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

[Charlie99]

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!!

2.5% of TOTAL weight -- you, your tank, and all your gear, including any lead you need for proper weighting.

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

63.95/62.293 = 1.0266, or seawater is about 2.66% more dense than fresh for the temps you chose.

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.

If you are properly weighted, at the end of a dive you+tank+gear+lead are the same density as the water. A more accurate way to measure your volume is to simply weigh all of you + all of your gear and divide by the weight per cubic feet. Now that you know your volume, you can do your calculations more accurately. Or you can just skip converting to volume and back and apply the correction directly to your TOTAL weight.

 

[Crazy Fingers]

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".)

Oh, sorry... I didn't realize you were talking about a rule of thumb. It's just so dang hard to read every post on these long threads!

 

[Crazy Fingers]

A more accurate way to measure your volume is to simply weigh all of you + all of your gear and divide by the weight per cubic feet. Now that you know your volume, you can do your calculations more accurately.

That would work, but only if you would be completely neutral in your setup without any air in the bladder when you measure your weight. Basically what you are doing is assuming that your average density is the same as the water, and then saying that since your densities are equal, your volume can be calculated from your weight and the known density of water.

I don't know if that's more accurate than measuring your volume directly and multiplying by the density difference. In fact, if we remove random errors and talk just about systematic ones, then measuring volume directly and multiplying by the fluid density difference is definitely more accurate. (Random being the difficulty of measuring your volume in your bathtub with a ruler, and which you can fix by getting better instruments. Systematic being the fact that you are assuming that the densities are equal, and which you can fix only by getting a better method.)

But I agree that a ruler and bathtub are crappy instruments to measure your volume with... anyone want to spring for a huge graduated cylinder?

And honestly, this argument is just academic anyways because proper weighting is so imprecise (+/- a pound or two... mostly because of exposure suit) that it doesn't much matter if you are off a little. So yeah, maybe 2.5% of total weight is a good rule of thumb. But let's not be using it if we're figuring out if a submarine can enter a freshwater river while submerged, mmkay?

 

[markfm]

No problem
I actually ended up putting together a little note on it, because it comes up as a question periodically:
http://markfm.dynalias.org/Weighting.pdf

 

[tremtech]

Wow, what ever the coaficiant of pi x radius x diameter divided by the viscosity of thirty weight oil added to 85 degree water equals you should all know how to do a bouyancy check in the water and then determine how much you need to add to compinsate for the salt water .