Cylinder type and proper weighting (SPLIT FROM 'Pull Dumps - Lose them')

[TrimixToo]

a full aluminium tank is buoyant on the surface, a steel tank of the same volume has a negative bouyancy empty. If you use steel twins of the same volume as an aluminium 80, not only will you not need lead to sink the AL80 but the extra weight of the steel twins will negate the buoyancy of other essential gear and dramatically cut the amount of lead needed. Also the aluminium tank is heavier to carry on the surface. The only reason aluminium tanks are used for diving is there cheap to buy and maintain so the dive resorts and schools like them.

Some aluminum tanks are negative empty and some steel tanks are buoyant empty. I don't see *any* AL80 tanks in this table that are buoyant when full; in fact, I see only one in the entire table, the 14CF Luxfer:

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

Dry weight varies, too.

Do you have a reference that disagrees this this one?

 

[boulderjohn]

a full aluminium tank is buoyant on the surface, a steel tank of the same volume has a negative bouyancy empty. If you use steel twins of the same volume as an aluminium 80, not only will you not need lead to sink the AL80 but the extra weight of the steel twins will negate the buoyancy of other essential gear and dramatically cut the amount of lead needed. Also the aluminium tank is heavier to carry on the surface. The only reason aluminium tanks are used for diving is there cheap to buy and maintain so the dive resorts and schools like them.

What you are saying here is different from what you said before and to which I originally responded. What you said before was that people are carrying too much weight because they are using aluminum tanks. Yes, people using aluminum tanks generally have to carry more lead than a diver using steel tanks--no question about it--but there is nothing in that statement that requires them to use too much weight. They need to have enough weight, but needing enough weight does not require you to use more than enough.

Since you bring up doubles...

I sometimes use aluminum doubles when diving in fresh water while wearing a dry suit. When doing so, I wear my steel back plate, put a 5 pound v-weight in the doubles, and usually wear a couple more pounds elsewhere. I am pretty properly weighted at the end of the dive that way.

I usually use my steel LP 108 doubles, though. When I do, I use my aluminum backplate to cut back on weight. I carry no other weights. Even so, I am still very much overweighted at the end of a dive. If my wing were to fail, I would have no way to drop weight, and there is no way I could swim that rig to the surface. I have to rely upon the redundant buoyancy of my dry suit, and hopefully that will not fail as well.

So there is a negative aspect to the weight of a steel tank. In my mind, the primary benefit to steel is not the weight as much as it is the greater gas carrying capacity.

 

[divad]

I agree... ditch the pull-dumps.

 

[Dan]

After reading all 25 pages (246 posts) of this thread & looking at my good-working BCD with pull dump that I never used, I decided to leave it as is. Why fix thing that ain’t broken? I’d just use it for emergency, just in case my power inflator fails open.

 

[Umuntu]

I had one with a CO2 cartridge in it back in the day. The Navy still uses them since some of their guys dive, shoot, get shot at, and leave by water again.

AP Diving still makes BCDs (Buddy and Commando) to which you can connect a small air bottle topped up from a tank. The bottles valve design looks like either a yoke or DIN regulator connector so it matches perfectly to be topped up from the tank. They have been making these for decades.

A much greener solution than disposable CO2 cartridges.

 

[Umuntu]

a full aluminium tank is buoyant on the surface, a steel tank of the same volume has a negative bouyancy empty.

Surely you meant an empty aluminium cylinder ?

Both LUXFER and Catalina now sell "neutral" cylinders which are much less buoyant when empty.

The only reason aluminium tanks are used for diving is there cheap to buy and maintain so the dive resorts and schools like them.

In warm salt water with a 3mm wetsuit and an AL80, I require no lead.
If they have little exposure protection most people generally need very little lead with AL80's in the tropics, if they are properly weighted.

 

[aquacat8]

In a runaway inflation scenario, I have a hard time understanding how exerting yourself to the maximum extent possible, (kicking down super hard) and then inverting yourself and pulling the dump valve is a viable solution.

The problem I envision, is that it probably takes 2 hands to disconnect an inflator hose. If one hand is on a rear pull dump, you might be able to prevent an ascent, but you don't seem able to actually resolve the problem.

