Is BCD with 20 lb lift adequate?

[shurite7]

Where does that come from?

The information came from an equipment seminar that I attended several years ago.

 

[Bowers]

Can you explain why it should be 10 lbs greater? It seems like if weighted for neutral bouyancy with used cylinder at the surface, then lift would only need to be inversly proportional to the weight you wear to be neutral at depth. 2-4 lbs extra lift would give more than enough room for error. Or is there something i'm not thinking of?

 

[tbone1004]

Can you explain why it should be 10 lbs greater? It seems like if weighted for neutral bouyancy with used cylinder at the surface, then lift would only need to be inversly proportional to the weight you wear to be neutral at depth. 2-4 lbs extra lift would give more than enough room for error. Or is there something i'm not thinking of?

depending on suit thickness, you want some extra room to keep your head clear of the water. It's not the amount of lead though, it's the amount of buoyancy change. I.e. diver 1 a manatee and are in a bathing suit and need 14lbs of lead. diver 2 is jacked and is in a 7mm suit and needs 14lbs of lead, both divers carry all lead on a weight belt, tank is an al80. Diver 1 does not need that 14lbs factored into their wing because that buoyancy is fixed. Diver 2 needs to factor it in because the wing will need to lift that lead at the bottom.

In OW scenarios, I typically do suit buoyancy plus gas in tank, plus 10lbs to give me enough lift to keep my head all the way out at the surface if I need to. In a cave it's not really necessary though having a buffer is good if you need to pick anything up

 

[doctormike]

Can you explain why it should be 10 lbs greater? It seems like if weighted for neutral bouyancy with used cylinder at the surface, then lift would only need to be inversly proportional to the weight you wear to be neutral at depth. 2-4 lbs extra lift would give more than enough room for error. Or is there something i'm not thinking of?

If you ever have to rescue another diver, it's very helpful to have some extra lift.

 

[Bowers]

depending on suit thickness, you want some extra room to keep your head clear of the water. It's not the amount of lead though, it's the amount of buoyancy change. I.e. diver 1 a manatee and are in a bathing suit and need 14lbs of lead. diver 2 is jacked and is in a 7mm suit and needs 14lbs of lead, both divers carry all lead on a weight belt, tank is an al80. Diver 1 does not need that 14lbs factored into their wing because that buoyancy is fixed. Diver 2 needs to factor it in because the wing will need to lift that lead at the bottom.

In OW scenarios, I typically do suit buoyancy plus gas in tank, plus 10lbs to give me enough lift to keep my head all the way out at the surface if I need to. In a cave it's not really necessary though having a buffer is good if you need to pick anything up

ok, so if it takes 14 lbs of lead to "sink" the 7mm wetsuit, then it would take 14lbs of lift to raise the weights once the wetsuit is collapsed. that is where i get the concept "inversely proportional". In the same scenario, at the surface the wetsuit offsets the weights, so now very little extra lift is required for the head.
I guess what im getting at is that the only time you need the extra lift for head is in the manatee example.
Im not trying to be rude; is there something else im missing or is the 10 lbs extra just for the guy with no wetsuit?

 

[Bowers]

If you ever have to rescue another diver, it's very helpful to have some extra lift.

that makes sense. thanks

 

[shurite7]

Can you explain why it should be 10 lbs greater Don't forget the tank? It seems like if weighted for neutral bouyancy with used cylinder at the surface, then lift would only need to be inversly proportional to the weight you wear to be neutral at depth. 2-4 lbs extra lift would give more than enough room for error. Or is there something i'm not thinking of?

The extra ten pounds is a guideline to keep a person afloat at the surface. From my experience it works. What one also has to take into consideration is the type of tank being used, which includes the weight of the gas inside. Other equipment as well as thermal protection also plays a role. Up here we dive in cold water and many of us use use steel tanks. As an example, one of the guys had a BCD that had a 30 pound lift capacity. He uses 28 pounds of lead and a steel tank. With the BCD on a ST100 tank and fully inflated the unit sank. Therefore, he switched to a BCD that has a 40 pound lift capacity and has had no issues.

When diving in the tropics this is less of an issue, especially when using an AL tank, but the guideline does offer useful information in terms of lift capacity.

 

[JamesBon92007]

How much buoyancy could a XXS, 7mm suit possibly lose at depth? 5 pounds maybe? An AL80 is about 5 pounds more buoyant when empty than when full. That makes 10 pounds. Add a couple more for good measure and that makes 12. OP already has and extra 8 pounds of extra buoyancy with a 20-pounds-of-lift BC (approximately) so what's the other buoyancy for? If OP is weighted properly in the first place so she is slightly negative at the safety stop and can compensate with 1-2 pounds of lift to become neutral then I think 20 pounds is more than adequate.

 

[Bowers]

The exact reasoning slips my memory. It was several years ago that I attended the seminar. From my experience the guideline works. What one also has to take into consideration is the type of tank being used, which includes the weight of the gas inside. Up here we dive in cold water and many of us use use steel tanks. As an example, one of the guys had a BCD that had a 30 pound lift capacity. He uses 28 pounds of lead and a steel tank. With the BCD on a ST100 tank and fully inflated the unit sank. Therefore, he switched to a BCD that has a 40 pound lift capacity and has had no issues.

When diving in the tropics this is less of an issue, especially when using an AL tank, but the guideline does offer useful information in terms of lift capacity.

gotcha. so lift should account for weights, tank, and any other negatively bouyant equipment you carry.

 

[shurite7]

gotcha. so lift should account for weights, tank, and any other negatively bouyant equipment you carry.

Yes. I changed my post slightly after pulling out my quickly jotted notes from the seminar. The lift capacity at the surface is the significant issue, especially at the beginning of the dive. While at the surface a drysuit or wetsuit will add lift capacity, but when one takes off the scuba kit the BCD will sink if there isn't enough lift capacity to hold up the weights and tank plus any other gear attached to the BCD.