Better formula for weighting people?

[divingyogini]

I'm sure that someone has come up with this already, but has anyone used a formula for weighting people that takes into account body composition (corrected from body mass index)

I know that the exposure protection is definitely a factor, but too often shops I've run into just use a person's weight, which is someone inaccurate given that two women can both weigh 140 pounds, but someone who exercises often will have far less surface area than someone who does not, given that muscle weighs more than fat. Has anyone come up with a formula that takes into account, for example, a person's height and waist size or bmi if available?

I guess what I'd be looking for is some formula that resembles something like this:

weight (math operator) height (math operator) body composition or even waistline size (math operator) factor for exposure protection - dry suit or mils (math operator) factor for tank metal composition and size

I haven't seen it in Dive Training, but if anyone has seen, or devised such a formula, I would love to see it.

Thanks, Kimberlee

 

[boulderjohn]

I am not sure from your description what you want. As you seem to understand from your description, the concept of the body mass index is the exact reason why it can't be used in determining your weighting. A person with a BMI of 30 is considered to be obese, and such a person would theoretically need a lot more weight than someone else. However, if you check NFL statistics (and I have), you will find that even some of the fleetest and fittest running backs score 30 on the BMI. These people would need very little weight--if any--to dive.

The BMI has limited usefulness in assessing overall fitness of large populations, but it is of relatively little value in assessing an individual's fitness.

A couple of years ago, DAN's Alert Diver magazine had an article on this, and it concluded that the BMI was the worst indicator of personal fitness of all the other methods commonly available.

A much better method would be to use body fat percentage. Unfortunately, that is very hard to measure. As the article mentioned, the popular skin fold caliper test is inaccurate.

 

[Ken abucs]

I believe that most people float with their lungs full, and sink with their lungs empty. If you fall into this category, you can consider yourself neutrally buoyant for the purposes of weighting.

You then need to figure out the buoyancy characteristics of your gear. I have a little table that has all the items that I use with significant buoyancy attributes. I can quickly figure out that If I use this exposure protection with this tank, this is how much lead I need. The tank figures assume an empty tank.

USD 80 -3.3
Faber 100 -7.3
PST 100 -1.3
Alum 80 4.4
Dry Suit 32
Full 7MM 24
Shorty 3MM 5

I tested all this gear in my backyard pool, so these are fresh water values. Salt water is ~1% more dense than fresh. When I dive in salt water, I have to add 1% more weight of the total package, my weight + all my gear. I'll tend to round up a little, so I add 3 -4 lbs for ocean dives.

 

[divingyogini]

The BMI has limited usefulness in assessing overall fitness of large populations, but it is of relatively little value in assessing an individual's fitness.

A couple of years ago, DAN's Alert Diver magazine had an article on this, and it concluded that the BMI was the worst indicator of personal fitness of all the other methods commonly available.

A much better method would be to use body fat percentage. Unfortunately, that is very hard to measure. As the article mentioned, the popular skin fold caliper test is inaccurate.

Yes, correction, instead of BMI I meant body composition. Though I do think that waist size is very relevant as two people can way the same, but one person's waist could be much bigger or smaller depending on their ratio of lean body tissue.

I'm going into lots of different conditions, with different exposure suits, and my log book is a reference, but on so many previous dives the instructors and dive shops had me weighted so incorrectly, that I'm looking for a more precise way to figure out my weight in different conditions (lakes versus salt), exposure suit, type and size of tank. So I was looking for a better way to do these calculations when I hit a new environment. I know that people do trim themselves and get their weight down, but usually it's just for their local diving.

 

[divingyogini]

I believe that most people float with their lungs full, and sink with their lungs empty. If you fall into this category, you can consider yourself neutrally buoyant for the purposes of weighting.

You then need to figure out the buoyancy characteristics of your gear. I have a little table that has all the items that I use with significant buoyancy attributes. I can quickly figure out that If I use this exposure protection with this tank, this is how much lead I need. The tank figures assume an empty tank.

