I am ashamed to ask, but... BCD weight question.

[trickery]

I am sorry to do this, but i have just recieved my OW certification, and i am thinking about buying my own equipment. during class we used 63, and/or 80c tanks. One of the bc's that i am looking is the dacor rav. i like the jacket style, and it nicely fits my budget. at the dacor internet site i noticed that the dacor small has a lift capacity of 17 lbs. now, if i am not mistaken the tanks we used were well over 17 lbs. does this mean the amount of weight that you can put in the bc pockets, or the actual tank mount backpack. please help, any advice is gladly accepted.

thanks,
trickery :06:

 

[sharkbaitDAN]

does this mean the amount of weight that you can put in the bc pockets, or the actual tank mount backpack. please help, any advice is gladly accepted.

Neither. It is the amount of positive bouyancy the bladder can lift. If you are over 17 lbs negative underwater this BC will not be able to lift you. This would be unlikely if you are doing tropical diving with a thin wetsuit and not much on a weightbelt. You will have no problem with any single tanks. Weight integration is rated seperately, and is usually 10-15 pounds each, left and right sides, split between front and back pockets. Basically, ten ponds on each side in the front and 5 pounds on each side in the back= 30 lbs total. Oh, and congratulations on the cert and welcome.

 

[trickery]

Neither. It is the amount of positive bouyancy the bladder can lift. If you are over 17 lbs negative underwater this BC will not be able to lift you. This would be unlikely if you are doing tropical diving with a thin wetsuit and not much on a weightbelt. You will have no problem with any single tanks. Weight integration is rated seperately, and is usually 10-15 pounds each, left and right sides, split between front and back pockets. Basically, ten ponds on each side in the front and 5 pounds on each side in the back= 30 lbs total. Oh, and congratulations on the cert and welcome.

i will be diving with a two piece 7mm farmerjohn/jane wetsuit
. if i understand correclty this would help things??

 

[sharkbaitDAN]

i will be diving with a two piece 7mm farmerjohn/jane wetsuit
. if i understand correclty this would help things??

It won't really help, since you have to wear weight to offset the positive bouyancy of wetsuit. I would want a little more lift in an emergency, but it would probably be adequate for most dives, now that I think about it. if you use a larger steel tank, it is possible that the BC will not have enough lift. If you stick with aluminum, there shouldn't be a problem.

I'm rambling a bit, I think its time to sleep.

 

[Charlie99]

..at the dacor internet site i noticed that the dacor small has a lift capacity of 17 lbs. now, if i am not mistaken the tanks we used were well over 17 lbs. does this mean the amount of weight that you can put in the bc pockets, or the actual tank mount backpack. please help, any advice is gladly accepted.

The weight in air of the tank doesn't matter, except for how difficult it is to walk to and from a shore dive.

What matters is the weight of the tank in the water --- aka buoyancy.

The lift of the BC just has to add enough lift so that you and your gear (including tanks, wetsuit, lead, etc.) are neutrally buoyant.

17 pounds of lift is fine for the tropics where you don't wear a lot of wetsuit.

In colder locations, if you wear a 7mm 2 piece farmer jane this will give you a lot of buoyancy on the surface. To counteract this, you will add lead weight. When you descend, however, the wetsuit compresses and loses buoyancy. This leaves you negatively buoyant and you add air to the BC to be neutral again.

A conservative rule is to have enough lift to counteract the negative buoancy of your lead weight and your full tank, while assuming that the wetsuit has 0 buoyancy.

This might be more than 17 pounds of lift. For a large guy, 17 pound BC most likely would NOT be enough with double 7mm wetsuit. For a small woman, it might or might not be. You need to look more closely, to determine whether or not 17 pounds of lift is suitable for your cold water diving.

 

[Pillow]

We dive with 6mm suits. It takes my buddy 20 lbs and me 22 lbs in cold fresh water. I doubt that you would have enough lift.

 

[miesemer]

does this mean the amount of weight that you can put in the bc pockets

The RAV is not weight integrated nor does it have trim weight pockets. It has one storage pocket (not for weights) on the left side.

 

[CD_in_Chitown]

We dive with 6mm suits. It takes my buddy 20 lbs and me 22 lbs in cold fresh water. I doubt that you would have enough lift.

It's not directly related to how much lead you use. If you're properly weighted, an extra 17 lbs of positive buoyancy will lift you from any depth. It's more related to the amount of lead it takes to get under, and the amount of bouyancy you lose in suit compression, air draining from the tank adds some back to that equation of course but if you factor this in you're relying on never having to call a dive, which is not smart.

Charlie99 explains it best, Trickery I would encourage you to rent and dive, or talk the shop into allowing you to demo the BC in question before making the purchase.

CD

 

[Lemonade]

Trickery,

BC’s main purpose in this world is to compensate for changes in diver’s buoyancy during dive.

The changes in buoyancy come from two factors:

1. As air supply is being depleted, the tank is becoming lighter, hence more buoyant
2. As diver goes deeper, the wetsuit compresses, becoming less buoyant

The weight swing of the tank can be easily calculated, it is the weight of the air it contains when full; for 80 cu.ft. tank it is 0.075x80 = 6 lbs.

Buoyancy swing of a wetsuit can only be estimated, but I am reasonably confident that for a 2-piece 6-7 mm farmer john it’s well above 11 lbs. that are left after subtracting tank swing from your BC’s lift.

So my firm opinion is that 17 lbs. of lift in BC is not sufficient to dive in a thick wetsuit, you described.

 

[LeFlaneur]

I have read that a "general rule of thumb" is that the lift of your bc should be 5-10 pounds greater than the maximum weight you plan on carrying. I assume this is a figure for rec diving with a single aluminum tank. A bc with only 17 pounds of lift does not leave a lot of room to grow as a diver.