Formula for Buoyancy - Initial weight baseline.

[Teamcasa]

Formula for Buoyancy - initial weight baseline.

I was asked the other day for the formula for determining the initial weight for first time divers. I searched hi and low and found no reliable information or chart. As most instructors do a terrible job at worst to a simplistic, weight them down, at best buoyancy drills. Most of the posts just say add a little or remove a little to obtain decent buoyancy. I realize that achieving good buoyancy and trim is a skill learned over time and after many dives. However, when asked, I would like to get closer based on a reliable formula that takes in diver size and weight, tank & BC configuration along with the exposure suit setup. Does it exist? Some place you can plug in the variable and a basic starting point is revealed.

I am making weights for friends and new divers as well.

Dave

 

[donacheson]

... However, when asked, I would like to get closer based on a reliable formula that takes in diver size and weight, tank & BC configuration along with the exposure suit setup. Does it exist? Some place you can plug in the variable and a basic starting point is revealed.

Never heard of one and I find it difficult to imagine that a good one (+/- 2 pounds) exists.

 

[ams511]

I would think it is impossible. The weight of the diver is not as important as the body composition. So a 200 lb football player would use less weight that a 200 lb out of shape overweight guy (muscle is more dense than fat). If fact one of the most accurate ways to measure body fat is by water displacement. The only more accruate method is by dissection.

The DIR fundies book gives a method of weighting that is better than the Naui/Pada method. At least it makes more sense to me. Most instuctors I have seen only guestimate the weight, if the student stays down all is well. If she doesn't then just give her more until she can sink.

I can only speak from my experience but the biggest mistake I see instructors make is not weighting the students in the gear they will use on the checkout dive. My pool training was in a heated pool so no exposure protection was required. For the open water dives wearing a 3/3 wetusit I had no clue over what weight I should use. I should also mention that the shop was part of a chain and used different instructors for the pool training and cert dives.

 

[rockhound13]

The wing calculator that is stuck to the top of the BC forum actually got me within 4 lbs. It's a program, but if you contact the author of the thread he may be able to give you some info.

 

[O2BBubbleFree]

A couple of years ago I posted a copy of the chart I got from the LDs when I got certified. It got mixed reviews: some said it was way off, others said it was just right... PM me if you want me to send it to you.

Just to throw a wrench in the machine, I'm probably over 20% body fat and am negatively buoyant (see my sig line). In fresh water with no neoprene I can sit solidly on the bottom for the rest of my life. With a 3mm shorty I can very nearly do the same. (I use 2# of lead because 1# is hard to trim...)

So, body composition makes a big difference, but you can't always tell by looking, either.

 

[ams511]

So, body composition makes a big difference, but you can't always tell by looking, either.

Not to fight with you O2 but body fat measurements are general at best. I have one of those scales and if I change by body type setting from "normal" to "athletic" my bodyfat drops from 35% to 19%. Personally, I think I am in between and only use the measurements to see if I am dieting in the right direction. The fitness mags back me up on this, unless you get an expensive water displacement test your measurements could be off 5% or more. So your 20% could really be 15% to 25%.

However you are right body composition is not the entire story. I think you would need to use some sort of weight / displacement method to be accurate. However I do not think this is what the original poster was looking for.

 

[Ben_ca]

Here in Northern CA most guys use 10% + 5 as a starting point... That's with a mil wetsuit... Not sure if this is what you are looking for...

 

[O2BBubbleFree]

<hijack continues>

Not to fight with you O2 but body fat measurements are general at best. I have one of those scales and if I change by body type setting from "normal" to "athletic" my bodyfat drops from 35% to 19%. Personally, I think I am in between and only use the measurements to see if I am dieting in the right direction. The fitness mags back me up on this, unless you get an expensive water displacement test your measurements could be off 5% or more. So your 20% could really be 15% to 25%.

However you are right body composition is not the entire story. I think you would need to use some sort of weight / displacement method to be accurate. However I do not think this is what the original poster was looking for.

Back when I was in the Air Force the caliper test they did had me at 20%. That was 10 years ago. Now I'm 6'8" and about 225# (about 20# heavier than I was then).

Now I know from fooling around with SCUBA that if I did a float test (assuming they float you in 'just' water) I would sit solidly on the bottom, which would mean 0%, right?

I'm not that muscular. Just dense.
<end hijack>

 

[Carribeandiver]

a person's size and weight cannot be considered as a factor in determining weight amounts. How solid or flabby is the individual? Doesnt muscle weigh more than fat? Isnt muscle ore dense and therefore not as buoyant?
A man 5'10 weighing 185 pounds but with 20-25% fat is going to use less weight than a man 5'10", 185 lbs that is only 10-15% fat.
The same for women, body fat contributes to the amount of weight needed. So, to get a formula, body fat has to factored in as well as size and weight.

 

[Teamcasa]

Well it seems there is no baseline chart - so far. This just might be an opportunity in the making! If they can hit an astroroid or a comet with a probe we should be able to make a buoyancy chart.

Take in all of the known variables and estimate the others based on some real data and we should be able to come closer than 10#!


Dave