Dumpable weight vs trim

[boulderjohn]

I am implying that if one needs X amount of weight to descend at the beginning of the dive while using a tank that has a buoyancy characteristic of being positively buoyant when near empty, that one would need X+Y amount of weight to be able to hold a safety stop within the top 33ft/10m of the water column at the end of the dive. Y would be roughly equal to or greater than how positively buoyant the tank is at the reserve pressure one plans to surface with.

This is a common belief, but it is not true. The buoyancy swing caused by the use of air during a dive is equal to the weight of the air that was used. Period. The buoyancy characteristics of the tank do not matter.

The buoyancy of a diver is determined by the total weight and the total volume of the entire package, not just the tank. If that total volume remains the same but the diver loses a total of, say, 5 pounds through the use of air, then the entire package will be 5 pounds more buoyant at that point in the dive. A tank's buoyancy will swing from one number to another, and it does not matter if the number zero was passed during the swing.

 

[Zef]

This is a common belief, but it is not true. The buoyancy swing caused by the use of air during a dive is equal to the weight of the air that was used. Period. The buoyancy characteristics of the tank do not matter.

The buoyancy of a diver is determined by the total weight and the total volume of the entire package, not just the tank. If that total volume remains the same but the diver loses a total of, say, 5 pounds through the use of air, then the entire package will be 5 pounds more buoyant at that point in the dive. A tank's buoyancy will swing from one number to another, and it does not matter if the number zero was passed during the swing.

It took a me a bit to understand what you posted but I think I understand. Please let me know if the following is correct:

The tank may itself be buoyant when empty but that does not mean it is material to the diver as a complete unit. What matters is the weight/mass of the air that is depleted from the tank....the weight/mass of the tank itself remains the same as at the beginning of the dive, the change is the weight/mass of the contents inside it and the impact of that on the diver as a unit.

-Z

 

[boulderjohn]

It took a me a bit to understand what you posted but I think I understand. Please let me know if the following is correct:

The tank may itself be buoyant when empty but that does not mean it is material to the diver as a complete unit. What matters is the weight/mass of the air that is depleted from the tank....the weight/mass of the tank itself remains the same as at the beginning of the dive, the change is the weight/mass of the contents inside it and the impact of that on the diver as a unit.

-Z

Yes.

As I said, this is a common source of confusion for divers. When a diver gets into the water, he or she is a collection of a wide variety parts, each with its own buoyancy characteristics. Some of it is neutrally buoyant. Some of it is positively buoyant (wetsuits, body fat, etc.) Some of it is negatively buoyant (lean muscle mass, bone, etc.). If the total weight of the diver is less than that same volume of water, then the diver is positively buoyant, cannot sink, and must add an appropriate amount of lead, which adds little volume and lots of weight.

When the diver descends, the diver adds air to the BCD in order to increase the volume and become neutrally buoyant. As the dive progresses and air is removed from the tank, the diver becomes more buoyant because while the tank loses weight, it does not lose volume. The diver must decrease the volume of air in the BCD to compensate for this. It does not matter what the tank is made of. As it loses the weight of its air without changing volume, the diver must adjust the volume of the BCD to compensate.

When the diver is near the end of the dive and nearly all the air weight in the tank is eliminated, the diver should have very little air left in the BCD. There is no point in having more than necessary. Every excess pound of weight results in nearly a point of unnecessary air in the BCD.

A major factor in this discussion is the wetsuit. A very thick wetsuit compresses as the diver descends, greatly decreasing the diver's volume. That means the amount of lead needed to descend near the surface is no longer needed at depth. Thus, all people who wear heavy wetsuits MUST be overweighted at depth. Unfortunately, they cannot hand that weight off to someone and get it returned later, because most of it will be needed to prevent a runaway ascent in shallow water as the suit returns to its original volume. It is thus more difficult for a diver with a thick wetsuit to determine proper weighting. I once worked with a student who swore that he knew he absolutely, positively needed 22 pounds while wearing a 7mm wetsuit in fresh water with an AL 80. Four training dives later he was happily diving with 10 pounds.

 

[TMHeimer]

One would think that if 42lbs of lead is needed to get down at the beginning of the dive that @TMHeimer would be positively buoyant and struggling to stay down at the end of the dive if his tank down around reserve pressure (50 bar / 500psi) given the buoyancy characteristics of an AL80.

