Help with BCD's

[Gnasher]

Hi

My better half has real problems being buoyant enough on the surface ie her mouth is barely clear of the water and we’ve be advised her BCD doesn’t have enough lift but it's working fine.

How do you work out what size/lift capability you need when choosing a BCD? Having read allsorts in manufactures catalogues about what lift a particular BCD has ie 15kg or 20kg etc they don’t tell you how to work it out.

How is it done?

Cheers

[Adrian Kelland]

Perhaps she is overwieghted. Has she done a weight test recently?

[Nigel Hewitt]

My better half has real problems being buoyant enough on the surface ie her mouth is barely clear of the water and we’ve be advised her BCD doesn’t have enough lift but it's working fine.Diving a single cylinder you need 14 or 15Kgs.

That covers all eventualities other than carrying too much lead.
It should put your head and neck out of the water even at the start of the dive.

Your head weighs 7Kgs (if you don't believe me cut it off and put it on the kitchen scales) and the gas in your tank weighs 3Kgs so you could get by with 10Kgs of lift but a suit flood might leave you in trouble so that you had to dump lead to get back to the surface.

Twinsets, where you can carry 7Kgs of gas on your back and another 5Kgs in your stages, need 20/22Kgs.

If she is too low in the water take some lead off. You can't buck basic physics.

[Mark]

It does sound like too much lead, I'd do a proper weight check.

Also, with some BC's if the straps (particularly the waste strap), are pulled too tight then the bc cannot be fully inflated, meaning that no matter what it's stated lift is, it isn't going to be able to achieve it.

[sharmite]

I agree that weight could be the problem but also bad fitting.Are you diving dry or wet as this obviously makes a difference to required weight.When wearing the bc how much space is left in the arm holes.If this is too big then you tend to slump lower in the bc,also some of the lift on some jackets is in this area which means the jacket is clear of the water but you are not.What make and model is it.I am sure a local shop or club would advise if you took it too them.
hope you can sort it without a longer snorkel..:D

[John Bantin]

Manufacturers usually calculate the lift of a particular BC by masuring the internal volume by filling with water. A litre equals a kilogram. When we do BC comprison tests we compare max lift (by using weights) and ride-heights. That is to say the height of the mouth above the water for the same diver and rig with each BC fully inflated. We get some interesting results. (see www.divernet.com comparison tests.)

[Nick Kay]

Twinsets, where you can carry 7Kgs of gas on your back and another 5Kgs in your stages, need 20/22Kgs.
Hi Nigel

Whilst I agree with you in principle, you have to take each case on its own

For example, I dive OC:
- Twin 12s, 232B
- twin 7s
- CD TCW
- Portland Engineering s/s backplate
- Mares Abyss regs
- DUI 200CFX, Weezle thinsulate
In this configuration, I need 0Kg/2Kg (even when down to 50B) in freshwater, 2Kg/4Kg in seawater

The strange thing is, that a mate dives virtually the same configuration - I think the only difference is the backplate - he needs 8Kg to hold 3m stops in freshwater - we've weight-checked him!

[Gnasher]

I was suspecting the same as you have all stated plus I felt her BCD wasit strap was too tight, her instructor tells her to keep it tight! :confused: We are diving again on Sunday and Monday so will first loosen the BCD and second try less weight, she was only carrying 2kg ;)

Thanks will let you know how she gets on but I would still like to konw how it's all worked out in a simple rule of thumb way.

later

[Nigel Hewitt]

Hi Nigel

Whilst I agree with you in principle, you have to take each case on its ownWooo.
You've jumped subject from lift to lead.

[Edward]

Here's a simple test.

Set up the aqualung, fully inflate the jacket, lay the weightbelt over the whole thing and see if it floats.

Somewhere on the jacket there should be a lable giving its capacity in Liters, as JB said, use 1 Lt volume for 1 Kg of lift.

Jackets designed for warm water may only have 12-15 kg of lift, for UK diving I would suggest a minimum of 18Kg. My own has 25Kg.

HTH

Edward

[Mike Halligan]

We are diving again on Sunday and Monday so will first loosen the BCD and second try less weight, she was only carrying 2kg ;)

Thanks will let you know how she gets on but I would still like to konw how it's all worked out in a simple rule of thumb way.


You can try something before getting wet. If your good lady would don the BCD and tighten it to the extent that she has understood her instructor wishes, you can then inflate the BCD until it overflows. Release the adjustments for fit, straps and cummerbund, even taking the BCD off altogether, and see whether it will accept further inflation. If it doesn't then at least you know that over-tightening isn't the problem.

If it will take more gas, then refit the BCD whilst truly full and mark where the adjustments lie, so that you both know what tightening to apply on Sunday. Depressurise the BCD and look carefully how the adjusters lie - this is their "right" place.

In cases of doubt, always 'fit' the BCD when full. Hope this helps,

[Gnasher]

Hi

Well its all sorted was a bit of both ie too much weight and BCD little too tight. She's now also started using her wet suit and her buoyancy issues have completely gone the BCD lifted her and her instructor. :)

Thanks for all your help.