Safety question about BCD lift capacity and weights

[Tatsubon]

Newbie here with an interesting question.

I had just taken a look at the specs. of my weight integrated BCD, showing total capacity of intergrated weight at 30#, and Lift capacity of 36#. This allows a safety margin of 6# of lift to get one positively boyant at the surface. I'm sure that this figure was calculated by the manufacturers for a good reason. My concern now is what happens when you do have to exceed this limit due to a shell type drysuit, thermal protection, and Al tank. I am sure that I am going to need extra lead to sink. Can I take into account that you can use your dry suit to give extra lift if needed, and not worry too much if you do lets say have to pack over the limit of 30#. Why I ask this is because I have an opportunity to pour custom ingots of lead to fit in my BCD pockets. Therefore exceeding the limit of 30# + 2# or 3# if needed. I do not want to wear a weight belt if possible.

Like to hear your thoughts about how much of a safety lifting margin should allways be taken into account with these types of BCD.

Thanks

Tatsubon

 

[EvaL]

I've been diving BCs with 5-6# of lift over my max weight in cold sw (although I wouldn't use a BC with less than 4# of lift over my max weight). In fact too much buoyancy is bad for reasons of bulk, less streamlining, improper diver positioning (on the surface and UW) etc.
Are you a very big guy and really need 30# of weight? The manufacturer likely designed the lift at 36# assuming you'll only max out the 30# of weights. If you go beyond this, then it might not be sufficient, and you should consider a BC with more lift.
Drysuits and wetsuits are nil to positive buoyant depending on thickness. These actually add to your lift so there shouldn't be any concern about lack of lift here. If you don't use the integrated eight pockets, just be certain you can dump your ingots from your pockets in an emergency.
Perhaps someone can correct me if I'm wrong on this statement: Generally, insufficient lift is a concern only when the equipment is grossly negative buoyant (such as tech - mutiple stage bottles, metal backplate and on very deep dives). Otherwise, a few # of lift from the BC is usually sufficient.
Dive safely,

 

[Dean810]

I'm wondering how much weight you think you need. You are diving salt water?

 

[Tatsubon]

Thanks for the quick replies!

To fill in some of the blanks for you.
6' tall and 170#.
Cold Salt Water diver (10-14'C)

Personal Kit:
BARE Nex Gen drysuit with T100 polarwear and underarmour.
Sea Quest Balance (ML)
Catalina 80 Al

By my calculations: 10% of body weight + 10# for shell drysuit + 5# for tank = 32#

From past drysuit dives with rental setup, I had difficulty even at 32# diving with my instructor. We had to add 2# to my tank to maintain negative boyancy when my tank droped below 1000 psi. So from then on I had been diving with 34#.
I do believe that in time I will be able to shed some more weight to around 30# or less. My legs hate it when I have to get out of the water, feels like I'm carrying someone else on my back.

Do you recon I am carrying too much weight?
I would like to hear how much some of you other drysuit divers carry out there.

Thanks

Tatsubon

 

[donooo]

Let's see,,, if you think it takes 32 lbs to sink you, and you go in with 30 lbs, how many lbs of lift in your BC does it take to float you ???? Answer = 0 It will take no air in your BC because you are floating. So if you add two lbs to sink,,, I'll bet it only takes two lbs of lift to bring you back to the surface. You need 30 lbs of lift if you are 30 lbs overweighted. I think in one of your classes, you were striving for being neutral, or in the advanced class only overweighted enough to compensate for the change in bouyancy in your tank, maybe 5 lbs. So you need 30 lbs of lift if you are 30 lbs overweight. In recreational diving you might be 3 - 10 overweight depending on conditions and experience. Thirty lbs overweight you have some extra tanks and regulators and no weight 's. Might be tech diving.


adios don O

PS, in a dry suit, 30 lbs. 6'0 190 lbs. 30 lb wing not a lot of dry suit experience

 

[hilti]

