Do your short doubles make you head heavy?

[BenjaminS]

I have a friend who is willing to sell me some PST MP72s cheap and I was wondering if they would make me head heavy when doubled. Feel free to post pictures of your short doubles. I've not been able to find pictures of short doubles online. Here's some information on these cylinders...

Diameter : 6.9"
Height : 21" (If I remember correctly)
Service pressure : 3000+10% psi
Capacity : 72 cu. ft. (@ 3300 psi)
Weight empty : 30 lbs
Bouyancy when empty : -6 lbs
Bouyancy when full : -11 lbs

I pitched my idea to an instructor I know who is a die hard DIR diver. He told me that short cylinders, such as HP80s, make you head heavy when using a Bp/W. Now I understand that DIR diving is less about experience and more about standardized use of simple rules. The guy I heard this from is pretty knowlegable, but it leads me to suspect that he has never used HP80 or MP72 doubles before by how strongly he is against them.

Also, are these cylinders good for overfills? I know a dive shop owner who is willing to overfill by a few hunered psi and I was wondering if I could give him any reason for why he should fill them higher than that. He doesn't want to be liable for failed hydros, but I'm not convinced this is a problem as these cylinders would cost me so little.

 

[Lorenzoid]

It seems like a matter of physics that, since the manifold weighs the same, and since the diver's proportions remain the same, and with top of the tank in the same position relative to that diver's body, the shorter the tank, the closer to the diver's head the center of gravity will be with respect to the diver's center of buoyancy and thereby tend to torque the diver headfirst.

 

[jeffmg]

http://www.indianvalleyscuba.com/se...ction/information/CYLINDER SPECIFICATIONS.pdf

puts the mp72s at 20.75" long without the valve

When I first made the switch to doubles I initially bought HP100s and while many claim they had great experience with those at 24" long I had trouble with trapped gas in the overlap of a 40lb wing on the bottom and found I had to loosen my shoulder straps really far down my back to get that weight in a comfortable position to where I wasn't head heavy with my head against the isolator. I'm 5'11" and about 190lbs. At my height I have had alot of luck with Al80s doubled as well as Lp85s.

If they're really that cheap, go for it and see if they work, if you have to sell them at a little bit of a loss because it didn't work just consider that money going towards a little knowledge.

 

[Diver0001]

I have a friend who is willing to sell me some PST MP72s cheap and I was wondering if they would make me head heavy when doubled. Feel free to post pictures of your short doubles. I've not been able to find pictures of short doubles online. Here's some information on these cylinders...

Diameter : 6.9"
Height : 21" (If I remember correctly)
Service pressure : 3000+10% psi
Capacity : 72 cu. ft. (@ 3300 psi)
Weight empty : 30 lbs
Bouyancy when empty : -6 lbs
Bouyancy when full : -11 lbs

I pitched my idea to an instructor I know who is a die hard DIR diver. He told me that short cylinders, such as HP80s, make you head heavy when using a Bp/W. Now I understand that DIR diving is less about experience and more about standardized use of simple rules. The guy I heard this from is pretty knowlegable, but it leads me to suspect that he has never used HP80 or MP72 doubles before by how strongly he is against them.

Also, are these cylinders good for overfills? I know a dive shop owner who is willing to overfill by a few hunered psi and I was wondering if I could give him any reason for why he should fill them higher than that. He doesn't want to be liable for failed hydros, but I'm not convinced this is a problem as these cylinders would cost me so little.

Get in the water with them. Don't buy anything until you do that.....It could suck balls but your buoyancy could also be perfect!

I've used these tanks in the past and they were ok. That said, they are not the "standard" tanks that lot of technical divers use. The volume is on the low side for technical application

R..

 

[Doby45]

Wow, perfect case of things are different for everyone. I am 5'11" and about 210lbs and use doubled HP100s and I do not find myself head heavy at all. I also use neutral fins, Deep6 Eddys. The best thing to do is figure out the tanks you need to be able to do the dives you want to do and then go from there. If you need 100s and you are a bit head heavy, go with some heavier fins.

 

[RyanT]

what @Diver0001 said. I dive a set of steel HP 80s. They are really short, but trim out nicely on me. The problem with making predictions is that tank trim varies not only with their length,but also volume, material, etc. Second, your own build can affect your trim. You really just need to try them.

 

[KDAD]

I have one of those that I dive as a single. I find it heavy especially going from an al80, n80, or lp72 which are some of the other singles that I dive. With the bouyancy characteristics, two of them doubled up are going to have you fairly negative compared to larger capacity tanks. Not to say they won't work, like others have said your trim may be fine with them and you certainly can shed some weight, depending upon what you are currently using. Try before buy.

 

[BenjaminS]

It seems like a matter of physics that, since the manifold weighs the same, and since the diver's proportions remain the same, and with top of the tank in the same position relative to that diver's body, the shorter the tank, the closer to the diver's head the center of gravity will be with respect to the diver's center of buoyancy and thereby tend to torque the diver headfirst.

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 ou head heavy.

well as Lp85s

I've been considering getting LP85s as they can be filled to 3100 psi for 100 cubic feet of gas each.

That said, they are not the "standard" tanks that lot of technical divers use. The volume is on the low side for technical application

I'm more interested in doubles for redundancy and to prime me for tech. And at 3600 psi they hold more than al80s do.

I had to loosen my shoulder straps really far down my back

Your shoulder straps should be at a length determined by your ability to reach the valves.

If they're really that cheap

They're not really that cheap, they're REALLY cheap. I could probably sell them at a decent price for a profit.

I dive a set of steel HP 80s

Would you mind sending me a picture of your kit?

I also use neutral fins

As do I.

 

[BenjaminS]

certainly can shed some weight,

I was thinking I would have to add a trim float, actually. lol. They're only like 3 lbs heavier than a LP72, they're around 10 lbs heavier than al 80s underwater.

 

[KDAD]

Edited:

Not sure where you are getting the LP72 buoyancy characteristics from but all LP72s are not created equal and can have varying buoyancy even between tanks of the same make. Most tend to be about 4# negative when full and near neutral when empty.

Are you going from double AL80s to the MP72s? You are correct that it is a 10# difference but that is per tank and does not include the valves and manifold in the MP72 buoyancy. Times two for the double and that is at least 20# difference which is a big jump. Even when empty they are still 6# negative each.

Make sure to adjust your weighting accordingly.

Mine *might* be a little head heavy now that I think about it; can't say for sure though. They're not bricks but they are on the heavy side relative to their capacity. Might work out well. I don't dislike mine just pointing out the bouyancy when full weight. Let me know if you don't pick them up I might be interested. With those numbers I could shed my weight belt and...