Buoyancy

[Electricgahs]

Hi,

I have been looking around to find a kind of " formula " to calculate, based on the kind of gear you are wearing during a dive, the ideal amount of weight which one has to carry in order to gently sink at the start of the dive, and to be almost neutrally buoyant at your last shallow decostop.
For some divers is a learning process : you keep adding or removing weights until you reach the best...some other divers seem to know exactly how many kilos they require.
Is there a kind of " checklist " to follow in order to reach this goal ?
If is not there, maybe we can try to put one together, so that everyone can benefit from it.

P

[kath2407]

Whilst the principle may be a good idea it would be nigh on impossible to create a forumla which fits all people.

the human body has variable bouyancy for each individual person as well as the 1000's of configurations of wet suits, dry suits, undersuits, steel tanks, ali tanks, steel back plates, ali back plates, 15l cylinder, pony, twins. The combinations are endless.

I have a rough idea for myself as to how different configurations of kit work for me, but to apply those princples to others would be difficult.

K XX

[Nigel Hewitt]

I'm a working physicist and I can't offer a formula that's good enough to dive on.
I log my kit/weight in the log book and I can usually turn up something similar but new kit needs a re-test in controlled water or I just dive overweight.

This still didn't stop me messing up the first dive of my recent holiday by being about 3Kgs underweight. :mad:

Few things are intuitive. I dangled my hood and gloves in the water-butt and measured 1.5Kgs of buoyancy, which was more than I expected, while my two 7L stages that are back-breaking climbing the ladder are virtually neutral in the water and empty.

[Electricgahs]

I log my kit/weight in the log book and I can usually turn up something similar but new kit needs a re-test in controlled water or I just dive overweight.


So you believe in doing a "try-dive" first, to check the buoyancy, and then going for a more demanding dive ?
I have done this with my actual kit configuration, and I reached near perfection...

[kath2407]

All new configurations should involve a weight check in sheltered water (peferably in salt water if your "more demanding dive" will be in the sea too)

I log my weight and air consumption for each dive I do and I can usually identify a pattern when I use similar or regular kit configurations.

K XX

[Richard Whitcombe]

No such thing as a weight formula. Despite the ones touted around in some warm water manuals these typically leave the person grossly overweighted and even then only applies to 3mm suit and aluminium tanks.

Weighting doesnt NEED a complicated forumula - it takes less than 5 minutes to sort out on a dive and is tailored to the individual.

Given every single person is a different size, shape, different bone density, different fat content, different fit suits, different types of kit you cant just shove numbers through an equation.

Get into the water but not wearing many weights, fully empty your wing & suit, breathe out and get someone to hand you weights until you have enough that you JUST start to sink. Then add to that figure the weight of gas in your tank (2-3kg if its a single) and you're good to go. Simple, quick, easy and it works. Unlike any magic formulae. Your goal with correct weighting is to be able to hold a shallow stop (say 3m) with a completely empty cylinder and no gas in the wing (and maybe just a bit in the suit for warmth).

[Vic]

I have been looking around to find a kind of " formula " to calculate, based on the kind of gear you are wearing during a dive, the ideal amount of weight which one has to carry in order to gently sink at the start of the dive

Yep. Dead simple.

m(pb) >= pV(t) - m(d) - m(k)

Where:
m(pb) is the mass of lead required
p is the density of the water you're diving in (don't know how to display a rho here)
V(t) is the total volume of the diver and his kit at the surface
m(d) is the mass of the diver
m(k) is the mass of his kit (less the lead you're calculating)

Alternatively, forget about formulae and go measure it.

Vic.

[Electricgahs]

Get into the water but not wearing many weights, fully empty your wing & suit, breathe out and get someone to hand you weights until you have enough that you JUST start to sink. Then add to that figure the weight of gas in your tank (2-3kg if its a single) and you're good to go. Simple, quick, easy and it works. :)

I guess that is the way...

[thereisnofishofsuchvirtue]

There are some basic guidelines in PADIs PPB course:

http://farm4.static.flickr.com/3157/3025070342_368d17ecef.jpg

[Richard Whitcombe]

Those are the ones i mentioned that would leave most divers grossly overweighted. Especially with steel tanks.

[nigelhoath]

Those are the ones i mentioned that would leave most divers grossly overweighted. Especially with steel tanks.

Yup according to that I would need 14 to 15 Ks but I just moved from 9 to 10 ks and everything is perfect :D

[Tony Dwyer]

Way over the top for me.

The recommendation for me in the sea would leave me 5 kgs heavy. NOT fun. :(

Body density is not a constant. There is a very wide range of density among any group of people.

Women tend to be less dense than men. (swmbo has been telling me that for years!)

[Mike Halligan]

That explains quite a lot. I'd be about 5kg heavy if I used that.

[nigelhoath]

So from three examples the formula is perfect providing we apply Einsteins missing constant (-5) :D

Or perhaps the author of the formula was using the old pre-decimalisation kilos ??

[paulg]

The new BSAC Buoyancy & Trim workshop suggests as a starting point use:
● UK 7% body weight (fresh)
● UK 10% body weight (sea)
then adjust for different kit.

See http://www.bsac.com/page.asp?section=1726&sectionTitle=Buoyancy+%26+Trim+Workshop for more on it.

Paul

[Mike Halligan]

The new BSAC Buoyancy & Trim workshop suggests as a starting point use:

then adjust for different kit.

See http://www.bsac.com/page.asp?section=1726&sectionTitle=Buoyancy+%26+Trim+Workshop for more on it.

Paul

So the quote suggests 5kg more than the BSAC, which seems to be broadly accurate. This translates to over a gallon of excess air being dragged around underwater, with buoyancy compromised accordingly of course. :cool:

Hmmm, I suppose there must be a reason.

[Richard Whitcombe]

All this highlights is how pointless formulae are in all this and how important a proper weight check is.

Its EASY to write a formula to guarantee people will sink very well. That however isn't what you want to be doing. Sadly, its what quite a few people do!

[PeteM]

Its EASY to write a formula to guarantee people will sink very well.

Yep: Weight required = 40Kg.

The formula under discussion are only slightly less useful.

[Electricgahs]

There are some basic guidelines in PADIs PPB course:

http://farm4.static.flickr.com/3157/3025070342_368d17ecef.jpg

I tried that and I would be 3.9 Kg over...