fresh to salt weight conversion

[bowtieman427]

Rule of thumb start by adding 2.5 - 3% more total weight. You do not need dispalcement or Volume to figure it out. You only need to know your total weight you + your gear.  First, you must get yourself perfectly neutral bouyent in fresh water. Next, with all your gear it took to do this go wiegh yourself and then all your gear add it up (make sure your gear is dry). The displacement or volume is not necessary. We know density of both fresh and salt water (volume cancels out) and can find your total weight with gear. given density of salt water = 64lbs/ft^3 density of fresh water = 62lbs/ft^3
(density of salt water / density of fresh water)
((64 / 62.4)X (you + all gear dry weight)) - ( you + all gear dry weight) =total weight you need to add to your neutrally bouyant fresh water wieght to be neutrally bouyant in salt water.
if you and your gear weighed 232 you would need to add 5.948lbs or 6lbs

[Seal]

Salt water for a 6.5 suit is 15% of your body weight plus 5 pounds assuming you are not diving in a current.
Fresh water for the same suit is 10% plus 5 pounds.  This is a pretty close in most cases in how to figure.

[arcFlash]

I'm 5'11 and 150lb, full wetsuit is 8 pounds freshwater, I use 22 for my drysuit. If I was to use 10%+5 in my wetsuit, I'd have a miserable dive. In a 3 mill shorty, I do not need weight.

I understand most people are heavier than I and 10% may be close for lots of people but it's not going to be correct for the athletic builds.

It's likely we can use the BMI calculator and then plot a line for 7mil suit. That way you look up height/weight and it gives weight belt size.

[Dragon Eye]

I got a chuckle on the AOW weight calculations- I know they are suppose to be approximate, but I would of been an anchor in all scenarios.

[mattd]

I take the same weight in a wet suit in fresh 7mm vs salt 3mm - 24 pounds.  If I dropped 12 pounds and jumped in fresh water in the 3mm, I'd be a cork for sure.  In a drysuit with a thick undergarment in fresh, I need 30 pounds (16 on a belt, 6 pound plate, 8 pound weight plates).  Add another 5 if I change out my steel tank to an aluminum.

I have yet to find a rule of thumb that works right.  I'll just go with doing weight checks every time I change conditions.

[DetectorGuy]

I agree that there are so many variables that could affect the amount of weight, that a weight check when you change configurations is key:
SS backplate weighing 6 pounds versus Alum backplate weighing about half that.
Steel tank versus alum.
Type and age of neoprene in a wetsuit.
Male diver versus female diver.
Individual body type differences.
Type and amount undergarments in a drysuit.
Buoyancy of pony or stage if used.
Buoyancy of extra equipment - canister light or video light batteries.
Even to a slight degree, temp of the water (no pun intended).
current of water.

I always chuckle when I hear the phrase "rule of thumb" since I heard the origin of where that phrase came from. In the old days it was illegal to beat your wife with a stick bigger than your thumb. Let the comments fly, but thats what I heard.

[arcFlash]

Hydro and I have been working some math.... ;D Geeks I guess.

So the basics of human buoyancy for me at least.

You need your average density so go get a bodyfat% from and your weight in Kg.
Mine is 68kg and 9%fat. Calculate an average density.
9% of 68kg is 6.12kg times that by the density of fat (.9g/ml). Do the remaining Kg (61.88) times the density of muscle (1.06) and divide the sum by weight.
68kg/(6.12kg*.9 + 61.88kg*1.06) = 1.04g/ml so I'm denser then water and sink.

My volume is then 68,000g/1.04g/ml = 65.384L = 17.2 gallons = 143.6lb of water and I weigh 150 so I do not displace enough water and would need 6.4lb lift to float. Average lung volume is 6L or 12Pint or 12pound so I float when I hold my breath.

Take my average breath at 3L is about 6pounds lift, I'm neutral. I'm really just compensating for the suit at 8lb lead.

If you do this it should give you an idea of what you need to do to offset your natural buoyancy but then 15 minutes at the lake side and you'd know as well.

I don't know if this helps anyone but it's fun knowing. And I may have made a math/logic error as well but I think it's correct.

[Terry]

                  Now Steve I'm impressed by the time you spent to screw around to the 100th of a kilo in your calculations, (not nearly as impressed as I was at the job you did rolling up my anchor rode and dock lines the other day, your welcome on my boat anytime!) but am wondering about your assumption that what isnt fat, is just muscle, what about factoring the weight vs density of blood and bone. I mean your in pretty good shape and all, but to say your body weight is 91% muscle, why then, arent you modeling in one of those muscle mags that the women like to rave about, having to beat the women back with a stick at the dock before you climb on my or Pete's boat? Not that I think that would be a bad thing for us single guys like Joe and I, you draw them in, Joe and I go in for the kill. Let me know your thoughts on this.      THANKS!      Terry

[mermaid]

This is a picture of Terry and Joe sharing ideas on how to "go in for the kill" >

[arcFlash]

Joe brought up that as well as trapped gas in your, um, bum. But we decided that average density is going to be fine and fat is the least dense and the greatest difference in one gender. We all have some bones and organs but that's going to be irrelevant as we all have roughly the same so we could simply assign the density of 1 to them.

Height/weight/waist ratios should even out most of the irregularities in all of us. But it is hypersensitive to Waist size so play with it until you get a number that isn't unbelievable and your likely within a few points of the true %.

It's also a very narrow window for most people float or sink on a breath. You can't be all fat at .9 and you can't be all muscle at 1.06 so you only have .16 of a gram to play with. I know you can walk and that's about 2/3 the muscle you have and your head is all meat so really you may have .08 to play with so getting into bone v. organ is adding complexity at the same rate of error.

Kind of a odd fact I ran into while looking for data. A body drown will sink because inhaled water displacing trapped air, while a body place in the water will float from the trapped air. Something like 7% will sink regardless. I happen  to be in the 8th percentile for body-fat and I'm 6.4lb negative so that makes sense and I'm a sinker.