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

[rongoodman]

I was responding to a poster who was complaining of not being able to position himself properly without using his hands to get the photo he wanted.

 

[dberry]

I was responding to a poster who was complaining of not being able to position himself properly without using his hands to get the photo he wanted.

I think that poster was me. But I wasn't complaining about needing to use my hands (I usually don't.) I was talking about maintaining really tight vertical control (to get a photo of a resting turtle) without touching anything. I did okay, but not as tight as I might like. Point taken, tho; I definitely need to improve my kicks, especially the back-frog kick. I almost kissed a big Moray hiding next to the turtle.

 

[TMHeimer]

Sorry Ron--I can see how helicopter can be of value to a photog.

 

[EFX]

The buoyancy "swing" for a steel (72) tank is only about 5-8 pounds but the swing for an aluminum 80 is about 11-13 pounds. When empty, a steel 72 has only about 1 pound of buoyancy and the aluminum 80 about 5 pounds.

Here are the buoyancy numbers from my spreadsheet:

Faber steel 72: full = -8.45, empty = -3.7. Swing = 8.45 - 3.7 = 4.75 lbs.
AL80: full = -1.4, empty = +4.4. Swing = 4.4 + 1.4 = 5.8 lbs.

When empty the 72 is 3.7 lbs negative, the AL80 is 4.4 lbs positive. The difference between the two is 8.1 lbs.

I think the standard rule of thumb is that you need about four more pounds of lead with an aluminum 80 compared to a steel 72. It does not make sense to me that if you remove 6 pounds of air from a steel tank you gain 6 pounds of buoyancy but if you remove 6 pounds from an aluminum tank you gain 12 pounds of buoyancy. These numbers are from the book Diving Science.

I think that standard is off. You need 8 lbs more lead going to an AL80 from a steel 72 all other things being equal. Removing 6 lbs of air from any tank of any size or material will make that tank 6 lbs more buoyant. To understand why look at this equation for total buoyant force B:

B = VD - W
where V = volume of object, D = density of water, and W = weight of object. VD then becomes the weight of the water displayed by the object.

When going from a full to empty tank VD doesn't change. The only thing that changes is the weight of the tank comprised of the weight of the tank itself and the weight of the air. The weight of the tank doesn't change. Therefore, the only thing changing is the weight of the air (6 lbs in your example). The direction of change is more buoyancy, i.e., W is getting smaller while VD is constant, hence B gets bigger.

The thing that puzzles me the most is the posts I've read on ScubaBoard where people say they are wearing something like 45 pounds of lead. Where is that extra 20-25 pounds of positive buoyancy coming from?

Compared to average sized people very large people displace more water, therefore their VD is high, while their weight doesn't go up as high comparatively. Add a dry suit which is bulkier (more volume) and more weight is needed. Of course, they could be overweighted.

 

[Bob DBF]

I think the standard rule of thumb is that you need about four more pounds of lead with an aluminum 80 compared to a steel 72.

I think that standard is off. You need 8 lbs more lead going to an AL80 from a steel 72 all other things being equal.

Back when there was only one steel 72, 6# was a good number for me. Now, with so many different 72s as well as all the other steel tanks, I just keep a cheat sheet.


Bob

 

[Colliam7]

the aluminum tank is larger and displaces more water so it has a greater buoyancy force being exerted upon it by the water.

True - compared to the LP steel 72. I fully agree. But, that is not the issue, is it? You explicitly stated - 'the swing for an aluminum 80 is about 11-13 pounds'. And, that is simply, and absolutely, not true. The difference for an AL80 is, as I noted, is ~6.24 lbs. I am curious - did the '11-13 lbs' number come from Diving Science, which you mentioned in a related post.? If so, can you cite the page? If that misinformation is in a publication, I am more than happy to alert the authors to a problem, even if the book is 12 years old..

Now, if your intended point is that there is a difference in buoyancy characteristics between 'old' LP72 steels, and AL80s, I will agree. The external volume of the LP72 is less than that of the AL80. And, there is a greater buoyancy force being exerted upon the AL80 by water a larger volume of water is displaced. Of course, that has nothing to do with the buoyancy 'swing' you described. The 'swing' has to do with the weight of air that is withdrawn from the cylinder during a dive. And, the absolute quantitative 'swing' associated with the withdrawal of 70, or 80, or 50 , or 20 cubic feet of air, will be identical for an 'old' LP72, an AL80, a HP100, a HP120, an AL80. Because, in all cases the external volume of the cylinder - whatever it is - does not change as gas is withdrawn. The only difference is in the overall weight of the cylinder.

So, I will take this opportunity to disagree with another statement: 'the difference in the weight of the compressed air is not all that great'. I would suggest that it is 'that great' - it is 6 lbs. And, if you are not properly weighted for it, you will have some difficulty maintaining a neutrally buoyant safety stop at 15 feet, when your cylinder is down to 500psi (or less). At 500psi, the weight of the AL80 has decreased (from full, at 3000psi) by 5.2 lbs. That, too, is 'that great', if you are not weighted for it.

And, as an aside, these issue are part of why - in contemporary OW scuba training - we go through a proper buoyancy check, 'early and often'.

 

[Shotmaster]

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!

 

[JamesBon92007]

True - compared to the LP steel 72. I fully agree. But, that is not the issue, is it? You explicitly stated - 'the swing for an aluminum 80 is about 11-13 pounds'. And, that is simply, and absolutely, not true. The difference for an AL80 is, as I noted, is ~6.24 lbs. I am curious - did the '11-13 lbs' number come from Diving Science, which you mentioned in a related post.? If so, can you cite the page? If that misinformation is in a publication, I am more than happy to alert the authors to a problem, even if the book is 12 years old..

It is on page 38. That's what I get for believing everything I read Looking back it actually says "11-13 pounds."

Diving Science



So, I will take this opportunity to disagree with another statement: 'the difference in the weight of the compressed air is not all that great'. .

What I meant was that the difference in the weight of the air between an aluminum 80 filled to 3000 psi and a steel 72 filled to 2475 is "not all that great." It's only about 1/2 pound. I will try to be more clear in the future

'

 

[TMHeimer]

What I meant was that the difference in the weight of the air between an aluminum 80 filled to 3000 psi and a steel 72 filled to 2475 is "not all that great." It's only about 1/2 pound. I will try to be more clear in the future

'

Yeah, either way, you get more air in the AL80 and the tanks cost a lot less. Thus my only motives for going all aluminum. The buoyancy "difference" shouldn't matter at all after you get a decent amount of experience.

 

[Bob DBF]

Yeah, either way, you get more air in the AL80 and the tanks cost a lot less. Thus my only motives for going all aluminum. The buoyancy "difference" shouldn't matter at all after you get a decent amount of experience.

That was my reason for buying AL for my first tanks, I could buy two AL for a small bit over the price of an old steel 72. Since it was a long ride to the fill station, two dives a day was better than one. I was younger so a bit more weight was less of a problem than it is now. As for buoyancy, neutral is neutral regardless of what you are diving, even if you can feeling them tugging.


Bob