How much BC lift do I require?

[ScubaWim]

Am looking for a BC and am interested in the Cressi J-107.

From a test in a German magazine I have learned that the lift of this jacket is limited to 12,1 KG (appr. 27 pounds).

I weigh appr. 175 pounds. Can anyone tell me if the lift of this jacket is insufficient or not? I will probably never dive deeper than 110 feet.

Furthermore I am of course interested in any experiences with this jacket.

 

[steevke]

12 kg is sufficient for typical single tank diving when wearing a drysuit. If you're wearing a thick wetsuit (double 7mm) you might want a bit more lift (say 40 pounds).

YMMV.

 

[IndigoBlue]

Am looking for a BC and am interested in the Cressi J-107.

From a test in a German magazine I have learned that the lift of this jacket is limited to 12,1 KG (appr. 27 pounds).

I weigh appr. 175 pounds. Can anyone tell me if the lift of this jacket is insufficient or not? I will probably never dive deeper than 110 feet.

Furthermore I am of course interested in any experiences with this jacket.

We are left wondering the following:

1) Where do you plan to dive? This matters because there is a big difference between diving warm water or cold water.

2) What type of thermal suit will you wear? This matters because there is a huge difference in how various types of suits handle compression due to depth.

3) What type of scuba tank will you wear? This matters because different tanks will have different buoyancy characteristics when almost empty (500 psi).

All of the above will determine how much if any lead weighting you will need to wear.

Once you have determined how much lead weighting you will need to wear, then by hanging your weight belt onto your B/C with your full tank attached, you will be able to see if your B/C has sufficient lift capacity to remain neutral with all of this weight attached to it.

Then that is where the current arguments begin, the question being, should your B/C be able to support all of your in-water weighting requirements alone? And how much more, if any, additional lift capacity is needed?

Then you will hear about a dozen or more different opinions. The question you asked comes up a lot.

 

[ScubaWim]

Mostly I will be diving in tropical conditions, or occassionally here in the Netherlandsbut never in watertemps below 60 degrees Fahrenheit. Suit will vary from 7 mm fullbody to 2,5mm shorty, wet or semi-dry. Tanks in Tropics mostly 12 Liters and mostly aluminium.

Option of testing in tropical conditions is not possible, will buy my BC here...

 

[Pez de Diablo]

Am looking for a BC and am interested in the Cressi J-107.

From a test in a German magazine I have learned that the lift of this jacket is limited to 12,1 KG (appr. 27 pounds).

I weigh appr. 175 pounds. Can anyone tell me if the lift of this jacket is insufficient or not? I will probably never dive deeper than 110 feet.

Furthermore I am of course interested in any experiences with this jacket.

For the style of diving you are planning, this should be lots of lift. Think about everything you bring into the water and the items that are buoyant vs the items that are negative and neutral.

Wetsuit 7mm, positive 6 lbs
Weltbelt, negative (just enough lead to counter all positive items)
Regs, negative 2 lbs
tank, alum - positive when near empty 4/5 lbs
Accessories, ?
You ? maybe 2 pounds buoyant with a full set of lungs

Remember your goal is to be at the surface only slightly negative, but keeping in mind the buoyancy characteristics of you tank if it's aluminum it will become buoyant when near empty.

So if you are just slightly negative at the end of your dive, how much lift do you need from your BCD? Not much. However also keep in mind that you need enough lift to insure you are positively buoyant at the surface and also you need a couple of extra pounds of lift for a fudge factor.

Another consideration is the depth that you will be diving to and the effects on your gear. For instance at depth your wetsuit will compress and loose some of it's buoyancy. So add another pound or two to the equation to compensate for this.

Just some of the items to think about. Then take a look at the net weight in the water, with a full tank, you will be more negative than with a near empty tank by at least the 4 to 5 lbs that it will become buoyant.

Now this is where your weight belt comes in. You only need enough to counter all the buoyant items and you don't ever want to carry to much lead and be too negative. Being to negative poorly effects your trim in the water increasing your drag and your work load, thus increasing the amount of air (gas) you will consume and shorten your dive time. Not to mention the safety issues.

