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.
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.