lift requirements of a BCD?

[29South]

I am new to scuba and would like to understand the objective reasoning behind statements of BCD lift requirements.

I am interested in a particular weight integrated travel BC. It happens to have a lift spec of 24lb of lift.

I keep hearing statements like "you need >>30lb lift in colder water" and while i dont discount me being an idiot, it does not make clear sense to me as a mandated statement...and yelling it didnt help me understand either

So i need help, bear with me...

As i understand, a BCD minimum lift spec would derive from three distinct requirements...

1. A diver bouyancy compensation/control device
2. A diver surface flotation device.
3. A rig flotation device.

#1. In its role as a bouyancy compensation device, lets assume you weight to neutral at eye level with zero BCD air.
Is the total negative bouyancy to be compensated for (aka lift requirement) equal to a) the lift to balance the subsequently added 5lbs weight for the full tank, plus b) the loss of bouyancy for the wetsuit at depth?... say approx 3lb for 3mm and say 10lb for a 7mm?.. in this case then only 12-15lbs lift meets the needs for bouyancy compensation..regardless of dive location.

#2. Additional surface flotation lift above that in 1. above is needed dependent upon the diver comfort level to allow him to float at ease on the surface.

#3. It is clearly also prudent for the BCD to be able to float the rig independently of the divers bouyancy. In this case, a weight integrated BCD loaded for cooler climes could result in sinking gear (oops). The diver can of course wear some of the weight on a separate belt to ensure his rig floats.

This would leave the minimum lift spec to be the greater of #1+#2 or #3?...

Do i understand the math here?.... I am certainly not adverse to wearing say 8-10lb on a belt locally to satisfy #3 if it means i can buy a BCD that will fit into something smaller than a steamer trunk for travel.

thanks!

 

[PerroneFord]

Looks good to me.

Of course you could just buy a BP/W and not worry about the steamer trunk.

 

[*Floater*]

I am new to scuba and would like to understand the objective reasoning behind statements of BCD lift requirements.

I am interested in a particular weight integrated travel BC. It happens to have a lift spec of 24lb of lift.

I keep hearing statements like "you need >>30lb lift in colder water" and while i dont discount me being an idiot, it does not make clear sense to me as a mandated statement...and yelling it didnt help me understand either

So i need help, bear with me...

As i understand, a BCD minimum lift spec would derive from three distinct requirements...

1. A diver bouyancy compensation/control device
2. A diver surface flotation device.
3. A rig flotation device.

#1. In its role as a bouyancy compensation device, lets assume you weight to neutral at eye level with zero BCD air.
Is the total negative bouyancy to be compensated for (aka lift requirement) equal to a) the lift to balance the subsequently added 5lbs weight for the full tank, plus b) the loss of bouyancy for the wetsuit at depth?... say approx 3lb for 3mm and say 10lb for a 7mm?.. in this case then only 12-15lbs lift meets the needs for bouyancy compensation..regardless of dive location.

Sounds about right to me, except your lungs can also be used for buoyancy, and you don't have to be neutral at surface... slightly positive with tank empty is okay. The important thing is to be able to comfortably hold your last safety stop, say at 10 ft., with an almost empty tank and wetsuit compressed 10 ft. worth of depth.

In addition, your wetsuit can supposedly loose all it's buoyancy at around 160 ft. so you should be able to compensate for that loss with your BC and lungs while your tank is still almost full.

#2. Additional surface flotation lift above that in 1. above is needed dependent upon the diver comfort level to allow him to float at ease on the surface.

Some people want enough lift so that their whole head stays comfortably above water with wing and tank/s full, so yeah, it may take a little extra from just floating at eye level.

#3. It is clearly also prudent for the BCD to be able to float the rig independently of the divers bouyancy. In this case, a weight integrated BCD loaded for cooler climes could result in sinking gear (oops). The diver can of course wear some of the weight on a separate belt to ensure his rig floats.

This would leave the minimum lift spec to be the greater of #1+#2 or #3?...

Do i understand the math here?.... I am certainly not adverse to wearing say 8-10lb on a belt locally to satisfy #3 if it means i can buy a BCD that will fit into something smaller than a steamer trunk for travel.

thanks!

The right amount of lift is difficult to estimate without getting in the water. For warm water diving 18# travel wings seem to be enough. For other rec diving 30# seems to be enough for almost everybody, at least according to JJ from Global Underwater Explorers - provided that you are correctly weighted. In theory, a large person using a heavy tank and lots of neoprene might need more, or a cold water wet-/drysuit diver who loaded all his weight on the rig and wanted it to float independently on the surface.