Fresh to Saltwater - nothing else changes - how much more weight???

[MXGratefulDiver]

Charlie99 is correct. The change in buoyancy from fresh to salt water has absolutely nothing to do with how much lead you need in fresh water and is strictly dependent on your total weight.

Now, if that's the physics - and immutable fact - why does Pickens feel that he's properly weighted with only a two or three pound addition when moving from fresh water to salt? Obviously there's some disconnect here

Actually, it has to do with the total combined density (not weight) of you and your equipment ... and the total amount of water you displace by getting into it.

Some of us are denser than others ... which means that for the same weight, we'd displace less water ...

... Bob (Grateful Diver)

 

[Rick Murchison]

Actually, it has to do with the total combined density (not weight) of you and your equipment ... and the total amount of water you displace by getting into it.

Some of us are denser than others ... which means that for the same weight, we'd displace less water ...

... Bob (Grateful Diver)

No, "it" being the difference in lead weights required to achieve neutral buoyancy for an object (human body and dive equipment) that is buoyant without any lead weight in fresh and salt water, "it" is strictly a function of total system weight. The density of the whole package is precisely 1g/ml* in fresh and approximately 1.025g/ml in salt water, regardless of weight. That density is neutral buoyancy.
Rick

*at 4C... there is a slight decrease in density either side of that temp, but not of any significance to us as divers.

 

[MXGratefulDiver]

No, "it" being the difference in lead weights required to achieve neutral buoyancy for an object (human body and dive equipment) that is buoyant without any lead weight in fresh and salt water, "it" is strictly a function of total system weight. The density of the whole package is precisely 1g/ml* in fresh and approximately 1.025g/ml in salt water, regardless of weight. That density is neutral buoyancy.
Rick

Hmmm ... not the way I learned my physics ...

When determining the buoyant state of a diver, one must consider both the weight of and volume of the water the diver displaces ... in other words, the density of the water.

Two people of exactly the same weight can displace different amounts of water, depending on the makeup of their gear and the different types of tissue in their body (e.g. fat, muscle, and bone). In which case, while the person with the greater aggregate density may achieve neutral buoyancy with a specific amount of lead, the person with the less dense makeup will require more lead to achieve the same state, because he is displacing a greater volume of water.

To achieve neutral buoyancy the weight of the diver must equal the weight of the water displaced ... and since the displacement volume can vary from one diver/gear to another, it is density (weight per unit volume) that determines your buoyant state.

... Bob (Grateful Diver)

 

[Charlie99]

Some of us are denser than others ... which means that for the same weight, we'd displace less water ...

Yeah, but then you wear less lead.

If you are properly weighted, at the end of the dive, in the 10 or 15' depth range and with a small amount of gas in our tanks, the overall density of you + wetsuit or drysuit + all your gear including lead and tanks will be equal to whatever sort of water you are diving in.

That is the direct result of having your weighting so that you will be neutral with little or no air in your BCD at the safety stop at the end of the dive.

Proper weighting means that the average density of you + all gear will be very close to whatever type of water you are diving in. How the different bits and pieces (such as your body) vary in density doesn't really matter.

 

[markfm]

Hmmm ... not the way I learned my physics ...

When determining the buoyant state of a diver, one must consider both the weight of and volume of the water the diver displaces ... in other words, the density of the water.
...

True enough, but the basic question was the delta weight needed when transitioning from fresh to salt. In other words, you have a diver who, kitted out, has achieved a neutral state. At that point, by definition, you've got a match for the initial fresh water condition, you've matched density appropriately, so your compensation to go salt is simply a 2.5% bump in system weight (1 pound for each 40).

Take someone who is very muscled, get him neutrally buoyant in fresh water, weigh him and his kit, you would add 2.5% of that weight to make him neutral in salt. Take another person, one who is adipose-enabled ; get that person neutral in fresh, then you would weigh him and his gear and add 2.5%. The 2.5% bump is used whether a person is rail-thin or rotund, given that you've got that person to an initial neutral state in fresh water.

 

[Rick Murchison]

Hmmm ... not the way I learned my physics ...

You're missing the point.
When a submerged object - in this case the "object" is the diver and everything attached to the diver - is neutrally buoyant its specific gravity is equal to the specific gravity of the liquid in which it is submerged.
Period.
In the case of fresh water, that specific gravity is 1. That's a density of 1g/ml; 1Kg/L; 62.4 pounds/CF
In the case of salt water, that specific gravity is approximately 1.025. That's a density of 1.025g/ml; 1.025 Kg/L; 64 pounds/CF.
It is no more complicated than that.
The difference in weight required to achieve neutral buoyancy is 2.5% of the total weight of the diver and all his/her gear including weights.
The "less dense" diver will have to carry more weight than the "dense" diver to achieve neutral buoyancy in the first place, but the difference needed to move from fresh to salt will still be 2.5% of the total weight. (Diver + all equipment, including weight)
That's the physics.
Rick

 

[HappyGirl1024]

if you take 5' 8" slightly top heavey woman, pour her into a 0.5 dive skin, add a quite nice Athena BC, a quality pair of fins, mask ans snorkel,

what amount of weigth is going to be needed to sink her like a stone? without preventing her from ever surfacing again?

 

[spectrum]

if you take 5' 8" slightly top heavy woman, pour her into a 0.5 dive skin, add a quite nice Athena BC, a quality pair of fins, mask ans snorkel,

what amount of weight is going to be needed to sink her like a stone? without preventing her from ever surfacing again?

Why on earth would I want to do that? :11: That's a perfectly good BC.

To the OP...

I'm with the 1:40 of total dry diver weight offset for typical seawater for fresh assuming no other changes. Add weight for the ocean. When going the opposite way the discrepancy is a rounding error.

The density of the diver surely effects the baseline weight requirement, that's from weight check 101.

Most males will fall into the 6-8 pound window.

pete

 

[Crazy Fingers]

Adding 2.5% of weight is the total wrong way to do it. In freshwater with a 3 mil I need roughly 10 pounds of weight. That doesn't meant that in salt water I need 10.25 pounds!!

What matters is the density DIFFERENCE between salt and fresh water multiplied by the volume of your body and of your gear.

Saltwater at 80 degrees (ocean) = 63.950 lb/cuft
-Freshwater at 70 degrees (spring water) = 62.293 lb/cuft
--------------------------------------------------------
= 1.657 lb/cuft.

Keep in mind that a cubic foot of volume doesn't sound like a lot but it actually is.

Multiply that times your body's displacement and your gear's displacement to get your overall figure. If you want to know that, fill your bathtub up with some cool water, enough to cover you completely when you submerge. Put a line there with a pencil. Then get in with all your gear and try to get as much of your body and gear in the water as you can. Put another mark where the water goes to. The area (sqft) x the length (ft) is the volume you occupy. Multiply that number by 1.657 to get how much lead difference you need.

For me it's about 4 pounds.

Numbers were from the Coast Guard: (http://www.chrismanual.com/Intro/prop.htm)

 

[markfm]

Crazy -- No, we understand the math, what we're talking about is 2.5% of your total weight, fully kitted. See my example on the first page, where my land weight comes out to about 240 lb (with everything, including tank), so I add about 6 lb (2.5%, or 1/40 [which is the same thing], of my total dry weight, with all gear).

(There's variability, some places with very high salinity water, but the 1/40 covers the oceans of the world, is "close enough".)