vjongene
Contributor
I was wondering if there is a good rule of thumb for choosing the size of a bladder or wing on a BC. Intuitively, the parameters would be as follows, assuming that the diver is weighted correctly, i.e. neutrally buoyant (or almost) with 30 bar in the tank at 3 m below the surface:
- compensate for the weight of the gas in the tank(s) at the start of the dive
- compensate for the decrease in the volume of the wet suit at depth, conservatively by compensating for the full volume of the wet suit
- if diving a dry suit, very little lift should be needed from the BC, because the volume of the suit will be more or less constant during the dive
If you add up these parameters, the amount of BC lift required during a "normal" dive should be very modest, even in cold water with a thick wet suit. A quick calculation would say no more than about 10 l if diving a single tank, with maybe an extra 2 l or so for each extra tank.
Talking to an instructor friend, he nevertheless argued that one never has too much lift, because of unforeseen circumstances. He cited two examples: having to bring to the surface a buddy with a BC problem (this happened to him: buddy's BC bladder had holes in it...), or wanting to bring a negatively buoyant object to the surface (also happened: found a weight belt loaded with 8 kg of lead). One additional consideration may be surface swims, where one wants to avoid any chance of sinking (this is very common where I dive - most of our shore dives start with at least 200 m on the surface). BTW, my friend argued against dumping the weight belt from the buddy with the BC problems, to avoid an uncontrolled ascent.
So what do SB members think? Better to calculate the lift based purely on the need to compensate for changes in buoyancy during the dive, or better to add a sizable margin of safety?
- compensate for the weight of the gas in the tank(s) at the start of the dive
- compensate for the decrease in the volume of the wet suit at depth, conservatively by compensating for the full volume of the wet suit
- if diving a dry suit, very little lift should be needed from the BC, because the volume of the suit will be more or less constant during the dive
If you add up these parameters, the amount of BC lift required during a "normal" dive should be very modest, even in cold water with a thick wet suit. A quick calculation would say no more than about 10 l if diving a single tank, with maybe an extra 2 l or so for each extra tank.
Talking to an instructor friend, he nevertheless argued that one never has too much lift, because of unforeseen circumstances. He cited two examples: having to bring to the surface a buddy with a BC problem (this happened to him: buddy's BC bladder had holes in it...), or wanting to bring a negatively buoyant object to the surface (also happened: found a weight belt loaded with 8 kg of lead). One additional consideration may be surface swims, where one wants to avoid any chance of sinking (this is very common where I dive - most of our shore dives start with at least 200 m on the surface). BTW, my friend argued against dumping the weight belt from the buddy with the BC problems, to avoid an uncontrolled ascent.
So what do SB members think? Better to calculate the lift based purely on the need to compensate for changes in buoyancy during the dive, or better to add a sizable margin of safety?