Weight calculations

You know those little formulae poeple use to guess how much weight they need? Well here's one that can tell you how much weight you need to go from fresh to salt that actually works (unless my napkin I threw this together on was wrong):

Wsw = Wfw + V * (Dsw - Dfw)

where Wsw/Wfw is weight needed in seawater/freshwater,
Dsw/Dfw is density of seawater (1.035 kg/L)/freshwater (1.0 kg/L),
V is the volume of the diver w/ gear.

Well, no one knows V but you can figure it out without getting into the tub (but you will need to gear up on a scale)...

If you need 2 kg (this is done in metric because the numbers are nice and round) to be neutral at 15', 500 psi then that means your density with those 2 kg weights is 1 kg/L (the approximate density of fresh water). If you, with your gear and 500 psi tank weigh 98 kg (without the lead), you must have 100 L in volume (98 kg+ 2 kg)/100 L = 1 kg/L - neutral. Of course, I'm discounting the volume of the lead but close enough.

Anyways, plugging all the number in, I get 5.5 kg.

Those of you who live by the ocean and are diving there pretty regularly could care less about this but the rest of us inland divers who only see the ocean once or twice a year may not want to mess around with a proper weight check.

I'm not sure of the error in using 1 kg/L for freshwater would be (that's the density of distilled, O2 free water at 3.98 C) but it surely varies from lake to lake and it should be pretty close (can any hydrologists verify this?)

Here are the numbers in imperial units:

Dfw = 8.34 lbs/Gal
Dsw = 8.64 lbs/Gal

You'll be figuring your volume in gallons and your weight will be in pounds.

The fabric pouch weight belts with weight bags are the easiest and most reliable way to find out what your weighting needs to be. You can do it at the pool and then multiply by 1.025641025 which comes from the quotient of the density of seawater over freshwater.

64 / 62.4 = 1.025641025

IndigoBlue once bubbled...
The fabric pouch weight belts with weight bags are the easiest and most reliable way to find out what your weighting needs to be. You can do it at the pool and then multiply by 1.025641025 which comes from the quotient of the density of seawater over freshwater.

64 / 62.4 = 1.025641025

Click to expand...

Nope, the difference in weight from fresh to sea water cannot be determined just by some multiplication factor. Let's say I have a 1 L box that weighs .9 Kg, I am going to need to add another .1 Kg to make it neutral. Now lets say I want to make it neutral in sea water, I am going to have to add an additional .035 Kg to make it neutral (density of sea water is 1.035 Kg/L).

Now let's say we have a 1 L box that weight .8 Kg, it is going to need .2 Kg to make it neutral in FW but it will still only need an additional .035 Kg to make it neutral in sea water.

In the first example, the ratio of the weights needed is .1/.135 but in the 2nd example, the ration of the weights needed is .2/.235, quite a bit different. The reason it is different is because we started out with boxes of different densities (just like divers and their gear have different densities). There is a ratio for the amount of extra lead you and your gear need for going from fresh to salt but that ratio will not work for the rest of us.

1. Find your weighting for neutral in freshwater.

2. Take your overall weight with full equipment and divide by 40.

3. Add that to your weighting and go diving in salt water.

If you're hitting the water at an overall weight of 240 pounds, adding six pounds will put you very close.

The amount of weight that is on a belt is a side issue and not something you need to worry about for overall weighting.

Don Burke once bubbled...
1. Find your weighting for neutral in freshwater.

2. Take your overall weight with full equipment and divide by 40.

3. Add that to your weighting and go diving in salt water.

If you're hitting the water at an overall weight of 240 pounds, adding six pounds will put you very close.

The amount of weight that is on a belt is a side issue and not something you need to worry about for overall weighting.

Click to expand...

Lets see if this works:

240 / 40 = 6 [method A]

240 x 1.02564 = 246

246 - 240 = 6 [method B]

Yes, it works perfectly!

Don Burke once bubbled...
1. Find your weighting for neutral in freshwater.

2. Take your overall weight with full equipment and divide by 40.

3. Add that to your weighting and go diving in salt water.

If you're hitting the water at an overall weight of 240 pounds, adding six pounds will put you very close.

The amount of weight that is on a belt is a side issue and not something you need to worry about for overall weighting.

Click to expand...

It took a few messages to get to this, the easy, direct, and correct way. <G>

Providing water temperature is the same.

devilfish once bubbled... Providing water temperature is the same.

Click to expand...

Over 0-30C, the difference is only about .5%. That's one pound in 200.

Over the range of temperatures most of us deal with, it's quite a bit less than .5%. I wouldn't be too concerned about it.

http://wow.nrri.umn.edu/wow/under/primer/page5.html

The salt content of the water you diving in makes a world of differance. When I dive in the Bahamams I have to add 2 extra lbs using the same cofiguration. If theres a way to figure that in then the famula would be great. Also gear configuration plays a great roll in what the weight would be. The gear absorbs water which makes it heavier. When you first enter the water your wet suit has air in it which is then forced out and replaced with water also makes a differance. I live by the simple rule of thumb add 6 to eight lbs when going in salt water for an average size man. Add 4 to 6 lbs for the average size woman.
For a wet suit add 1 lb per mill. thickness. Then add 2 lbs for the other diver thats under weighted and forgot how to figue it out.
That way you can give it to them without calling the dive.
Works everytime HEHEHEHE

Fred

fgray1 once bubbled... The salt content of the water you diving in makes a world of differance. When I dive in the Bahamams I have to add 2 extra lbs using the same cofiguration. If theres a way to figure that in then the famula would be great. Also gear configuration plays a great roll in what the weight would be. The gear absorbs water which makes it heavier. When you first enter the water your wet suit has air in it which is then forced out and replaced with water also makes a differance. I live by the simple rule of thumb add 6 to eight lbs when going in salt water for an average size man. Add 4 to 6 lbs for the average size woman.
For a wet suit add 1 lb per mill. thickness. Then add 2 lbs for the other diver thats under weighted and forgot how to figue it out.
That way you can give it to them without calling the dive.
Works everytime HEHEHEHE

Fred

Click to expand...

I would think the easiest way to figure for different salinities would be to jump in the water.

If you want to come up with a magic number for conversion, I suppose you could get yourself neutral in a swimming pool and then go jump in the ocean. Your overall weight divided by the amount you needed to add for the ocean would yield some number in the region of 20-40.

Water that goes into free-flooding portions of your gear won't make any difference because the volume it occupies would either reduce the displacement or offset the displacement, depending on how you want to look at it.

If you have a salinity and want to do the math, one fortieth of overall weight is pretty good for a salinity of 35 parts per thousand. Since zero additional weight is needed for freshwater (zero parts per thousand), coming up with a coefficient from a salinity won't be too hard.