-Z

Somewhat correct. At the end of my (shore) dives (which is all I do now), I may pop up if I am close to the surface. If I am on the bottom (even at 10'), not a problem.

 

[TMHeimer]

Ok so we know an aluminum 80.
What about BC? What type and what brand? Jacket - s/s plate?
Farmer John suit, beaver tail - step in?
Hood - hooded vest or bib style?
Body composition? How much body fat? At 190 a 205 I suspect not much but an insulation layer could have an effect.
You still need to answer about your 15’ stop at the end of the dive, do you have any air in your BC or not?

BC- Phantom, bought used in 2005. Jacket. Plastic plate (small, I guess).
Farmer john-- step in.
Hood- bib.
Body comp.- Not too much fat, other than a bit of a gut.
15' stop-- haven't boat dived in 5 years, so only gradual ascent to shore.
When I was boat diving I had no trouble holding safety stops, but of course never emptied my BC.

 

[Eric Sedletzky]

BC- Phantom, bought used in 2005. Jacket. Plastic plate (small, I guess).
Farmer john-- step in.
Hood- bib.
Body comp.- Not too much fat, other than a bit of a gut.
15' stop-- haven't boat dived in 5 years, so only gradual ascent to shore.
When I was boat diving I had no trouble holding safety stops, but of course never emptied my BC.

The jacket might be anywhere from 2 to 5 lbs buoyant and need that amount of weight to offset it. The aluminum tank will need about three extra lbs. to offset it.
Make sure you’re BC is empty at 15’ at the end of your full length dive. The reason I say at the END of the full length dive is because wetsuits not only compress but they also cool as the dive goes on making the gas bubbles smaller and making the rubber harder and more stretch resistant which means as you come up from depth after a time frame your suit will not bounce back and re-inflate at the same rate that you come up. If you take a fresh suit that’s been sitting in the sun and a suit that has just come up from a full length cold water dive, the suit that just came up will not be as bouyant as the fresh suit. So with this in mind you can actually lose a few more lbs. off your belt to offset this phenomenon.
But in order to know exactly, you must do the 15’ stop and no air in BC test to know for sure. If you get that number then at least you will have a baseline to know what the absolute minimum is.

 

[ATJ]

This is a common belief, but it is not true. The buoyancy swing caused by the use of air during a dive is equal to the weight of the air that was used. Period. The buoyancy characteristics of the tank do not matter.

The buoyancy of a diver is determined by the total weight and the total volume of the entire package, not just the tank. If that total volume remains the same but the diver loses a total of, say, 5 pounds through the use of air, then the entire package will be 5 pounds more buoyant at that point in the dive. A tank's buoyancy will swing from one number to another, and it does not matter if the number zero was passed during the swing.

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.

 

[Eric Sedletzky]

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 think what he meant was that the buoyancy characteristics of a tank don’t matter as far as the weight of The gas being used. Buoyancy characteristics and weight swing are two separate issues.
With an aluminum 80 you need to add additional weight to your person to make up for the lightness of aluminum. The bigger the cubic foot volume of the tank the bigger the weight swing. I think we can all agree on that.
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. The weight of the gas included in that does not matter except at the very end of the dive at 15’ to be able to hold a stop with no air in the BC. At this point is where the remaining 500 or so PSI gets included into the weighting formula to determine perfect weighting.

 

[TMHeimer]

The jacket might be anywhere from 2 to 5 lbs buoyant and need that amount of weight to offset it. The aluminum tank will need about three extra lbs. to offset it.
Make sure you’re BC is empty at 15’ at the end of your full length dive. The reason I say at the END of the full length dive is because wetsuits not only compress but they also cool as the dive goes on making the gas bubbles smaller and making the rubber harder and more stretch resistant which means as you come up from depth after a time frame your suit will not bounce back and re-inflate at the same rate that you come up. If you take a fresh suit that’s been sitting in the sun and a suit that has just come up from a full length cold water dive, the suit that just came up will not be as bouyant as the fresh suit. So with this in mind you can actually lose a few more lbs. off your belt to offset this phenomenon.
But in order to know exactly, you must do the 15’ stop and no air in BC test to know for sure. If you get that number then at least you will have a baseline to know what the absolute minimum is.

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).

 

[TMHeimer]

I'd suggest shallow, and then try from gradually deeper depths until you reach the maximum depth you dive recreationally.

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?