L13
Contributor
While you shouldn't be weighted more than you can kick back up without your BC, you shouldn't actually do it that way unless you have a BC failure. Swimming up to control your descent vs using your BC is a bad idea. Use your BC to reduce work during all phases of your dive.I get that, and I understand the principal behind it.
My understanding was that while some air would need to be added at depth to maintain buoyancy, you should never be so over weight that the air is required to arrest the descent vs using kicks to arrest the descent and then air in the BCD to reduce the work of staying at the desired depth.
The difference in weight of air in your tank between full and safety stop is approximately 4.5 lbs which requires approximately 2 quarts more air in your BC at the beginning of the dive than at the safety stop. As you descend from 15 ft to 65 ft, that air will compress to 1 quart, so you will have to add another quart to stay neutral. As you continue to descend from 65 to 130 ft, it will again compress to 1 quart and you will have to add another quart. If you were to ascend immediately to 15 ft, with the tank still full, you would have to vent 6 quarts of air at 15 ft to restore neutral buoyancy with 2 quarts left in your BC.
As you breath down the tank all those numbers will get smaller, and the extra air in your BC to compensate for tank air weight will drop from 2 quarts to 0. Suit compression and other similar effects will result in you needing to add even more air than listed above descending, and vent more ascending. The above numbers are just the effects on BC volume due to the weight of air in your tank.
Swimming up 4# of air plus 8# of suit compressing during the decent at the beginning of your dive is a bad idea, even though you can swim up 12# in an emergency. You would reach your target depth with a high CO2 loading and be more prone to narcosis, paranoia, and panic.