights despite trying. People panic on the surface; buddies panic on the surface. Ditching your rig underwater with a 15+ pound buoyancy difference between yourself and your rig could cause fatal problems.

 

[Jax]

Gary - to answer your specific question -- how much air difference at 3000 psi - you need to know the cubic feet at 3000psi. To know the cf, you must know each tank's Tank Factor. Please see Scuba Cylinder Specifications from Tech Diving Limited - 928-855-9400. Tank factor is = capacity / rated pressure.


Most 80's actually carry 77.4 cf. 77 / 3000 psi = .0256
100's: 100 / 3442 = .029
120's: 120 / 3442 = .0349

AL80: .0256 * 3000 = 76.8 cf
100: .029 * 3000 = 87 cf
120: .0349 * 3000 = 104.7cf


Disclaimer - fat fingers apply, check the math.

 

[eelnoraa]

Aquaregia has good point on ditchable weight. You probably don't want to have all your ballast on your rig.

As for HP100 vs HP120, it also depends on your build. HP120 is a very long tank, even longer than AL80 if my memory served while HP100 is 2" shorter than AL80. If you are relatively tall and head heavy in water, HP120 probably make a good choice. If you are not tall, leg heavy, then HP100 will trim out better.

 

[GaryBDavis]

I think you'd need more datapoints to establish the relation. It's quite possible to have wildly different SACs for different parts of the dive (ascent, descent, etc). I'd try to dive the tanks first. I only did one dive in a HP120 but I found that I didn't care for it much, whereas the HP100 is quite nice. If I recall correctly, the price difference between similar tanks isn't huge, so I'd get the biggest one you like to dive that you can stand to carry.

If your goal is to have no ditchable weight, make sure that's really what you want. I had an incident where a friend descended with her valve off and could not reach it to turn it back on. There was also a recent fatality in Florida wherein a victim was unable to ditch weights despite trying. People panic on the surface; buddies panic on the surface. Ditching your rig underwater with a 15+ pound buoyancy difference between yourself and your rig could cause fatal problems.

I'm getting my dive buddies start and end pressures from his log book so I'll have more data points. We just did a trip with 7 dives together, so that should do it.

What didn't you like about the HP120? Too tall? Didn't trim well?

I've seen a couple of DIR/HOG setups with steel tanks and BP/W with no ditchable weights. Those rigs belonged to DMs that seemed to be pretty experienced so I'd expect them to be safe configurations. I'd like to streamline my new setup as much as possible which is pretty much based on the Hogatharian configuration. Is a rig with no ditchable weights considered unsafe? For my rig, my test points will be:

1. Can the rig float by itself with a full tank (wing inflated)?
2. Will I be neutral buoyant at my safety stop with a very low tank (perhaps slightly negative, but not positive)?

Perhaps another question is can I swim from depth to the surface with a complete wing failure with a full tank?

 

[DA Aquamaster]

Currently at the end of a dive your buddy seems to have about 700 psi more gas than you, or about 18 cu ft.

You are correct that if you can only get 3000 psi fills, an HP 100 will only hold about 87 cu ft - but that is still about 10 cu ft more than an AL 80. And if you can get 3300 cu ft , you'll have 95 cu ft and make up the difference. The issue then is whether boat can't pump more than 3000 psi or just won't pump more than that in a 3000 psi tank. Boats are mobile so the DOT regs apply and an overfill is a distinct violation where in a dive shop it is not really an issue for anyone other than the shop owner. In short, if the boat can pump 3442 or close to it and will do so for a HP tank, then you are good to go with an HP 100 and will save some total weight doing it if you are carrying a few pounds of lead already.

An HP 120 / X7-120 is pretty long and may not trim well unless you are tall, but you can get a bit more capacity in an X8-119 (123 cu ft) anyway. With the X8-119 you get a shorter tank with an 8" diameter in what is basically an LP 95 sized package. 3000 psi in an X8-119 is 107 cu ft. For that matter an LP 95 holds 95 cu ft at 2460 psi, so a full fill won't be a problem on a boat and it gets you the same 95 cu ft you need to hang with your better sac rated AL 80 breathing buddy.

-------

Dumping weight at depth is poor form and not really needed if you are properly weighted.

Ditchable weights however do allow a diver or an assisting diver to get immediate positve buoyancy at the surface and thus they are considered a safety feature.

With a BP wing and a hogartian/DIR style webbing harness, it is very quick and easy to just cut a strap and relieve the diver of the weight of the tank, plate and reg, so ditchable weight is not a consideration in an emergency as you will cut the diver out and leave them on the surface in just their buoyant exposuie suit.

The three considerations you lay out are precisely on point.

 

[GaryBDavis]

Thanks DA Aquamaster and other posters for your comments. The only thing that keeps me from considering the 119 is the 8" diameter and how some boat's tank racks are set for 7.25". Decisions, decisions....

 

[JEC11718]

I dive lp120's (8 inch dia) and boat dive alot, I have never had a rack not fit my giant 32 inch tall tanks. I am 6'4 and love that extra 40 cuft of air!!!

 

[halocline]

Thanks DA Aquamaster and other posters for your comments. The only thing that keeps me from considering the 119 is the 8" diameter and how some boat's tank racks are set for 7.25". Decisions, decisions....

I think you'll be happiest if you find a tank that you enjoy diving with and not worry too much about the boat. I think a lot of divers fail to appreciate the big impact on trim that different tanks have. Try the HP100 first; it's really versatile, pretty easy to manage, overall a really popular tank.

As far as your gas consumption numbers, the usage between you and your buddy doesn't seem to be really even, so something's going on. Usage is linear in the sense that for a given tank, X PSI equals X cuft at a given depth; it doesn't matter whether its a full or near empty tank.