How much air in your BC?

[elan]

Even with all that I am failing to see how the buoyancy swing will be different in an aluminum cylinder vs. steel. The metal the cylinder is made from isn't going to affect the buoyancy from full to empty, since the metal isn't changing throughout the dive.

The ONLY thing changing throughout the dive is how much gas is inside the cylinder, which effects the weight and average density (and thus the buoyancy) of the cylinder. The inherent buoyancy characteristics of the cylinder's metal isn't going to change.

I think you can see that all in the formulas. The final result is right the difference between steel and al due to the same air lose. Though the volumes stays the same over the dive but the difference in densities changes of the 2 tanks what makes the difference as far as I can see. The density of the tanks changes differently as one of them are bigger but loses the same amount of air.

With HP80 the situation will be even worse for AL as the tank will be even more smaller.

 

[SparticleBrane]

I think you can see that all in the formulas. The final result is right the difference between steel and al due to the same air lose. Though the volumes stays the same over the dive but the difference in densities changes of the 2 tanks what makes the difference as far as I can see. The density of the tanks changes differently as one of them are bigger but loses the same amount of air.

With HP80 the situation will be even worse for AL as the tank will be even more smaller.

You are still looking at absolutes. YES, some cylinders will be more positive or negative than others, but the change in buoyancy will be the same for equal volumes of air.

 

[elan]

You are still looking at absolutes. YES, some cylinders will be more positive or negative than others, but the change in buoyancy will be the same for equal volumes of air.

No I'm not looking at absolutes. The equation takes into account the fact that in the beginning of the experiment all the tanks expel the same force, for example neutrally buoyant or equally negatively buoyant. That means all tanks has additional weights added if necessary to make them equally buoyant. I only look at the difference in between the forces at the end of the experiment. You can see it in the formulas

 

[SparticleBrane]

Adding weights totally screws everything up.

 

[elan]

An update, I was looking through the formula again when I got spare time and though the formula is correct the final conclusion is wrong. The formula just shows that there will be a constant difference in the buoyancy between 2 tanks as the change of air mass is not involved in the final result. Meaning no matter how much air mass changes in both of them as long as the change is the same in both tanks there will be a constant buoyancy difference between the 2 which is in our sample is 4.4lb. This difference can be compensated with the lead at the beginning of the dive. Thus the change of buoyancy does NOT depend upon what tank is used. I think the case can be closed now Anyone seeing any mistakes here ?

P.S. As a cross check if we add the weight difference between the 2 tanks which is approx 1.7lb to our 4.4 lb we get that ~6lb buoyancy difference. Everything seems to fall in place.

 

[elan]

Adding weights totally screws everything up.

In fact it does not It just shifts the forces.

 

[Web Monkey]

Makes sense. I forgot to add that we rarely dive AL cylinders but mostly steel and there is very little difference in buoyancy between the full and empty cylinder.

Any cylinder ever made will become lighter by about [SIZE=-1]0.08 lbs. [/SIZE]for every cubic foot of air removed from it. It doesn't matter if the cylinder is made from steel, aluminium or titanium.

Either way, I do think that you are all correct and that I should rather adjust my weighting to be suitable for a neutral safety stop rather than a neutral bottom time.

You need to be neutral at your safety stop, or you won't be able to stop unless you can hold on to something or want to spend the next 3 minutes upside down and finning.

This is what I was hoping to achieve, a point where I have a BC but don't really need a BC - and I think I've been getting close.

You don't need a BC as long as you have no need to be buoyant on the surface and can compensate for the buoyancy change in your tank and wetsuit with your breathing. This isn't too difficult with smaller tanks, but becomes more difficult as the tanks get larger.


Terry

 

[Teamcasa]

...You don't need a BC as long as you have no need to be buoyant on the surface and can compensate for the buoyancy change in your tank and wetsuit with your breathing. This isn't too difficult with smaller tanks, but becomes more difficult as the tanks get larger.

It's becomes more difficult with a thick or layered wetsuit as well.

 

[spectrum]

neutral at 15 feet with 500 psi and no air in the bc is proper weighting. add about 4.5 pounds for the air in a full 80 and youve got it. that means just a coupla liters in your bc at depth with a full tank (depending on wetsuit compression of course). anything more is too much, requiring a lot of adjustments for depth changes, making proper trim harder, and increasing your cross section, which means more energy (=more air consumption) to swim.

You have one extra step in there. With the weight check done on a 500 PSI cylinder you are already tuned to the lightest condition. You do not want to be adding anything else to compensate for air. When you go to a full cylinder you will then be heavier and need to compensate for that weight with the BC.

Pete

 

[BobinNC]

Wow this conversation went way more complicated than it had to be, but it was interesting to read. I think most people have it right and the gist of the discussion is the weight of air and how much is used by the end of the dive. Steel, Aluminum and air volume do play into the weighting at the start of the dive but the real issue is how much difference is there in weight from when you start your dive to the time you reach your safety stop. Timing was perfect as I am testing this out myself with a bit of different gear in a quarry.