How will my weight loss affect weights needed?

[CamC]

20 # - I'm jealous! Is your trip to familiar waters? I have to adjust by 2-3 # just for salinity differences between the Red Sea and the Indian Ocean, diving exactly the same kit. This more than offsets any changes from my position in the binge/diet cycle

 

[decompression]

My 2 psi, if you haven't changed exposure suit size then, no not really. 20 lbs of body weight is mostly fat which is around 60% water anyway. Maybe a pound or two.

 

[industrious95]

Why can't you tinker on a shore dive? It my not be as practical as having a mate on a boat hand you an extra bar of lead, but with a little ingenuity you'll figure it out. Find a couple of 1 lb rocks that you can put in your BC pocket if needed, or just abandon in the surf if you don't need them.

And congrats on the weight loss.

 

[Mr Carcharodon]

1 pound less ballast is about right. While fat is a bit less dense than muscle they are both close to the density of water so it takes quite a bit of weight loss to make much of a buoyancy difference.

 

[LeadTurn_SD]

My 2 psi, if you haven't changed exposure suit size then, no not really. 20 lbs of body weight is mostly fat which is around 60% water anyway. Maybe a pound or two.

This is a good point. And that has been my experience after losing 20 pounds.... I was able to drop about a pound off the weight belt. I'd hoped I'd be able to drop more ballast weight, but I think unless I drop to the next smaller size wetsuit that will not happen.... that neoprene, even though it fits much looser now, still has the same buoyancy as before, and 20 pounds of fat in my case did not make a huge difference in my overall lead requirement.

Buts congrats to the OP for losing the weight! Good work!

Best wishes.

 

[SWAMPY459]

the way to do a proper weight check is to wait till the END of your dive, when all the air is out of your wetsuit, and on your safety stop at 15 feet let ALL the air out of your bc with a nearly empty tank of air (don't go under 500 psi). You should be neutral under those conditions. If you are perfectly neutral then you have what is known as a balanced rig. If you sink and have to fin up you are overweighted. If you have to grab something to stay down you are underweighted.

 

[dumpsterDiver]

the way to do a proper weight check is to wait till the END of your dive, when all the air is out of your wetsuit, and on your safety stop at 15 feet let ALL the air out of your bc with a nearly empty tank of air (don't go under 500 psi). You should be neutral under those conditions. If you are perfectly neutral then you have what is known as a balanced rig. If you sink and have to fin up you are overweighted. If you have to grab something to stay down you are underweighted.

No way this is correct.

If you have a thick wetsuit, and you use most all your air, you will be unable to stop from floating up from5 or 10 feet. What if there is a boat coming? Do you want to be pinned to the surface? Also this advice is ridiculous because you will be completely unable to hold down an smb. You need some weight to hold the smb vertical...

 

[dmoore19]

The density of mammalian skeletal muscle tissue is about 1.06 kg/liter. This can be contrasted with the density of adipose tissue (fat), which is 0.9196 kg/liter

 

[EFX]

If the dive kit doesn't change we only need to find the change in buoyancy due to changes in body volume and weight. We know the dry weight changed from 210 lbs to 190 lbs. If we can find the volume we can calculate the buoyancy.

B = VD - W

According to the equation the buoyant force B is the difference between the weight of the water displaced by the object (VD) and the dry weight (W) of the object. If B is zero the object is neutrally buoyant and will neither sink nor float. If B is negative (-) the object will sink and if B is positive (+) the object will float.

Let's calculate the buoyancy (B) for both dry weights and see what the difference is. We can find the approximate volume by dividing the weight by the density of the human body. The density is 1 g/cm3 (from How to Calculate the Volume of a Person | eHow). The procedure to find volume is:

1. Measure the weight in lbs.
2. Convert the weight to grams (W lbs x 453.6 g/lb).
3. Get volume (in cm3) by dividing weight by density (V = W/D).
4. Convert volume in cm3 to ft3 using 28,316.8 cm3/ft3.

W[210] = 210 lbs x 453.6 g/lb = 95,256 g
W[190] = 190 lbs x 453.6 g/lb = 86,184 g
V[210] = 95,256 g / 1 g/cm3 = 95,256 cm3 x 1 ft3 / 28,317 cm3 = 3.36 ft3
V[190] = 86,184 g / 1 g/cm3 = 86,184 cm3 x 1 ft3 / 28,317 cm3 = 3.04 ft3

Using the equation B = VD - W and 64 lbs/ft3 as the density of salt water the B for each weight is:

B[210] = 3.36(64) - 210 = 5.04 lbs.
B[190] = 3.04(64) - 190 = 4.56 lbs.

Dropping 20 lbs in body weight lowers your buoyancy by 0.48 lbs (a 9.5% change). This suggests reducing your lead weight by only 1 lb. If you are interested in the buoyancy in fresh water use 62 lbs/ft3 for the density of water. (Just for comparison B[190] in fresh water = -1.52 lbs).

 

[sphyrnidus]

Interesting all these calculations. But are they significant? I think it's mostly trial and error. If I try these calculators I would sink like a brick , they want me to take way more weight than I use on a normal (wetsuit dive).
What about this? The SO and me we have the same equipement. We have the same wetsuit. We both dive with a BP/W of the same size and weight. The same tanks too. We differ in weight by 40 kg and I'm 40 cm taller. She is very well balanced and one of the most relaxed divers I know, she is a DM with about 1500 dives. Yet I use only 1 kg of lead more than she does. Here in Holland, being properly weighted is extra important as we do most dives in rather shallow water, that is between 2-10 meters as below there is only mud and darkness. All life is in the shallow parts. So much for using the calculators. We go to Egypt quite often. Of course you need less weights over there, but still the difference between us stays about 1 kg.