Master.........Really?

[Xx_Holy Diver_xX]

couldn't agree more. i find this sadly relevant.

 

[Shotmaster]

couldn't agree more. i find this sadly relevant.

Hahahahaha!

 

[EFX]

Aluminum sucks in my opinion. For those that like them, you can have my share. I switched to Faber 95s, after too many issues with aluminum. When I did use aluminum, I had a weight belt for the tank so I could get better trim. If all the extra weight was hanging on my around my mid-point on a weight belt. Towards the end of the dive as the tank became more buoyant it would lift my head upwards and ruin my trim. Putting weights higher on the tank helped solve the problem, I liked the steel tanks even distribution and constant negative eliminated the bobbing cork feeling I got from aluminum. I also got rid of the extra 6 pounds of lead required, the extra steps needed to put a "tank" belt on. I also now dive with almost no air in my BCD. With aluminum you have to over compensate before the dive by adding extra weight and wind up overly negative requiring a bunch of air in your BCD and later as the tank becomes positive during the dive you have to re-trim by dumping air. I can dive a steel tank without a BCD, it would be damn near impossible with an aluminum tank unless you were a very strong swimmer and it wouldnt be an enjoyable dive. The Buoyancy Compensator Device allows the use of aluminum. It is also the reason that proper weighting gets neglected. I am glad I learned to dive when I did. You don't know, what you don't know!

So, how do you get rid of the 7 lbs of buoyancy at the end of your dive, diving with the Faber 95?

Let's look at some numbers:
Faber 95: empty = -1.20, full = -8.33. Swing = 7.13 lbs.
Luxfer AL80: empty = +4.40, full = -1.40. Swing = 5.80 lbs.

You say you dive with almost no air in your BCD. At the end of the dive you are now almost 7 lbs. more buoyant. You've got to be diving with a dry suit which you don't mention. The additional buoyancy of a dry suit would keep you neutral with that heavy full tank. But, you'll have to dump air from both the dry suit and BCD to stay neutral at the end of the dive. The swing in buoyancy is actually greater than the AL80. So, again, how do you account for the 7 lbs?

 

[boulderjohn]

Let's make sure there is no misunderstanding here about different tanks and weighting.

To be neutrally buoyant during a dive, we have to achieve a balance between the parts of our body and gear that are pulling us down (like lead, dense bones, and muscle) and the things pulling us up (air in the BCD, bubbles in a wet suit, body fat). The compressed air inside a scuba cylinder is puliing us down, contrary to what most common sense will tell you. therefore, as you dive, you are getting lighter and more buoyant. If you lose 6 pounds of air during a dive, you are 6 pounds more buoyant at the end of the dive than you were at the beginning, and it does not matter what your tank was made of.

You must therefore be able to compensate for that shift in weight. That means that you must have 6 pounds more weight than you need at the beginning of the dive when your tank is full. One way to do that is by having more lead than you really need at the beginning of the dive. Another way to do it is to have a heavier steel tank that is more negatively buoyant.

Some people have the mistaken notion that steel tanks do not change buoyancy during a dive. They change just as much as an aluminum tank would upon losing that same amount of air. The difference is simply that you don't have to use as much lead to achieve the balance of neutral buoyancy.

 

[TMHeimer]

Agree of course on all that John. Maybe it was my inexperience a few years back, but I tended to "float" more near the end of the dive and in shallow water with the AL80 than with the steel 72. Maybe it's because of the buoyancy "characteristics" of the AL being more buoyant toward the rear (bottom of tank?)--that people talk about. I always varied the 6 pounds more or less when switching tanks. My theory back then was that even though all you say above is true, the steel is still a bit negative when near empty and the AL somewhat (more) positive, so the AL is working against you staying down and is a big thing, while the steel is not (and is also smaller). With experience, the difference apparently is less noticeable. I guess.

 

[boulderjohn]

the steel is still a bit negative when near empty and the AL somewhat (more) positive, so the AL is working against you staying down and is a big thing, while the steel is not (and is also smaller). With experience, the difference apparently is less noticeable. I guess.

Yes, this is true, but...

Remember that the operating factor is Archimedes' Principle. If you and your gear weigh the same as the same volume of water, then you are neutrally buoyant. If you and your gear weigh less than the same volume of water, you will float.

