Is there a rule of thumb concerning travel BC's vs full BC's as far as bouyancy?
To elaborate.....does a travel BC require more or les weight to achieve the same bouyancy as a full BC?
I have a Sherwood Freedom that I need 16# to get perfectly bouyant.
Will I need more weight to get the same result from an older Zeagle travel BC?
"Travel" bc's usually feature less lift than "conventional" bc's because most people choose to travel warm sunny places.
Warm water means less exposure suit. Thinner suits will have less of a change in buoyancy than a thick wetsuit.
Buoyancy
Compensators are used primarily to compensate for changes in the buoyancy of the divers exposure suit.
Any bc needs to meet two criteria; offer enough lift to float the diver's gear at the surface if they ditch it. This should be calculated with a full tank.
I deal with BP&W's so I'll use one in this example. A medium SS plate and harness is about -6 lbs, a reg is about -2 and a full al 80 is about -2. That totals -10 lbs. In theory only 10 lbs of lift is necessary to float this rig at the surface with a full tank.
With an empty tank this rig will be about -4 lbs.
If you assume that this diver is in tropical conditions and is wearing a full 3mm wetsuit, their suit will only be about 4 lbs positive. This suit cannot loose more buoyancy than it starts with, that is a max of 4 lbs. This diver needs only 4 lbs of lift to compensate for a fully compressed suit.
In short in a thin suit with buoyant tanks our "example diver" only needs about 12-17 lbs of lift.
Contrast this with a cold water diver in a suit that might be 20+ lbs positive, and who might be using negative steel tanks. With a large single steel cylinder the rig could be -18 lbs or more if the diver adds ballast to the rig. The diver's suit can now possibly loose more than 20 lbs of buoyancy via compression.
In this case our diver needs more lift, with a 26-30 lbs lift bc a reasonable choice.
Tobin