Buoyancy I’m confused

[Tank601]

So I went to do some pool work in my new suit and I was shocked at the amount of weight it required. It’s a mako spearfishing 5mm suit, my backplate weights 9 lbs and I’ve always heard 2 lbs per mm. So I expected to put 1 lb on the belt. But I needed 12lbs! That’s in fresh water! I don’t understand. Is open cell worse?

 

[BoundForElsewhere]

Umm, 2 pounds per mil would be 10 pounds.

 

[MargaritaMike]

Umm, 2 pounds per mil would be 10 pounds.

5mm × 2# = 10# - 9# backplate = 1#

I think his and my arithmetic is correct. HOWEVER, I would have thought 10 more lbs. if he was buoyant prior to donning his 5mm wetsuit.

Cheers - M²

 

[M-Cameron]

I though 2lbs per mm was only for converting between suit thicknesses ( is, going from 7mm to 3 mm)

I mean 12 lbs isn’t a crazy amount.... Hell i need 35lbs in my 7 mm

Also does the backplate weigh 9 lbs.... or does the whole bcd weigh 9 lbs?

 

[mi000ke]

12 lbs total or 12 lb on the belt?
I use 12 lb total when wearing my 5 mm.

 

[TMHeimer]

I have only this experience:
Used 7 mil farmer john wetsuit bought in 2006 required 42 pounds (jacket BC, AL80 tank).
Same suit & set up 10 years later-- 37 pounds.
--Fairly new used farmer john bought 2016, same set up-- back to 42 pounds.
First suit lost "integrity" over 10 years, I conclude.

 

[BoundForElsewhere]

5mm × 2# = 10# - 9# backplate = 1#

I think his and my arithmetic is correct. HOWEVER, I would have thought 10 more lbs. if he was buoyant prior to donning his 5mm wetsuit.

Cheers - M²

M-squared? Now you're just being a math showoff.
I stand corrected.

 

[Ghost95]

Just because your backplate weighs 9lbs dissent mean that is 9lbs negative. Buoyancy is the sum of all parts. Positive of the wing and harness, negative of the backplate, positive of the naturally insulated diver plus his suit.

Sounds like your weighting is pretty good.

I need almost 20 with a full 3mm suit and hooded vest.

 

[lexvil]

It all depends, the 2# is a guide line, if you have more neoprene on you than a smaller person it doesn’t work the converse is true too. And the buoyancy will change as the suit breaks in.

 

[Esprise Me]

Your backplate is probably only 6 lb. negative if it's steel; 2 lb. if it's aluminum. Buoyancy and weight are very different. A 5-gallon water jug weighs about 40 lbs. when full, but would be neutral or even a bit positive due to the small amount of air in the neck if you threw it in a swimming pool. What kind of tank were you using? I would expect most people diving in a 5mm with an AL-80 would need more than 10 lbs.; I certainly need more than 12 in my 7mm.