Trapped air

[CoralSeaMan]

I dive salt water for one or two weeks a year. The rest of the time, year round I dive in fresh water. I have adopted the following procedure to adjust my weights in sea water. On the first day of diving I go a little heavier than my last salt water dives. I then drop weight usually to my same weight I was diving at previously. I then try to end my week by dropping a little more weight. It almost always works.

Lots of things affect bouyancy and therefore weighting, including relaxation and comfort level, newness of neoprene gear, diving conditions, physical fitness, etc. With every dive I am able to improve my bouyancy. Practice is the key.

 

[tfsails]

I think that if you swim above enough divers their bubbles will displace so much water that you can actually sink a bit from swimming through them. This would be sort of like a boat sinking if it were to sail into the gas bubbles from an undersea volcano eruption. Obviously air bubbles won't support one's weight as well as liquid water will.

 

[Walter]

Depends entirely on tank choice. Its perfectly possible to be overweighted without a wetsuit or any weights and in that case you will require air in the BC.

Perhaps you missed this part of my post........

Next lesson - if you are not wearing a wetsuit and you are weighted properly, there's no need to put air in your BC.

Yes, it is possible to be overweighted with no lead, but that's another point.

 

[Ayisha]

yeah generally I keep almost no air in my bc but my dive buddy tended to overweight himself so as a general rule I carried an extra two to four pounds in order to help them at the end of the dive if necessary. We were using aluminum tanks so he tended to get particularly buoyant toearss the end. The first few dives he forgot the extra weight so I started packing it just in case. Thus the extra bit of air

bold added

This sounds like your buddy may be UNDER-weighted rather than over-weighted.

I am assuming that he is positive even if his BC is completely empty and he can't hold a safety stop without a line. Then he is underweighted.

What happens to students or new divers a lot, however, is that they are over-weighted. So they add lots of air to the BC to stay off the bottom. Then, when they go a little shallower, they can't dump air fast enough to compensate as it expands and end up having a runaway ascent. The cycle continues. This is because they are over-weighted.

I like to just stop and hover horizontally without ANY movement to see if I am perfectly neutral. If I start sinking or have to kick to maintain position, then I know I am not neutral. I would add a little air. If I have to add a lot of air to the BC at depth, then I know I am overweighted. If I have to adjust the BC often, then I know my weighting is not correct. Most of the time, my lungs are able to compensate. I try to dive with just enough weight to be able to descend and be able to hold a safety stop without a line. You can refine this weighting over numerous dives with the same equipment (exposure protection/type & size of tank, etc.) and in the same environment (salt/fresh water).

Your friend should sort out what is causing the issue and deal with it appropriately rather than both of you being incorrectly weighted to compensate. If you were one day to not be right beside him, he would have no one to hand him the weight at the end of the dive, and it could cause an accident. Hope the above info helps.

 

[scubadiver888]

OK, I'll correct you. Most empty aluminium tanks will float and most empty steel tanks will sink, but that's only part of the story. To compensate for that, you'll use less lead with a steel tank than with an aluminium tank. Assuming the tanks hold approximately the same amount of air, the weight loss from using up the air in the tank will be approximately the same. If you use 6 lbs of air, it doesn't matter what type of tank it was stored in, that tank is still 6 lbs lighter.

I've always heard the opposite from people. I just started my AOW and realized the book has a chapter on Peak Performance Buoyancy so I checked it out.

In my PADI book it talks about how, if you are using an AL80 tank, you may need to add five pounds to your buoyancy calculations. "This is due to the fact that the tank becomes more buoyant as you breathe air from it." The next sentences are, "You may not need to add anything with other cylinders. In fact with steel cylinders, you may need to remove weight from your system."

The weirdest thing, on the following page, "your buoyancy will increase as you consume your air, no matter what type of tank you're using."

Now I'm really confused. When I think about what you said it makes more sense to me. ALL tanks do not change size as you consume the air BUT the weight decreases. Same volume but less weight equals more buoyant. So the steel tank should also require you to be overweight at the start of the dive. This way you will be properly weighted at the end of the dive.

Darrell

 

[Walter]

I've always heard the opposite from people. I just started my AOW and realized the book has a chapter on Peak Performance Buoyancy so I checked it out.

In my PADI book it talks about how, if you are using an AL80 tank, you may need to add five pounds to your buoyancy calculations. "This is due to the fact that the tank becomes more buoyant as you breathe air from it." The next sentences are, "You may not need to add anything with other cylinders. In fact with steel cylinders, you may need to remove weight from your system."

The weirdest thing, on the following page, "your buoyancy will increase as you consume your air, no matter what type of tank you're using."

Now I'm really confused. When I think about what you said it makes more sense to me. ALL tanks do not change size as you consume the air BUT the weight decreases. Same volume but less weight equals more buoyant. So the steel tank should also require you to be overweight at the start of the dive. This way you will be properly weighted at the end of the dive.

Darrell

You've got the concept down. Next concept, buy the NOAA Diving Manual and toss out the PADI book. If you don't want to spend that much money for a book right now, get Graver's SCUBA Diving, not as complete as the NOAA book, but much better than what you're using now.

 

[doctormike]

I've always heard the opposite from people. I just started my AOW and realized the book has a chapter on Peak Performance Buoyancy so I checked it out.