OK, Every dive is different from my perspective. Sometimes I carry a pony, sometimes I use an aluminum tank, sometimes steel.
If I use aluminum, I strap on a weight belt for that dive to accomodate the positive bouyancy I experience at the end of the dive~ steel requires nothing. I dive with a neoprene dry suit, for warmth and the positive aspect in the event of a valve failure or suit flood. I plan my weight system based on the circumstances for the dive I'm going to make, to me the ditchable weight system makes you positive in the event of an emergency based on what you choose to carry(understanding that your amount of positive bouyancy is related to your depth) deeper equals less positive bouyancy.
To me everything's fine, there are no issues, until something goes wrong~ that's what you have to plan for, you're reasonable worst case scenario.
Lately I've been thinking about drills for ditching weights that are attached to a reel, so you still have a connection, and some control, over your bouyancy as you near the surface.
In the old days dry suits were the bouyancy compensators.
As I understand it, bouyancy compensators are used for bouyancy compensators as the accepted rule today, drysuits are thermal protection.
Those are my thoughts~ Jim

 

[String]

A common mistake is to assume a BC or wing needs to lift the total of the lead a diver wears. So if you need 30lb to sink the BC needs to lift 30lb.

In reality this isnt the case. The lead is there to counter the buoyancy mainly of the exposure gear and the gas in the tank (and tank if using aluminium).

So for a properly weighted diver there is only really the buoyancy of gas in the tank (about 3kg) and tank (maybe 1kg if aluminium) to need supporting as the rest of the weight is just nullified by the suit and so on.
So assume a properly weighted diver needs 12kg, the BC only needs to be able to lift 3+1 == 4kg NOT the full 12 as the rest is just cancelled out neutralising the exposure protection. You then add a bit on (maybe up to 50%) to give good surface support. So then for a single tank diver they need about 4kf of lift (9lbs), double it for surface support and you get 18lbs. So from there you can see even 30lb lift figure for most single wings and small BCs should be more than adequate.

Massively overweighted divers obviously require more lift from their BC as they're carrying more lead that isn't needed but buying a big BC isn't solving that problem - its just ignoring it.

Basic fact is a BC doesn't need the capacity to lift all the lead a diver carries. For single tank diving a 30lb lift figure is more than adequate.

(If using weight integrated it needs to be able to lift all the lead you've stuffed into its pockets or it'll sink without kit on though. Personally i hate WI jackets).

Dont rely on using suit AND drysuit to supply the buoyancy between them - if the suit leaks/fails you still need to get buoyant.

Example with me yesterday, 15l steel tank, 12kg (26.5lbs) of weight, 30lb lift wing. Far more than enough lift both underwater and surface even with suit fully vented.

Moral of this story:- Get properly weighted. THEN things like this become a non issue!

 

[Gil57usa]

Newbie here with an interesting question.

I had just taken a look at the specs. of my weight integrated BCD, showing total capacity of intergrated weight at 30#, and Lift capacity of 36#. This allows a safety margin of 6# of lift to get one positively boyant at the surface. I'm sure that this figure was calculated by the manufacturers for a good reason. My concern now is what happens when you do have to exceed this limit due to a shell type drysuit, thermal protection, and Al tank. I am sure that I am going to need extra lead to sink. Can I take into account that you can use your dry suit to give extra lift if needed, and not worry too much if you do lets say have to pack over the limit of 30#. Why I ask this is because I have an opportunity to pour custom ingots of lead to fit in my BCD pockets. Therefore exceeding the limit of 30# + 2# or 3# if needed. I do not want to wear a weight belt if possible.

Like to hear your thoughts about how much of a safety lifting margin should allways be taken into account with these types of BCD.

Thanks

Tatsubon

Frankly, I can't think of any reason you would need 30 pounds of lead. Instead of looking at it from a max weight / lift capacity, figure out how much weight you really need to achieve neutral bouyancy.

 

[MSilvia]

Frankly, I can't think of any reason you would need 30 pounds of lead. Instead of looking at it from a max weight / lift capacity, figure out how much weight you really need to achieve neutral bouyancy.

Ever dive in an XL 2 piece 7mm wetsuit with 5mm hood, boots, and gloves? It used to take 36# on my belt to get neutral with an AL80.

 

[jon m]

yes, my wife has a 7 mil "farmer john" (and all the gloves, hood, ect) and she needs 32lb with an aluminum80... i only need 28lb with my one peice 7mil... and i know i displace more water than her, but that two peice just has so much more neoprene to balance out....