Dumpable weight vs trim

[Eric Sedletzky]

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.

Because the plate is 6 lbs. and the strap harness not much measurable amount plus stainless cam band buckles and stainless waist band buckle, handful of s/s D-rings and keepers, all of it adds up. The smaller donut wings are also less buoyant than full wrap around bladders, because of less material and less trapped air.
Jacket BC’s typically have quite a bit of padding and oversized wings with side inflation, plus an overall abundance of plastic. Some of those jackets take 2-5 lbs of added weight just to sink them alone and make them neutral depending on model/brand etc.
So add 6 lbs for the plate plus the 2-5 lbs for the jacket that you would need in extra weight and I came up with 7-10 lbs. instead of the obvious 8-11 lbs. (on the conservative side). It could be more than this.

 

[boulderjohn]

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.

We had a communication breakdown.

I was originally responding to the belief that there is a greater buoyancy swing during the dive when using an aluminum tank rather than steel. When you challenged my response, I assumed you were still on that topic and missed your comment that you were not talking about a buoyancy swing. You threw me when you stated the fact that the aluminum tank ends up being positively buoyant. That does not matter. A cylinder that is 6 pounds less buoyant than another cylinder will require the diver to wear 6 more pounds of lead regardless of the final buoyancy.

Yes, a diver using a tank that is 6 pounds more buoyant at the beginning, middle, and end of the dive will need to have 6 more pounds of lead throughout the dive, all other things being equal. (Technicality--a diver with a thick wetsuit will not need that weight in the middle of the dive, regardless of the tank composition.)

 

[TMHeimer]

Because the plate is 6 lbs. and the strap harness not much measurable amount plus stainless cam band buckles and stainless waist band buckle, handful of s/s D-rings and keepers, all of it adds up. The smaller donut wings are also less buoyant than full wrap around bladders, because of less material and less trapped air.
Jacket BC’s typically have quite a bit of padding and oversized wings with side inflation, plus an overall abundance of plastic. Some of those jackets take 2-5 lbs of added weight just to sink them alone and make them neutral depending on model/brand etc.
So add 6 lbs for the plate plus the 2-5 lbs for the jacket that you would need in extra weight and I came up with 7-10 lbs. instead of the obvious 8-11 lbs. (on the conservative side). It could be more than this.

OK I do follow all that. But would you agree that for whatever amount of positive buoyancy you have (body type, type of BC, maybe positive fins, etc.), you need a set amount of weight to balance that out and achieve neutral buoyancy, regardless of whether it's in the form of lead or something else-- like metal (backplate, D rings, huge knife, whatever)?

 

[boulderjohn]

OK I do follow all that. But would you agree that for whatever amount of positive buoyancy you have (body type, type of BC, maybe positive fins, etc.), you need a set amount of weight to balance that out and achieve neutral buoyancy, regardless of whether it's in the form of lead or something else-- like metal (backplate, D rings, huge knife, whatever)?

That is correct. As I have been saying throughout this thread, your buoyancy is a result of the weight and volume of the entire package. If the total weight is equal to the weight of that same volume of water, you are neutrally buoyant. Some parts of the package make you more buoyant, and some parts of the package make you less buoyant.

What follows is NOT a response to anything anyone has said in this thread; it is added information related to some confusion frequently found in newer divers, and I am adding it in case any such confused diver is tuning in.

In OW class, students learn that if they inhale while neutrally buoyant, they will ascend. If they exhale, they will sink. Some divers misunderstand why, thinking it is analogous to adding or subtracting helium or hot air to a balloon--they think adding the air to the lungs makes them lighter and exhaling it makes them heavier. What actually happens is inhaling increases your volume as your chest expands. Inhaling does not change your weight at all, since those molecules of air were in your tank previously and so were part of your total weight. By increasing the total volume without changing the weight, you have increased your buoyancy. When you exhale and sink, you are sinking despite the fact that you have become very slightly lighter by loss of the weight of the air you exhaled. You are sinking because you now have less volume with nearly the same weight and are therefore are less buoyant.