BC bouyancy at depth

[scubafool]

Something I have thought about a time or two, and I was wondering if anyone has wasted the brain power on this.

If I understand the principle of bouyancy correctly, a BC that has 30 lbs of lift at the surface fully inflated does not have as much lift at, say, 99fsw fully inflated. Reason being that air is compressed by the increasing pressure as you descend, and you have to add more air to the BC to maintain inflation. The air doesn't dissappear, it just shrinks. Therefore, a fully inflated BC at 99fsw should weigh more than it did at the surface.

Yeah, I know, nitpicking over nothing, but my small brain found it interesting.

 

[cool_hardware52]

Something I have thought about a time or two, and I was wondering if anyone has wasted the brain power on this.

If I understand the principle of bouyancy correctly, a BC that has 30 lbs of lift at the surface fully inflated does not have as much lift at, say, 99fsw fully inflated. Reason being that air is compressed by the increasing pressure as you descend, and you have to add more air to the BC to maintain inflation. The air doesn't dissappear, it just shrinks. Therefore, a fully inflated BC at 99fsw should weigh more than it did at the surface.

Yeah, I know, nitpicking over nothing, but my small brain found it interesting.

The air shrinks? Is this the small brained way of saying increased density?

You are of course correct, because of the increased density of the air the effective lift is decreased.

Lets see how much. 30lbs / ~64lbs cuft = ~.47 cuft.

Air weighs, at 1 atmosphere ~ .08 lbs / cuft.

Surface = 1 atm, 99 ft = 4 atm, 4-1=3

3 x .47cuft x .08 lbs /cuft = .05 lbs

At 99 ft you have lost .167% of your lift,

Better order that 90 lb wing tomorrow


Regards,




Tobin George

 

[scubafool]

And I just bet you have one laying around that you will let go cheap, huh?

The weight of air at surface level was what I didn't know, therefore I had no waying of figuring out what the actual difference was. Thanks.

 

[Diver0001]

Something I have thought about a time or two, and I was wondering if anyone has wasted the brain power on this.

If I understand the principle of bouyancy correctly, a BC that has 30 lbs of lift at the surface fully inflated does not have as much lift at, say, 99fsw fully inflated. Reason being that air is compressed by the increasing pressure as you descend, and you have to add more air to the BC to maintain inflation. The air doesn't dissappear, it just shrinks. Therefore, a fully inflated BC at 99fsw should weigh more than it did at the surface.

Yeah, I know, nitpicking over nothing, but my small brain found it interesting.

This is a great question for H2Andy. LOL

Meanwhile can you exaplain what you mean by "weighs more"? I'm not following what you mean.

R..

 

[scubafool]

OK, I will try. I am not the most literate, so bear with me.

As you descend, the water pressure around you increases, compressing the air in your BC. The air doesn't loose weight, it looses volume. When it looses volume, the BC looses lift, so you add more air to compensate. The air you added also has weight, so you have just made the BC heavier. So suppose for example that it took all 30 pounds of lift to make you neutral at the surface (hope not, but suppose). When you reach 99 fsw (4 atmospheres), you have to have 4 times as much air in the BC to fully expand it. Therefore, the air that the BC contains is 4 times as heavy as it was at the surface. The amount of water the BC displaces is the same as it was, but since it weighs more it is less bouyant.

And I am going to blame all of this on my arch nemesis H2Andy. If it hadn't been for reading his water weight thread, I wouldn't have remembered my previous mental wanderings, therefore I wouldn't have been posting about this at 2:40 in the morning!

 

[Diver0001]

Ok, got it. So the difference in "weight" is the extra air in the BCD. Sounds logical to me. I wonder how much 100 litres of air weighs.

R..

 

[Seabear70]

0.00129 gram/cubic-centimeter (g/cm3) at zero degrees Celsius and under one standard atmospheric pressure.

I'm too tired to do the math, especialy in the metric system.

 

[Diver0001]

Hmmm. air is heavier than I thought. 100 litres of air would weigh 129 grams then....

Of course at 4ata (99ft) 100 liters of air would create 25kg of lift so even though it makes your bcd heavier by 129 grams it isn't going to help you sink.... , which corresponds to what I *think* happens when you add air to a bcd at 99ft..... LOL. Of course maybe if your name is Andy you may get visions of making millions selling "air weights" but, well, you know, some people are just more....creative....than others. (sorry Andy, couldn't resist).

R..

 

[scubafool]

That was just plain low, LOL.