Simple salt/fresh wt. questions

[TMHeimer]

I should know this from taking the "old" PADI DM course with all that theory, but--
We know completely fresh (distilled) water has a Specific Gravity of 1.00 and a cubic ft. is 62.4 lbs.
Salt water (depending on how close you are to the poles or equator) has 1.03 and 64.
Not much difference. Two questions:
1. Why must you drop 5-6 lbs. going salt to fresh (like say from 40 to 35--that's a 12.5% difference --way more than the stats above)?
2. Does the % vary according to exposure suit? Ei. In just a bathing suit, say, you need 8 pounds in salt--would you need 3 in fresh. Is there a difference in %, or is it a straight plus or minus 5-6 pounds?

 

[dberry]

Because your displaced volume is probably ca. 3 cubic ft, so multiplying 3 times the difference between 62.4 and 64 gives you 4.8 lbs difference in buoyancy (or we can just call it 5 lbs

EDIT: I could do some more algebra to explain why change in weight isn't scaling the same as the change in water density, but that would require more mental energy than I can muster before dinner.

Regarding your Q2: the differential in weight will depend on your displaced volume, not the absolute amount of weight you need to carry in salt vs. fresh. I.e. if a 3 cubit foot diver needed 0 lbs in fresh water he'd need 5 lbs in salt... don't even try to calculate the percentage change THAT represents

 

[redacted]

I drop 7 to 8 lb but I weigh 220 and my gear adds another 30, so my gear and I displace about 4 cu ft.

 

[tbone1004]

so @dberry hit on it a bit, but this may help.

Weight is different than mass. Weight is essentially a function of your mass and the density of your body against the density of the medium that you are in.
If your body has a mass of 192lbs, and displaces 3 cubic feet of water, then you will be exactly neutral in salt water, and roughly 5lbs negative in fresh water. The amount of water you displace is 3 cubic feet in either direction, and the mass of your body is always 192lbs, but 3 cubic feet of fresh water weighs 187.2lbs, vs 3 cubic feet of salt water that weighs 192lbs.

Think about it in terms of a scale balance. You have you and your gear on one side and on the other is a vessel with volume equivalent to the amount of water you displace. In the example above, you have a mass of 192lbs, and the vessel is 3 cubic feet. In fresh water, the mass of that vessel is 187.2lbs and with salt the mass is 192lbs.

When you change your exposure protection you are changing the amount of water that you displace and replacing it with a combo of neoprene, nylon, polyester, and air. The volume that the suit displaces is equivalent to the amount of lead you have to add. The "rule" is roughly 2lbs per mm of neoprene in salt water. We can approximate from this that 1mm of neoprene has a volume of roughly 2/64cf. A 8mm suit *for easy math* in that case would displace 16/64cf of salt water and require 16lbs of lead to sink. In salt water it still displaces 16/64cf of water, but to get the amount that it would require in fresh water we have to use the following. 16*62.4/64 which is 15.6lbs to adjust.

The real difference starts to show up when you have to adjust for your total body and rig weight. If I'm diving in salt water with my 6.5mm, the math is crazy. I weight 270lbs, my wetsuit weighs about 10lbs, and my doubles rig weighs roughly 120lbs. Let's assume I'm neutral in fresh water, so my rig and body has a mass of 400lbs. 400lbs means I'm displacing 6.41cf of water. In salt, I still displace 6.41cf, but the mass of that water has changed from 400lbs, to 410.24lbs so I have to add 10-11lbs of lead to my rig to allow the balance to level itself.

The basic algebra requires the following
mass of diver+rig+exposure protection weighted appropriately for either salt or fresh*in air on the surface that is weight on a scale btw*
mass of 1cf of salt water=64lbs *both masses of water will be slightly different based on salinity, mineral content, etc*
mass of 1cf of fresh water=62.4lbs

If weighted for fresh
massdiver*64/62.4

If weighted for salt
massdiver*62.4/64

Most people aren't going to put themselves on a scale in all of their gear to make the adjustment, and if you infrequently dive one or the other, adding 6-10lbs for salt, and removing 4-6lbs for fresh is usually sufficient. In either circumstance you'll be slightly overweighted by 2-4lbs, but it is often impractical to do a full weight check, especially for the salty stuff if you dive it infrequently.
Note that 64/62.4 is 1.026, and 410.24/400 is also 1.026, hence the 3% rule.

none of this is meant to replace true weight checks, but that's the science behind it. I'm very tired from a long day of work, so if I missed something, please call me out and correct it

 

[tursiops]

Here's the quick answer, OP.
Yes, the difference is 3%, but it is 3% of you and your gear, not 3% of the lead you are carrying.

 

[dberry]

tbone's explanation is certainly correct, but I think tursiops cut to the heart of it.

 

[TMHeimer]

Thanks all for both the detailed and simple explanations. I get it all. I'm sure that was covered in the "old" course, but in a more practical way such as "This guy (idiot) dropped a 200 pound motor in 100fsw it displaced...blah blah" --How did I ever get most or all of those physics problems right?......
I'm still waiting for that $800 motor recovery gig.

 

[RayfromTX]

tbone's explanation is certainly correct, but I think tursiops cut to the heart of it.

I love the human mind and the different ways in which we view a problem.

 

[tbone1004]

@TMHeimer of important note for the motor recovery, and I apologize for the narrative. I'm an engineer, and just got off of 2 weeks of running open water training, and am not running training sessions at work, so teacher engineer brain is taking over.

bcd's and lift bags don't have "weight" of lift *kg's or lbs depending on where you are*, they have volume of displacement *liters or cubic ft*.

what is important to note with this is that the "lift" actually changes based on the kind of water you are in. The way Deep Sea Supply measures their wings is by putting the wing on a backplate, strapped to an 8" tank and worn by a diver. The diver then stands on a scale and the scale is 0'd. The wing is then filled with water and the mass of water that is able to get into the wing equates to the amount of lift that it has.
The wing is filled with fresh water from a hose.
If a wing is rated at 62.4lbs, it means it displaces 1cf of water, and will actually have 64lbs of lift when in salt water.

I do not know of any other manufacturer that measures their wings or bcd's this way. Not to say they don't, just that I don't know of any that do.

The issue here is that since the ratio of amount of lift vs. mass of the object is very lopsided so while the wing experiences an increase in lift in direct proportion to the increase in mass required to offset the change in buoyancy, since the absolute change is very different, you may need to adjust the size of your wing.

 

[Bob DBF]

I'm still waiting for that $800 motor recovery gig.

Yeah me too, I've been lucky to get beer money.


Bob