In my PADI book it talks about how, if you are using an AL80 tank, you may need to add five pounds to your buoyancy calculations. "This is due to the fact that the tank becomes more buoyant as you breathe air from it." The next sentences are, "You may not need to add anything with other cylinders. In fact with steel cylinders, you may need to remove weight from your system."

The weirdest thing, on the following page, "your buoyancy will increase as you consume your air, no matter what type of tank you're using."

Now I'm really confused. When I think about what you said it makes more sense to me. ALL tanks do not change size as you consume the air BUT the weight decreases. Same volume but less weight equals more buoyant. So the steel tank should also require you to be overweight at the start of the dive. This way you will be properly weighted at the end of the dive.

Darrell

Real simple. You are getting confused about the difference between the total weight of your rig (which you determine by trial and error, and adjust by adding or removing weight), and the CHANGE in weight between the beginning and end of the dive.

You start off perfectly neutral at the beginning of the dive, by figuring out how much weight you need for your specific combination of exposure suit, body fat, tank, etc... You breath air during the dive and blow it out into the ocean. The air that leaves the system weighs about 6 lbs. SO, when you are done with the dive, your total weight is 6 lbs less, and you will be 6 lbs more positive. It is the CHANGE in weight that determines the difference in buoyancy between the beginning and the end of the dive.

Imagine a tractor trailer that weighs 20,000 lbs when it is hauling a single empty scuba tank. Fill the tank. The trailer now weighs 20,006 lbs. Empty the tank. It is 20,000 lbs again. 6 lb weight change.

A steel tank really just moves weight from your weight belt to the tank. In fact, that is why some dry suit divers like them, because if you have to carry 24 lbs of weight, it's hard to stuff that much lead into a BC. Same logic for a backplate - it moves weight from your weight belt (or integrated pockets) into a place where it is more comfortable and more convenient. Downside is that it is non-ditchable weight, but as long as you can ditch enough to make yourself positive in an emergency, you don't need to have all of your weight be ditchable...

Mike

 

[Walter]

A steel tank really just moves weight from your weight belt to the tank.

Not really. It's more complex that that. You can often switch to a steel tank that is the same weight or lighted than the aluminium tank you had and still remove lead from your belt. It's not about weight, it's about density.

 

[doctormike]

Not really. It's more complex that that. You can often switch to a steel tank that is the same weight or lighted than the aluminium tank you had and still remove lead from your belt. It's not about weight, it's about density.

From a physics point of view, this is certainly correct, and I of course ultimately defer to Walter's guru status!

However, I think that we are making two different points - one about switching between tanks, and one about buoyancy changes during a dive. And using "weight" when describing a tank instead of "buoyancy" is confusing terminology... I may have caused that problem in my last post, but I was discussing change in total "underwater weight" (buoyancy) between the start and the end of the dife.

To clarify this important issue, here are some numbers taken from some published specs.

A high pressure steel 80 weighs (on land) about 32 lbs, and is -7 lbs empty buoyancy. A low pressure steel 80 weighs close to 30 lbs, but is physically much larger, displacing more water (due to lesser mean density - it has the same amount of steel, but the steel goes into making it taller instead of having thick walls). Therefore it is -2 lbs empty buoyancy. An aluminum 80 is larger still, but about the same weight, and is +4 empty. So even though all three tanks weigh pretty much the same on land, they have different displacements due to their different densities.

The underwater "weight" (buoyancy) is the weight of the tank minus the weight of the water it displaces. So you can make any object more buoyant by either making it lighter (e.g. removing lead), or by making it larger (e.g. an ocean liner, which displaces enough water to compensate for it's weight).

But getting back to Walter' point, using a denser steel tank will give you more negative buoyancy (both at the start and the end of the dive), and therefore you will need less weight on the belt (i.e. some of your needed negative buoyancy is displaced from the belt to the tank.

So to determine whether you need to add weight when you switch to a significantly lighter steel tank, you need to know more than just the weight. If the lighter tank is the same physical size (e.g. thin walls), then the buoyancy would be more and you would need to add weight.

I think in practical terms, the reason that steel tanks are used to displace weight from the belt is that people use tanks that are much less buoyant than the average aluminum tank. For example, I own a high pressure steel 120 which is very heavy on land (48 lbs), and -7 lbs empty. Note that this is the same -7 lbs as the much lighter HP steel 80. Why? Because it is larger, and displaces more water than the 80, otherwise it would be even less buoyant. But both the steel 80 and the steel 120 allow me to take 11 lbs off my weight pockets as compared to an aluminum 80 (-7 vs. +4).

And as to the first questions, the weight difference due to air consumed would determine the change in buoyancy between the start and the beginning of the dive for any given setup....

 

[Charlie99]

It's not all that complicated.

When looking at tank specs to figure out how much lead you need to add or remove as you change from one type of tank to another, there is only one spec you need to look at ----- buoyancy when empty.

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A few other basic observations:

The change in buoyancy of the tank as it goes from full to empty is determined by the weight of the air, which is independent of the type of tank.

If what concerns you is the total dry land weight that you have to haul to and from the dive site, simply add the required lead to the weight of the tank to get the total tank+lead dry land weight. You can easily see that the "figure of merit" in minimizing lead+tank dry land weight is tank_weight + empty_buoyancy (algebraic addition -- a negatively buoyant empty weight gets subtracted from the tank weight).

If one is properly weighted, then one can achieve positive buoyancy on the surface, even with a full tank, if you have ditchable weight equal to the weight of the air. No other convoluted calculations are needed.