Weight of the air in the tank.

[decompression]

Now calculate how to put 6 lbs of air in your BCD.

 

[runsongas]

Air weighs around 800g per 1000 liters..
Converting that to cubic feet works out to around 22.7g a cubic foot

Michael

Where are you getting that number? ISA for 1.01325 bar/15C is 1.225 g/l

 

[tursiops]

Where are you getting that number? ISA for 1.01325 bar/15C is 1.225 g/l

Because air actually weighs about 1293g/1000 liters.

 

[tursiops]

Now calculate how to put 6 lbs of air in your BCD.

Why?

 

[Charles2]

Because air actually weighs about 1293g/1000 liters.

Watch those zeros!

I think it is more like 12,927 grams per 1,000 liters.

366.05 grams per cubic feet

Or something like that

My brain hurts now.

 

[Alan Browne]

Don't try too hard for precision. Remember you can control several pounds with just your lungs, breathing off nearly empty or nearly full lungs.
In round numbers, carry 5-6 extra pounds for the AL80, to account for the weight lost from it during the dive, and 6-7 pounds for the AL100. That means 6 pounds extra will probably be very close no matter your AL tank.
For steel tanks, it is the same, because the air weights the same.....what changes is not the extra weight you need at the beginning, but rather the base weight you are carrying in additional to the extra.

I get all that, just seems I've probably been carrying 2 - 3 Lbs more than needed.

 

[BRT]

Don't try too hard for precision. Remember you can control several pounds with just your lungs, breathing off nearly empty or nearly full lungs.
In round numbers, carry 5-6 extra pounds for the AL80, to account for the weight lost from it during the dive, and 6-7 pounds for the AL100. That means 6 pounds extra will probably be very close no matter your AL tank.
For steel tanks, it is the same, because the air weights the same.....what changes is not the extra weight you need at the beginning, but rather the base weight you are carrying in additional to the extra.

I'm sure you meant the extra weight you need at the end.

 

[Jiminy]

I get all that, just seems I've probably been carrying 2 - 3 Lbs more than needed.

Have you done a buoyancy check at the end of the dive with 500 psi in your tank?

 

[tursiops]

Watch those zeros!

I think it is more like 12,927 grams per 1,000 liters.

366.05 grams per cubic feet

Or something like that

My brain hurts now.

YOU should check your zeros....

 

[tursiops]

I'm sure you meant the extra weight you need at the end.

What I meant was:
You carry two quantities of weight: a base weight to account for suit and equipment and your buoyancy, plus extra weight to account for the air to be lost during the dive. Without the extra weight, you are too buoyant at your safety stop, which means you are too heavy at the beginning of the dive...by 5-6 pounds for an AL80. The AL/Steel comment above is that the extra weight is the same for the same volume steel tank, but the base weight changes because the steel tank is heavier in the water than an AL tank is.