Dumpable weight vs trim

[boulderjohn]

Maybe I'm not reading what your saying correctly or perhaps you didn't word it correctly but the buoyancy characteristics of a tank definitely matter.

I'm not talking about the buoyancy swing but the absolute buoyancy of the tank when (near) empty.

A Faber 12.2L standard cylinder (steel) has a buoyancy of -0.75kg (1.7lb) when empty.
A Catalina S80 cylinder (aluminium) has a buoyancy of 1.27kg (2.8lbs) at 500 psi and 1.86kg (4.1lbs) when empty.

This means that you can carry up to 2.5kg less (extra) weight when diving with the steel 12.2L tank versus the 11.1L aluminium tank.

I am sorry you are having trouble understanding this. I must not be explaining it well. I will try again.

Your buoyancy depends upon the weight and volume of the total package--you, your gear, and and your wetsuit, your tanks--everything. So let's say diver A and Diver B are doing the same dive, and they miraculously have exactly the same buoyancy characteristics in every way except their tanks. Diver A is using a Faber 12.2L standard cylinder (steel), and Diver B is using Catalina S80 cylinder. Since you did not give this information, I will assume that the tanks both have the same external volume. You also did not give their weights when full, so I am going to guess that the Faber weighs about 6 pounds more than the Catalina. That means that at the beginning of the dive, both divers have the same total volume, but Diver A is 6 pounds heavier than than Diver B. Diver B must therefore add 6 pounds of lead to have them both the same at the beginning of the dive.

Let's say that they now have the same volume and weigh the same, and let's make that 226 pounds. As they dive, they both use up 6 pounds of air. At the end of the dive, they will both still have the same volume, and they will each weigh 220 pounds, because they lost 6 pounds of air during the dive.

How will the difference in the composition of the tanks affect the change in the buoyancy of the total package as the divers use their air?

 

[boulderjohn]

I was able to swim up my 42 pounds with empty BC today from 30'. That is my usual max depth, though I imagine I could do it from deeper. Lots of kicking, but not a problem. What does this indicate--good? bad? in reference to my weighting?

First, it is great you were able to kick you way up. That is very good news.

Here is the question you want to answer. You will answer this best if you have a buddy, but you can get a good idea on your own. When you are doing a safety stop with the normal remnants of air in your tank, how much air is in your BCD? If you have the buddy, have the buddy give your BCD a good going over to determine the size of the bubble. If you are on your own, you can get a decent idea by feel, but it will not be as accurate. At the safety stop, you should have very little air in the BCD.

With the amount of wetsuit you wear, I would expect you to have a pretty fair amount of air in the BCD at depth, but you should not have much at the safety stop.

 

[ATJ]

How will the difference in the composition of the tanks affect the change in the buoyancy of the total package as the divers use their air?

It doesn't at all. I think we agree on that point.

However, the composition of the tank WILL make a difference to the amount of lead the diver will have to carry to be neutrally buoyant on their safety stop with a near empty tank and empty BCD. As I said, I'm talking about the absolute buoyancy of the tank not the buoyancy swing.

If we take TMHeimer's case, if he really does need 42lbs when he's diving with an aluminium tank, he might be able to dive with only 37lbs with a steel tank.

 

[ATJ]

The heavier the TANK is underwater due to the material it’s made out of determines how much extra or less weight is needed to balance the diver out - overall.

Which is what I said.

 

[Eric Sedletzky]

First, it is great you were able to kick you way up. That is very good news.

Here is the question you want to answer. You will answer this best if you have a buddy, but you can get a good idea on your own. When you are doing a safety stop with the normal remnants of air in your tank, how much air is in your BCD? If you have the buddy, have the buddy give your BCD a good going over to determine the size of the bubble. If you are on your own, you can get a decent idea by feel, but it will not be as accurate. At the safety stop, you should have very little air in the BCD.

With the amount of wetsuit you wear, I would expect you to have a pretty fair amount of air in the BCD at depth, but you should not have much at the safety stop.

The way I do it is just to dump all my air out the normal way at 15’ and see if I start rising or sinking. If there’s left over air in a wrinkle or high spot then it’s there and not a lot I can do about it, it just becomes an inherent part of the equation. At least the method I use to dump all my air will be consistent. If I start sinking and can’t regain lost footage by inhaling deeply that means I still have extra weight needing to be shed, so I get rid of a few lbs and try it again. If I fight to stay down at 15’ and have to hold a shallow breath or flip over and fin down then I need a few more lbs.

 

[Eric Sedletzky]

Which is what I said.

