Is my weighting and BCD lift logic flawed?

[savechief]

New diver (just completed OW cert last weekend), and I've been doing a lot of reading in preparation for buying some equipment. A lot of that reading has had to do with 1) proper weighting and 2) the minimum lift that I will need from my BCD. The numbers below are approximations (but I would say pretty reasonable), but let me know if my logic is flawed.

Proper Weighting (neutral buoyancy @ 15ft w/500psi in salt water)
+2, me
+4, fins/3mm gloves/6.5mm boots
+16, 7mm full wetsuit
+4, hooded vest
-2, 1st stage/2nd stage/octo
+2, empty BCD
-7.3, nearly empty steel 80
Total = +18.7 * 1.025 (salt water) -> need to use 20lbs of weight

Minimum BCD Lift (maximum negative buoyancy @ 60ft)
+2, me
+2, fins/3mm gloves/6.5mm boots
+6, 7mm full wetsuit (66% buoyancy loss from +18 @ surface)
+2, hooded vest
-2, 1st stage/2nd stage/octo
+2, empty BCD
-13.2, full steel 80
-20, weights (I know that buoyancy is < -20, just don't know how much)
Total = -21.2 -> BCD must be able to provide at least this much positive buoyancy

Seem reasonable, or am I missing something?

John

 

[Fire_medic_RescueDiver]

What I use for full 7mm wet suit is 10% my weight + 3lbs trim weight on each side of BCD tank. A 3mm wet suit I use 6% my weight + the same trim weight. I have never had a problem going down or come up. But this is just the way that I do it.

 

[Tassie_Rohan]

Numbers seem ok as a starting point.

Under water if you are weighted correctly the BC only needs to make up for wet suit compression at depth and the weight of air in your tank.

However the extra lift is needed when on the surface: If you have your weighting correct 20 lb max lift will float you above the surface but a bit more will lift you a bit higher and make sitting around in heavy seas while waiting for a boat slightly more enjoyable.

Other ways to assist weighting:

Place your wetsuit and empty BC into a mesh bag and throw into the pool - see how many weights it takes to sink after any trapped air has escaped. Gives you a rough idea how bouyant your kit is.

Put your suit, mask and fins on and do the surface test (float at eye level and sink when exhale). Then allow for what ever tank your are using (its bouyancy when empty)

Spend some time at the end of a dive at 5 metres with a near empty tank and add/remove weights from your pockets until your can hover effortlessly with no air in your BC.

Cheers,
Rohan.