Weight calculations

[fgray1]

First did you have the same equipt. on ?
Did you have the same tanks?
Your big differance was the Wet suits, depth of dive and water temp.
As you desend the deeper you go the less bouyant you become.
Wet suit compresses.
You hade a total of 8mm on with air pockets in the suit. Makes you alot more bouyant. Give me more info and I might be able to help. You need to establish a base line weight first. This is done in a pool or sallow water with your basic gear setup. Using a tank with no more than 500 psi in you tank. If you want more info let me know.
It's time for me to go to bed. I have to get up at 4 am for work.
Good night
Fred

 

[Charlie99]

fgray1 once bubbled...
Your big differance was the Wet suits, depth of dive and water temp.
As you desend the deeper you go the less bouyant you become.
Wet suit compresses.

The water temp doesn't make much difference. How much wetsuit you wear does.

DEPTH of dive should NOT matter. Yes, the wetsuit compresses as you go deeper, but you should be weighted so that you can make a nice controlled ascent from safety stop.

Charlie

 

[donacheson]

RiverRat once bubbled...
On a side note, when going from full 7mm, hood and gloves in FW to a 3mm shortie in salt, does anyone know roughly how much the 3mm shortie will offset my buoyancy from the full 7mm, hood and gloves? Couple, 3, 4 pounds I'm guessing? I'm trying to come up with a good estimate for a G. Cayman trip. To complicate matters I'm going from rental BC's (28# lead fw) to a new BP/W (FredT 9# SS, new oxycheq 45#wing. 2# on the STA's) so I'm told I may loose a few lbs. of "hidden" buoyancy that was in the BC (trapped air?) as compared to the BP/W.

FWIW:

I dug out my old 1/4-inch wet suit (a farmer john), which I haven't used in years, and made some measurements:

jacket-torso - 250 in^3
-arms - 140
pants-torso - 150
-legs - 225
------
total 765 in^3

In fresh water, at the surface the suit would displace about (765/1728)x62.4=27.5 pounds of water. Since the suit weighs about 9.5 pounds, it requires about 18 pounds of lead to compensate for its buoyancy at the surface. (I have no idea how much weight would be required at other depths.)

Extrapolating, noting that 1/4-inch=6.4 millimeters, a similarly sized 5 mil full wetsuit would require about 14 pounds of lead and a 3 mil shortie between 4 and 5 pounds.

That wet suit fit me when I weighed about 210 pounds. Since (to a first approximation) body weight varies with the cube of body dimensions and body (and wet suit) surface area with the square of body dimensions, the weights for a 200-pounder should be about 97% of mine; for a 150-pounder, about 80%; and for a 100-pounder, about 60%.

It's a crude calculation, but it ballparks reality.

 

[Charlie99]

How much does the suit weigh when saturated with water? That's what matters at the end of the dive.

TIA,

Charlie

 

[AquaBob]

It doesn't matter what it weighs out of the water at the end of the dive. Buoyancy is directly related to the density (mass / volume) of a material related to the density of liquid it is immersed in.

Since it's mass if the wetsuit is fixed and the volume only changes with depth, the density of the suit should be the same at the end of the dive as it was at the beginning.

Of course, there are a few occassions when the buoyancy of a wet suit has not been the same at the end of a dive as at the beginning. But even in these cases, the buoyancy returns when the body is recovered.

 

[Charlie99]

AquaBob once bubbled...
It doesn't matter what it weighs out of the water at the end of the dive. Buoyancy is directly related to the density (mass / volume) of a material related to the density of liquid it is immersed in.

Since it's mass if the wetsuit is fixed and the volume only changes with depth, the density of the suit should be the same at the end of the dive as it was at the beginning.

Hmmmmm. The volume of the wetsuit doesn't change as it saturates with water, but the the total weight inside that volume increases as water saturates the suit. To me that indicates that it loses buoyancy --- even though I haven't made the direct measurement.

It sure seems that I continue to lose buoyancy for the first few minutes of the first dive of the day, even at a constant depth.

YMMV, but the physics doesn't. Perhaps someone has actually measured the buoyancy of a wetsuit when first immersed, and then after being in the water a few minutes.

Charlie

 

[Don Burke]

Charlie99 once bubbled... Hmmmmm. The volume of the wetsuit doesn't change as it saturates with water, but the the total weight inside that volume increases as water saturates the suit. To me that indicates that it loses buoyancy --- even though I haven't made the direct measurement.

It sure seems that I continue to lose buoyancy for the first few minutes of the first dive of the day, even at a constant depth.

YMMV, but the physics doesn't. Perhaps someone has actually measured the buoyancy of a wetsuit when first immersed, and then after being in the water a few minutes.

Charlie

I don't need to measure how much lower my wetsuit sits in my soak tank after a few minutes to know what it means.

The water entering the suit can either be figured as reducing the displacement or increasing the weight. The first reflects the change from a temporary condition to a normal condition. The second will lead you down a dead-end path.

Bringing that water out of the tank doesn't really accomplish anything since the dry weight of the entire diver/gear reflects the displacement after the first few minutes of the dive. All the wet weight would tell you is the displacement at the instant the diver enters the water, assuming you weighted for that.

 

[AquaBob]

Charlie99 once bubbled...
Hmmmmm. The volume of the wetsuit doesn't change as it saturates with water, but the the total weight inside that volume increases as water saturates the suit. To me that indicates that it loses buoyancy --- even though I haven't made the direct measurement.

It sure seems that I continue to lose buoyancy for the first few minutes of the first dive of the day, even at a constant depth.

YMMV, but the physics doesn't. Perhaps someone has actually measured the buoyancy of a wetsuit when first immersed, and then after being in the water a few minutes.

Charlie

You're absolutely right....your initial buoyancy is slightly different than after a few minutes, but it isn't because the suit is changing.... you're just getting rid of the bubbles stuck to the neoprene, and in your boots, and gloves, and hood, and under your gauges, and stuck to your wing, and in your pockets and .... well you get the picture.


Buoyancy is more than just a function of weight. It's the ratio of the density of the object over the density of the liquid it's in.

Since the extra weight that you are talking about is water, and the liquid that it is in is water, the ratio is water over water or 1/1.

Of course, this assumes no destruction of the neoprene. Obviously, over time, the neoprene begins to fail. Some wrinkles and creases never completely go away, and some people have reported poorer insulation after a few hundred dives. But the rate of decay or change in volume or density of a suit is negligable between successive dives.

This kinda reminds me of the guy who asked, "I wonder how much deeper the oceans would be if there weren't all those sponges soaking up all that water."

 

[donacheson]

Charlie99 once bubbled...
How much does the suit weigh when saturated with water? That's what matters at the end of the dive.

TIA,

Charlie

My assumption is that neoprene foam doesn't soak up water. The stuff is closed cell and not like a sponge. Water will, however, wet the nylon covering, but that's thin and probably pretty close to neutral.

Obviously, the estimates I made don't apply to Polartec - the suit I have is incomressible. It's not necessary to add/remove air to/from my BC when changing depth once water gets in and soaks the material.