Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.
Benefits of registering include
Ability to post and comment on topics and discussions.
A Free photo gallery to share your dive photos with the world.
8 lbs. 10 lbs. 20 lbs. Who knows? I think the only thing we can say with some certainty is that you'll probably need more lead weight when diving the 7mm wetsuit vs. the 3/2mm wetsuit.
+1 to what ajduplessis wrote. Please do an in-water weight check! There is no substitute for it. Afterward, be sure to record environmental conditions (salt vs. fresh water), your gear configuration (incl. exposure protection, tank, etc.) and weighting requirement in your divelog.
If you can't remember how to do a weight check, consult your basic OW manual.
Unfortunately, I've spoken to lots of certified divers who don't have a clear idea about how to do one properly.
No rule of thumb or generic weighting cheat sheet will be that helpful.
If you didn't specifically construct the weighting cheat sheet with your own personalized gear, then the cheat sheet is worthless. Throw it in the trash.
People conduct weight checks in different ways. It might be more convenient to: (1) start too heavy and remove weight or (2) begin too light and add weight.
Spending time trying to figure out a "starting number" is wasted time, IMO.
Other things to consider:
Is the 7mm wetsuit new? Positive buoyancy of a new 7mm wetsuit can be significantly greater than that of an old, well-used 7mm wetsuit.
Are you certain that the rest of your gear (tank, in particular) will be identical? Changing from an AL80 tank to a steel tank can make a big difference in weighting requirements.
Are you going to be diving in the same kind of water (salt or fresh)? If not, you'll have to adjust for the different density of the water.
tI think that Bubbletrouble raises an important fact o consider- the age of the wetsuit. My wife wore a 3 mil in the trip to Mexico from which we just returned, and it was new. We added 2 lbs just because she went from a 150 dive plus suit to a brand new suit. As has been said, there is no substitute for an in water weight check. I would suggest l in doing that check that you let the suit get permeated.
A 7 mil will take a little time to do that. If you jump in, do an immediate weight check, and proceed, you will probably be a bit overweighted- especially if it is a new suit. As to starting point, start at +3 or +4 pounds from what you used with the 3 mil rig, and add if necessary. 8 seems like too high a starting point to me, all other factors being equal.