Right

 

[Eric Sedletzky]

I see what you're saying, and have noticed over the years that even with my 42 pounds my first descent with a dry wetsuit takes a bit longer than the descent on my second dive of the day. So you're saying I should be able to drop a few pounds, though it means the first descent is more difficult (or impossible?)? My wetsuit doesn't compress a whole lot as most of my dives are in the 20-30' range, and often times I surface to check location (I get turned around at times searching for shells and surfacing is easier than using the compass).

It all hinges on what happens with your buoyancy at 15’ at the end of the dive. For me I have to tip forward at the beginning of the dive and kick down freediver style until I get deep enough to get some pressure on the suit to neutral out. During the dive the suit will cool and the bubbles in the foam will shrink, and by the end of the dive I’ve just traded the lost gas in my tank for the reduction in buoyancy of the wetsuit. My dives are also in the 25 to 40’ range lately.
So in your case, if you went to a negative steel tank you could probably drop about 4 lbs.
If you went to a 6 lb backplate you could probably drop another 7 -10 lbs because the jacket is bouyant on it’s own. I don’t Know what fins you have but jets will be heavy too so there’s a possible 1 lb there (if you’re wearing something light.)
So with all these things you could potentially shed about 15 lbs. and be down to 26 to 28 lbs on your belt. If you’re heavy at your safety stop you might even be able to drop more.

 

[ATJ]

I realise I would be better off comparing a Faber 10.5L to a Catalina S80 as they both provide around about the same amount of air.

Faber 10.5L weighs 11.3kg (24.9lb) empty. Catalina S80 weighs 14.2kg (31.3lb) empty.
Faber 10.5L has buoyancy of -1.0kg (2.2lb) empty. Catalina S80 has a buoyancy of 1.86kg (4.1lb) empty.

So, using a Faber 10.5L instead of a Catalina S80 means you could not only dive with almost 3kg less lead, you'd be another 3kg lighter out of the water.

Sources:
Faber 232 bar Steel Cylinder Range and Specifications Faber Steel Cylinders - 232 bar - The Scuba Doctor Dive Shop
S80 - High Pressure Aluminum Gas Cylinders | Composite Cylinders

 

[TMHeimer]

It all hinges on what happens with your buoyancy at 15’ at the end of the dive. For me I have to tip forward at the beginning of the dive and kick down freediver style until I get deep enough to get some pressure on the suit to neutral out. During the dive the suit will cool and the bubbles in the foam will shrink, and by the end of the dive I’ve just traded the lost gas in my tank for the reduction in buoyancy of the wetsuit. My dives are also in the 25 to 40’ range lately.
So in your case, if you went to a negative steel tank you could probably drop about 4 lbs.
If you went to a 6 lb backplate you could probably drop another 7 -10 lbs because the jacket is bouyant on it’s own. I don’t Know what fins you have but jets will be heavy too so there’s a possible 1 lb there (if you’re wearing something light.)
So with all these things you could potentially shed about 15 lbs. and be down to 26 to 28 lbs on your belt. If you’re heavy at your safety stop you might even be able to drop more.

I used to have a couple of steel tanks and did use 5 or so fewer pounds with it.
If I went to a 6 pound backplate why could I drop 7-10 lbs. of weight and not 6 (also, why a range of 7-10?)? Either way, doesn't the weight of the backplate just offset the amount of lead you can drop? The whole package weighs the same, no? I could be missing something.
I use split fins that are negatively buoyant.
I see what you're saying about descending freediver style. Most likely I could drop a few pounds and do this. Not something that is taught when doing a weight check where you descend feet first.
Never thought of it, maybe because I only shore dive now and just descend from the start (3-5' water). Obviously easy if pulling yourself down an anchor line as I've done in the past, as long as you can still hold a safety stop.

 

[TMHeimer]

First, it is great you were able to kick you way up. That is very good news.

Here is the question you want to answer. You will answer this best if you have a buddy, but you can get a good idea on your own. When you are doing a safety stop with the normal remnants of air in your tank, how much air is in your BCD? If you have the buddy, have the buddy give your BCD a good going over to determine the size of the bubble. If you are on your own, you can get a decent idea by feel, but it will not be as accurate. At the safety stop, you should have very little air in the BCD.

With the amount of wetsuit you wear, I would expect you to have a pretty fair amount of air in the BCD at depth, but you should not have much at the safety stop.

Thanks. Yeah there is quite a bit in there at depth. I haven't done a safety stop in years with only shore diving, but I can simulate what you advise of course at 15' near the end of the dive.
In the shop today the Mgr./instructor wasn't surprised at my 42 pounds. Said when he teaches OW, at the checkout dives he starts someone about my size (basically same equipment) with 40 pounds. Then does the weight checks, figuring if anything he may have to add a pound or 2.
I'll hang up on my weight questions now. Thanks guys.