Do your short doubles make you head heavy?

[Lorenzoid]

Yea, but the center of gravity would be about 2.5" higher than, say, the LP85s I've been looking at. This is what I mean by DIR diving being more about following rules rather than experience. What I'm looking for is if that is enough to make you head heavy.

I think you'd just have to try them out and see for yourself. I don't think there is a good way to figure it out mathematically based on the CG being 2.5" higher than some other tanks, because there are other variables, and some of it is just feel. Yes, a higher CG means being more head heavy, but the question is whether it's so much more that you feel like you can't keep in trim. There's also the saying (which I don't particularly care for but will repeat) that you dive the equipment, the equipment doesn't dive you. Meaning that some divers could manage to keep in trim with tanks that other divers would not tolerate.

I am 5'8" and am able to keep in trim with double LP85s (without resorting to a tail weight), but only after placing them so low that I have a hard time reaching my valves. From what I understand, HP100s are similar in size to the LP85s. I can't imagine having to deal with a still shorter tank. But I'm a relative newbie, and until I improve my skills and get more comfortable I am finding that to a large extent the gear sure does dive ME, not the other way around.

 

[BenjaminS]

HP100s are similar in size to the LP85s

They are similar in volume. 85s have a higher tank factor (volume/psi) so that they can hold the same volume at a lower pressure. At 3100 psi, 85s hold the same volume that 100s hold at 3442 psi. Their diameters are about the same (85s are 7" while 100s are 7.25") and their lengths are a little less close (26" vs 24", respectively). I heard 85s are best when used with heavier fins, such as jet fins, which most people use when diving dry rather than something like slipstreams, which are better when diving wet.

 

[RyanT]

BenjaminS, I've got a picture somewhere of my double HP 80s, I'll try to find it to post. In my experience, trying to find the center of gravity for a set of tanks alone, etc. was not helpful. What was helpful was just jumping in and trying the rig. Ideally just having access to a pool someplace to try it. The other thing about a BP/W is that you can adjust the position of the tanks on the BP. Amazingly, just moving the tanks up or down the couple of inches between between the positions on the plate can make a huge difference. I really wish there was some magic formula that would work for all divers; unfortunately, the reality is that it takes a little trial and error for most folks.

 

[Michael Guerrero]

If they do make you a little head heavy just add a little tail weight. Problem solved. As far as pressure, I fill my 2400 psi rated steel tanks to 3600+ on a regular basis and no issues. Cave divers have been doing this for a long time, and if the hydro is current it's probably safe.

 

[Scared Silly]

IMHO the buoyancy characteristics of these cylinders make them unacceptable for doubles for most people. So being head/tail heavy could be least of your problems. The cylinders alone are 23 lbs negative. Add in the rest of your gear and you could be 35 pounds negative.

My wives dives with LP66 which are less negative and being shorter her trim is fine. I have dove with them as well nd can trim out fine. Also at 3000psi they hold 75cuft.

 

[BenjaminS]

unacceptable for doubles

I figured that. I also thought about LP66s but I they're like unicorns. Plus they're a little too small.

 

[rx7diver]

Do your short doubles make you head heavy?

BenjaminS,

Depends. I have worn HP 80's (20"), HP 100's (24"), and HP 120's (28") in a BP/W configuration. All are PST 3,500 cylinders. I have no problem with "trim" with either of these sets of doubles--except when I adjust them so that I can *easily* reach my manifold valves, which means moving the 80's and the 100's up my back, closer to my head, which cause me to be head heavy.

In fact, I especially love the 100's for both their capacity and size (7.25" OD, 34# empty weight, -1.5# negative buoyancy empty in salt water), and they would be perfect for me for extended range, drysuit diving in fresh water--except for the above. I sold my HP 100's and purchased the HP 120's, instead, because of this.

Oh, I am 6'2".

Hope this helps.

Safe Diving,

rx7diver

P.S. At the risk of stating the obvious, if I were interested in doing only really long, no-decompression, drysuit dives, then, absolutely, I would prefer the double HP 100's (or the double HP 80's) over the *much* heavier double HP 120's. Maybe dive them as independent doubles. In fact, I have only ever dove the HP 80's as independent doubles.

 

[Lorenzoid]

They are similar in volume. 85s have a higher tank factor (volume/psi) so that they can hold the same volume at a lower pressure. At 3100 psi, 85s hold the same volume that 100s hold at 3442 psi. Their diameters are about the same (85s are 7" while 100s are 7.25") and their lengths are a little less close (26" vs 24", respectively). I heard 85s are best when used with heavier fins, such as jet fins, which most people use when diving dry rather than something like slipstreams, which are better when diving wet.

I think you have to look at the tank manufacturer to figure out just how similar they are. I have Worthington LP85s, which have a tank factor of 3.14 psi/100cf, so at 3100 psi they hold 97.34 cf. I was comparing them with Faber FX100s. Worthington LP85 is 24.7 inches in length by 7.25 inches in diameter, and Faber FX100 is 25.39 inches in length by 7.25 inches in diameter. In-air weights are identical at 34.3 lbs. Since Worthington is out of the scuba tank business these days, Faber is your only option if buying new.

For me, I went with the LP tank, since I dive both the FL cave country where 4000 psi fills are not unheard of, as well as places where the rated pressure is all they're going to give me.

 

[RyanT]

Here's the picture of my HP 80s, notice I put a block of wood under them to support them during the dive briefing. As I mentioned, they trim out nicely for me. Again, for most rigs I suspect just moving them a notch up or down on the BP will help, but you have to try it.

 

[Scared Silly]

I figured that. I also thought about LP66s but I they're like unicorns. Plus they're a little too small.

Small in what sense? Dimension wise they are basically the same, their dry weight is less, better buoyancy. They do hold less gas at their std. WP but as I mentioned fill them to 3000psi they hold 75 cuft of air. At 3000 psi MP 72 hold 65 cu ft. (It is at 3300 psi that MP72 hold 72 cuft)

For Sale - OMS Faber 66cu FT Twin setup