USD 80 -3.3
Faber 100 -7.3
PST 100 -1.3
Alum 80 4.4
Dry Suit 32
Full 7MM 24
Shorty 3MM 5

I tested all this gear in my backyard pool, so these are fresh water values. Salt water is ~1% more dense than fresh. When I dive in salt water, I have to add 1% more weight of the total package, my weight + all my gear. I'll tend to round up a little, so I add 3 -4 lbs for ocean dives.

This is a great way to test in fact, test the gear first. Unfortunately, I'm one of those people who sinks like a stone even in fresh water.

The formula I've been told, at least for cold water diving, is to take 10% of your weight and add about 5 pounds to compensate for tank drain at the end of the dive, but that doesn't hold true for warm water diving, and the type and size of tank can really change that formula.

I was just wondering if someone had come up with a more general purpose formula, but it sounds like a lot of it, when you travel and obviously cannot bring your own tanks, is taking the time to do a buoyancy check on the surface (which some dive operators are not too keen on because they're running two trips a day).

I live in a cold area and don't like cold water diving, so I'm really looking for a good formula I can use for warm water to account for the differences in the tank sizes and types that I will use depending on the dive operator.

 

[do it easy]

I tested all this gear in my backyard pool, so these are fresh water values. Salt water is ~1% more dense than fresh. When I dive in salt water, I have to add 1% more weight of the total package, my weight + all my gear. I'll tend to round up a little, so I add 3 -4 lbs for ocean dives.

I think salt water is about 2.5% more dense, so this might explain why you round up?

This is a great way to test in fact, test the gear first. Unfortunately, I'm one of those people who sinks like a stone even in fresh water.

Fresh water provides less buoyancy than salt water, so sinking like a stone in freshwater is not as strong a condition as sinking in saltwater. You have to be more negative to sink in saltwater, all else being equal. In otherwords, if you are negative in saltwater, then you will be negative in freshwater, but not necessarily the other way around.

OK, pools open!

 

[Garrobo]

When I first started diving the instructor came up wth the 10% plus five pounds for me too. I weigh 180 and have a little fat around the middle. I ended up carrying 22 pounds of lead because of it. After diving for a while I have ended up with 14-16 pounds in a 3mm suit in salt water. I plan to use a half mm suit on my next dives in the Keys next week. I hope to drop a few pounds of lead on these dives.

 

[GrumpyOldGuy]

Formula's don't work for most people, unless you happen to very Joe Average. Why not just rig up a few weights on clips. You can check you bouyancy properly and leave any unused lead clipped to the line or hand them up to the deck. It should only take a minute.

 

[divingyogini]

When I first started diving the instructor came up wth the 10% plus five pounds for me too. I weigh 180 and have a little fat around the middle. I ended up carrying 22 pounds of lead because of it. After diving for a while I have ended up with 14-16 pounds in a 3mm suit in salt water. I plan to use a half mm suit on my next dives in the Keys next week. I hope to drop a few pounds of lead on these dives.

In Seattle, the shops really overweight people. During my open water class, the instructor had 32 pounds on me (and I'm 5'3 and was < 125 pounds at the time) in a 7 mil farmer john with an aluminum 63. Using the formula I should have been at 18 pounds (rounding up for good measure) I know they need you to sink and stay down during open water, but when you're an open water student and really new to it, you might think that's really the weight you need to dive.

In the Carib, I'm at 6 pounds in a 3 mil with aluminum 80s and my control was great. I'm trying to figure out the compensation for when I switch to steel tanks down there, and trying to figure out a good formula for when we switch back and forth between cold water lakes and cold water salt. Some of the dive boats I've been on, because of drift, don't give you time to do a buoyancy check, it's hurry up and get in the water now, which is why I'm trying to find a more scientific way of doing it.

 

[alphonsis]

It would seem that most formulas are very basic ballparks after which you trim your weight as you dive more. However, to take it to another level, if you knew your body fat content and your weight, you should be able to dial it in much closer on paper. Unfortunately, the most accurate way to measure your body fat content is hydrostatic weighing in which they stick you in a special tub of water with scales to determine your density.

At which point, it's getting redundant, and expensive.