Both a steel tank and an aluminum tank start the dive by being among the things pulling you down. As noted in post #493 above, a Faber 95 is about 8.3 pounds negative when full, and an AL 80 is roughly 4.4 pounds negative. If they both lose 6 pounds of air during the dive, the Faber will be about 2.3 pounds negative, and the AL 80 will be about 1.6 pounds positive. Thus, the steel tank will still be among the things pulling you down, and the Aluminum tank will now be among the things lifting you up. However, the net effect is the same--in each case, you and your gear will be 6 pounds lighter at the end of the dive than at the beginning.

 

[Bob DBF]

The major factor is that you feel the AL tank tug on your harness/jacket as it became positive. Back when I didn't have an SPG, it was a handy indicator and was reassuring, some folks don't feel that way.


Bob

 

[Shotmaster]

So, how do you get rid of the 7 lbs of buoyancy at the end of your dive, diving with the Faber 95?

Let's look at some numbers:
Faber 95: empty = -1.20, full = -8.33. Swing = 7.13 lbs.
Luxfer AL80: empty = +4.40, full = -1.40. Swing = 5.80 lbs.

You say you dive with almost no air in your BCD. At the end of the dive you are now almost 7 lbs. more buoyant. You've got to be diving with a dry suit which you don't mention. The additional buoyancy of a dry suit would keep you neutral with that heavy full tank. But, you'll have to dump air from both the dry suit and BCD to stay neutral at the end of the dive. The swing in buoyancy is actually greater than the AL80. So, again, how do you account for the 7 lbs?

I switched to aluminum from steel 72 doubles, after battling the aluminum for while I went back to steel 72's and stayed with them before going to single steel 95's, as my typical dive profile changed from a variety diving to just spearfishing and I find that the 95's give me plenty of reserve if I need it and are a lot less drag than doubles and easier to haul around.

So if you want to poke me in the eye again, recalculate with a single LP 95 pumped up to cave fills. 3mm wetsuit. Scubapro classic BCD, I'm 5'6" 185 lbs, size 48 chest, size 32 waist. I will have to pull my weights and tell you how much I use to get underwater, since I figured this all out years ago.

 

[EFX]

Hey, I didn't mean any malice. I was just curious given the buoyancy numbers for the Faber 95 and your claims about diving without a BCD. Given you have practically no buoyancy in your wet suit and the tank is heavier than what I cited above it is even more incredible. You have a large chest given your waist size so maybe you're neutral at the end of the dive and you make up buoyancy at the start of the dive with chest/lung volume.

I agree with your reasoning in regards to switching to steel tanks. You can take off lead weight which can screw with trim. This is the reason that dry suit divers prefer steel is that additional weight needed to offset that extra buoyancy can be made up by heavier tanks. OTOH, aluminum's buoyant characteristic is why divers prefer these tanks as stage bottles. No one wants to add 10 lbs additional weight. So, the aluminum tanks get you at the end of the dive and the steel tanks get you at the beginning of the dive.

 

[Shotmaster]

My luxfer and Catalina 80's were both around 32 lbs, and they were both about 2 lbs negative when full, but both went positive by about 4 lbs when close to empty. I found the tugging upwards upset my trim when I slowed down between fish chases, I solved the problem by adding tank weights, remember this was for doubles so double the 4 lbs positive which is 8 lbs for the set. My Faber holds more air than the advertised 95, so the 7 lbs is actually 8.5 lbs, but the important point is it never goes positive. When empty it is still almost 2 lbs neg. Technically i start the dive more or less -8 lbs, if I have my weights correct for my wetsuit. I have a 3mm and a 5mm, my new scubapro Novas are a little positive vs my Jet Fins which were negative. I use my fins to control my descent, when I reach my target I am usually finning around hunting so being negative is not that noticeable, if I am stalking something I may hit the button a couple of short bursts to add a little positive buoyancy. At the end of the dive I am still negative but only by a couple of pounds. I control my ascent by using my fins until I get to my safety stop, where I add a couple of shots of air to get neutral and hang out. I like the control, and that was the way I was taught. I don't dive the Faber without the BCD, I do dive my steel 72s without one.