Lift capacity BCD

[DiveNav]

Hi there!

I noticed that BCD manufacturers rate the lift capacity in pounds.

I am wondering if the number given is for salt water or fresh water ...

In theory the BCD generates a volume displacement and the lift is equal to the weight of the water displaced. And we all know that the weight of fresh water is less than salt water.

Any input?

Thanks

I_AM

 

[H2Andy]

i don't know, but i would imagine fresh water (easier to test)

according to Dive Rite, the proper way to measure lift is by taking a bladder (or BC) and inflate it fully ... then start adding weight to it -- the weight applied to submerge the air cell while keeping the aircell fully inflated is the lift of the aircell.

so, if it takes 30 lbs to sink the BC, the BC has 30 lbs of lift

since it takes more weight to sink the aircell in salt water (compared to fresh water), it will have more lift in salt water than in fresh water ... by about 3% .... but i am not sure about that ... how did i get there?

since a cubic foot of fresh water weighs 62.4 lbs and a cubic foot of sea water weighs 64 lbs, a cubic foot of salt water will weight 3% more and thus it will take more weight to sink a bladder in salt water... whereas it will take 3% less weight in fresh water to sink the same bladder

so, if a bladder has a lift of 30 lbs in fresh water, it will have a lift of 30.9 lbs in fresh water -- not a huge difference

someone please check my logic

 

[Charlie99]

The difference between the lift in seawater and in fresh water is less than 3%. The unit to unit variation in BCD lift can far exceed that. Indeed, there were some reports of one manufacturers older 36 pound wings only having about 30 pounds of lift.

An alternative to seeing how much lead it takes to sink the BCD is to see how many pounds of water you can pour into the bladder.

1. Weigh the BCD.
2. Unscrew a shoulder dump and start pouring in water.
3. When full, reweigh the BCD and see how much weight it has gained.

The weight gain is the buoyancy of the BCD (assuming that you haven't added extra weight by getting the BCD wet while filling it --- in that case, empty out the bladder and check the empty weight again to figure out how much water was inside).

Another method of checking buoyancy is to see what volume of water it takes to fill the BCD. The metric rating for BCDs is "liters of capacity" and "Kilograms of lift". For practical purposes, they are the same since water is about 1kg per liter. See how many 1/2 liter bottle of water it takes to fill the BCD. Each 1/2 liter bottle equals 1.1 pounds of lift.

Charlie Allen

 

[cool_hardware52]

Hi there!

I noticed that BCD manufacturers rate the lift capacity in pounds.

I am wondering if the number given is for salt water or fresh water ...

In theory the BCD generates a volume displacement and the lift is equal to the weight of the water displaced. And we all know that the weight of fresh water is less than salt water.

Any input?

Thanks

I_AM

Salt water is about 2.5% more dense than fresh (depending on just how salty)

A bc with 30 lbs of lift in fresh water will have ~30.75 lbs of lift in salt water, no a very big difference.

What is siginifcant however is the difference in how buoyant you and your gear will be in fresh vs salt water.

Consider a diver, with all his gear, who weights 240 lbs. Assume he is perfectly weighted for salt water. Now put him in fresh water. 240 x .025 = 6

He will be 6 lbs overweighted for fresh.

Rule of thumb: 1 lbs of ballast difference between salt and fresh for every 40 lbs of diver and gear (1/40 = .025)

Tobin

 

[DiveNav]

Hi Tobin,
thank you for the clarification.

Considering that my reference BC is actually a DSS one ..... let me ask few questions directly:
When you state 40LB as lift capacity of your BC, do you mean lift capacity in fresh water or lift capacity in salt water?

One more question:
What is the lift variation from unit to unit?

Regards,

I_AM

 

[cool_hardware52]

Hi Tobin,
thank you for the clarification.

Considering that my reference BC is actually a DSS one ..... let me ask few questions directly:
When you state 40LB as lift capacity of your BC, do you mean lift capacity in fresh water or lift capacity in salt water?

One more question:
What is the lift variation from unit to unit?

Regards,

I_AM

We use fresh water to measure lift, and there will be some variation due to differences in cutting out the pattern, and sewing accuracy. Commonly this is about 1 lbs in the smaller wings and 2 lbs in the larger doubles wings.

This can be dealt with when calculating your required lift.

Tobin

 

[DiveNav]

We use fresh water to measure lift, and there will be some variation due to differences in cutting out the pattern, and sewing accuracy. Commonly this is about 1 lbs in the smaller wings and 2 lbs in the larger doubles wings.

This can be dealt with when calculating your required lift.

Tobin

Tobin,
thank you.

I got my answer: fresh water.

Regards,

AM

 

[ScottZeagle]

according to Dive Rite, the proper way to measure lift is by taking a bladder (or BC) and inflate it fully ... then start adding weight to it -- the weight applied to submerge the air cell while keeping the aircell fully inflated is the lift of the aircell.

so, if it takes 30 lbs to sink the BC, the BC has 30 lbs of lift

That is how we do it at Zeagle as well....in fresh water.

Thanks,

Scott