Exhaling completely, laying spread eagle on your back, dumping air from the inflator (via a pull dump or otherwise) - while you strive to disconnect the hose seems a much better, quicker and less strenuous solution. An Air 2 type connector is operable with one hand, but the "normal" ones aren't - in my experience.

Also, most stuck inflators I have seen are not blasting air 100% at full capacity. It is often a slight or moderate leak (that may not be so evident until you find yourself floating up in an unwelcome manner). So once you dump the excess air from the bC, are spread eagle to make a ton of drag - exhale (the fastest way to loose buoyancy), you have some time to disconnect the hose. That can all be done with near zero physical exertion. There is probably no need to shoot down as fast as possible, but rather to arrest the ascent ASAP, disconnect the hose and then you have some time to check air supply, depth, time and determine how you want to proceed.

Well to each his own, to me turning over and getting spread eagled which puts both my dump valves under me and hard to know in that position which one is higher sounds like task loading and I would feel pretty powerless, whereas finning down while venting the butt dump is completely natural, my preferred way of dumping. In fact I could probably dive a BC with just a butt dump. I do have a AIR2 so I guess it is not hard to disconnect compared to others , and if it did take a while I could be controlling position by finning the whole time. But I’m a swimmer, I swim 45 mins to an hour several times a week, swimming is like walking to me. Being upside down is one of my favorite things too. So dive and let dive, great to learn about multiple solutions to problems, so thank you! Your technique sounds great too and totally counterintuitive to me.

 

[Celt]

Surely you meant an empty aluminium cylinder ?

Both LUXFER and Catalina now sell "neutral" cylinders which are much less buoyant when empty.



In warm salt water with a 3mm wetsuit and an AL80, I require no lead.
If they have little exposure protection most people generally need very little lead with AL80's in the tropics, if they are properly weighted.

no i meant full, the average aluminium 80 is buoyant when full, the new luxfer 80 is advertised as neutral but luxfer built 5 pounds weight into it giving it a dry weight of 35 pounds empty, when you check the specs from luxfer you get 4.1 pound positive empty and 1.4 pound negative when full don't know whats going on there. If you can sink a full 3mm wetsuit and an al80 with no lead you must be all muscle and bone and have a huge negative body bouyancy. what do you mean by if they are properly weighted?

 

[Dan]

no i meant full, the average aluminium 80 is buoyant when full, the new luxfer 80 is advertised as neutral but luxfer built 5 pounds weight into it giving it a dry weight of 35 pounds empty, when you check the specs from luxfer you get 4.1 pound positive empty and 1.4 pound negative when full don't know whats going on there. If you can sink a full 3mm wetsuit and an al80 with no lead you must be all muscle and bone and have a huge negative body bouyancy. what do you mean by if they are properly weighted?

You mean if you place an AL80 tank full with 3000 psig air in a pool, it will still float?

 

[Celt]

What you are saying here is different from what you said before and to which I originally responded. What you said before was that people are carrying too much weight because they are using aluminum tanks. Yes, people using aluminum tanks generally have to carry more lead than a diver using steel tanks--no question about it--but there is nothing in that statement that requires them to use too much weight. They need to have enough weight, but needing enough weight does not require you to use more than enough.

Since you bring up doubles...

I sometimes use aluminum doubles when diving in fresh water while wearing a dry suit. When doing so, I wear my steel back plate, put a 5 pound v-weight in the doubles, and usually wear a couple more pounds elsewhere. I am pretty properly weighted at the end of the dive that way.

I usually use my steel LP 108 doubles, though. When I do, I use my aluminum backplate to cut back on weight. I carry no other weights. Even so, I am still very much overweighted at the end of a dive. If my wing were to fail, I would have no way to drop weight, and there is no way I could swim that rig to the surface. I have to rely upon the redundant buoyancy of my dry suit, and hopefully that will not fail as well.

So there is a negative aspect to the weight of a steel tank. In my mind, the primary benefit to steel is not the weight as much as it is the greater gas carrying capacity.

'To heavy getting in the water, if there was no air in the bcd he still should not have sunk from the surface." that was my original post. If your the right weight at the start of a dive you cant possibly be to heavy at the end of it, unless your refusing to let go of that porthole your carrying.