I dive with an Abyss 45 lbs wing, a drysuit and a set of E8-130's and I can safely carry a steel 80 stage bottle. The 45 lbs wing is enough lift for this equipment.

I hope this helps you to understand how to go about choosing the proper amount of lift required.

 

[steevke]

Wim,

don't forget to take the set itself into account. If attach a heavy HP steel tank to the BCD, a camera and maybe a tank mounted light (style Metalsub), you want your set to float if you dunk it into the ocean. 12 kg is more than sufficient for diving in tropical conditions, but for diving in Holland I'd go for just a little bit more lift.
YMMV.

 

[fdog]

Wim:

My BCs have lift for the worst-case scenario, which is at depth with full tanks. Here my suit is squeezed enough that it provides no lift, so my BC has to provide lift for my tank(s), lead, and equipment.

Indigo Blue wrote:

Once you have determined how much lead weighting you will need to wear, then by hanging your weight belt onto your B/C with your full tank attached, you will be able to see if your B/C has sufficient lift capacity to remain neutral with all of this weight attached to it.

This is an easy way to see if you have a BC that can at least meet the minimum.

I personally add 11 lbs (5 kg) of reserve bouyancy to the minimum needed, to accomodate extra equipment, or small items I might pick up from the bottom. The number comes from the weight of the human head, which is a good thing to float out of the water at the surface.

A direct application of this came when my wife and I began planning a trip to a location that uses steel 104s. She wears 16 lbs with her wetsuit and core warmer, and her BC has 21 lbs of lift. After consulting a chart of tank specs a good tank chart here
we realized her BC was too small, since her worst-case scenario is 27 lbs. So, we are getting her one with larger lift.

All the best, James

 

[ScubaRon]

There is a good calculation in this thread .

 

[Wijbrandus]

How do you figure out how much lift you should have?

So far, I'm a warm water diver. But, living in Colorado means cold water if I want to dive locally. I'm getting my AOW this spring, and fully intend to jump into cold water starting soon afterwards once the ice breaks.

For single-tank diving (just AL80 for now), how much lift would be needed for cold water? How does one figure it? Does it change much when I move to a steel HP tank?

The reason I'm asking is I'm preparing to purchase my own bp/w before my AOW class starts, and I don't want to spend the money on the wrong wing.

Seems to me that the 30# size would be appropriate, but is that suitable for drysuit diving in cold mountain lakes?

Thanks!

 

[ScubaRon]

This question has been posted dozens of times, but I'll repeat the calculation again:

Following is the formula to calculate the lift capacity of your BC for recreational diving:

weight of the air in the largest tank you'll dive with at max. pressure minus 500 psi [A]
+ buoyancy of the thickest wetsuit you'll dive with x 0.7
+ buoyancy of any articles you may be carrying or picking up UW [C]
+ safety margin [D] )
x 1.1 [E]
= BC lift capacity needed.

[A] At the start of your dive this is how much "overweight" you are; your BC needs to compensate for this. E.g. 80 cu. ft. of air at 2500 psi (=3000-500) weights 4.8 lbs. 120 cu. ft. at 2500 psi weights 7.2 lbs.
At 132 ft. your wetsuit will lose approx. 70% of its buoyancy. If you know how much weight you need wearing this wetsuit in the ocean and how much weight you need wearing just swimming trunks in freshwater (e.g. in a pool), your wetsuit buoyancy is the difference. At 132 ft. your wetsuit will loose about 70% of it's buoyancy due to compression. Your BC needs to compensate for this.
[C] Camera's, reels, lights etc.
[D] A safety margin of at least a few lbs is recommended, just in case you need to help your buddy carry something, go a little deeper or whatever.
[E] BC's typically have a 10% tolerance in their lift capacity. So a BC advertised with a 40 lb. capacity, may in reality give you anywhere from 36 to 44 lbs. To be on the safe side we assume the worst.

So a diver with a wetsuit with 20 lbs buoyancy, diving 120 cu. ft. tanks with a max. pressure of 3000 psi, carrying a 1 lbs camera and using a 5 lbs safety margin will need ( 7.2 + (20 x 0.7) + 1 + 5 ) x 1.1 = 30 lbs of buoyancy.