First ocean water dive weight question?

[MaxTorque]

You need to account for the extra water mass you displace in salt water, ie your total volume. As you are aiming to be neutrally buoyant, ie you and all your gear has the same overall density as the water around you, that volume is also your weight when in fresh water with a density close to 1kg/litre.

so for example, you, and your gear weight 100kg (nice round numbers, sorry, metric cause i'm in the uk), so to be neutrally buoyant, you and your gear need to displace exactly 100 litres of fresh water, and so create a buoyant uplift of that same 100kg.

In salt water, that is roughtly 2.5% more dense than fresh water, your 100 litres of displacement will create a buoyant uplift of 102.5 kg, so roughly (and not accounting for the extra volume of that extra weighting) you'll need to add 2.5 extra kg to your weighting system to be once again neutrally buoyant


Hope that all makes sense

 

[Diverlady13]

I have done my pool dives and I am going to do my ocean dives soon. I am 210lbs, with an 80 cylinder, 3mil hood, gloves and booties. At the pool I would have 10 lbs back trim weight and 16lb weight in the from making it 26lbs in fresh water. Towards the end less than 1200 psi, I had a hard time staying down, I was naturally buoyant. This made it very tough to do the excersizes where I had to take the BCD off and weights off and put them back in. I felt like I was rolling and floating in space because I did not have good footing on the ground. The instructor said to add 4-6 lbs for the salt water. I now have 20lbs in front and 14lbs in back making a total of 34lbs. My front bc pockets only take 10lbs each and my back trim pockets the most i could fit in was 7 lbs in each pocket. This is a wing BCD. I will not know if this is a good weight until the dive. Does this sound about right? Any other recommendations? I think being heavier would be better so i can do the skills on the ocean floor and actually get down during the end of the tank.

Any help would be appreciated.

I'm sure someone will come along to correct me if I'm off base, but that seems like a lot of weight to be carrying. I wonder if something else is happening. Are you/your instructor positive that you are expelling enough air from your BC when trying to descend? The way my gear is configured, I it's a lot easier to release air and completely empty my bc if I'm upright at least partially, though I can release some when horizontal. For my first maybe 25-30 dives after being certified, I was definitely overweighted until I figured out that despite thinking that my BC was empty, it did in fact contain enough air to make me float too much at the end of the dive. I was also having difficulty descending without that extra. Now I think that I could still take off a couple of pounds of weight since my buoyancy is better, having had a good number of dives this year. Still, at 180 lbs I carry 16 lbs of lead. I'm naturally buoyant even though I'm very muscular. I have about 15 extra pounds on me right now, so if I can get that off I could likely go down in weight again. My BC is a back inflate style. I distribute my weight to put a couple of pounds on my upper tank strap to counteract my upper body buoyancy. In fresh water I have been wearing 8 lbs.

Also for comparison, my husband weighs about 260 lbs over a 6 ft. 4 in. frame. He carries 14 lbs when wearing his 3 mil wetsuit and regular gear in salt water. He also wears a back inflate BC.

 

[Coztick]

Scuba diving weight belts SUCK! But that's how you equal out the ballast.
Consider buying a freedivers belt. Apparently, they are stretchy/rubbery and much better. Ideally, you should be neutral at depth without your rig.
Add the 2.5% as recommended but share your concern with the dm so you can simply ask for more when needed.
How cold is the water where you're going?

 

[lowwall]

When we were at the end of our tanks about 1k psi they did a weight check in the pool. The had us so our eyes were level with the waterline completely exhausted and with no air in bcd. They determined my weight should be 14lbs.

That actually sounds reasonable given a 3 mil and your body size.

Many, if not most, new divers have trouble getting down and staying down at their correct weight. It's a combination of involuntary finning while descending in a vertical position and a failure to fully dump the air from the BCD.

 

[johndiver999]

That sounds like too much lead for a 3 mm suit , 210 lbs and aluminum tank and freshwater. However, it is possible that you need that much weight. Another sanity check might be to take all the lead they say you need (34 lbs) maybe reduce that amount by 5-6 lbs and place it all on a weight belt (or two). Then put all that lead on and get in the pool and snorkel (without a bc or tank).

Are you comfortable with around 29 lbs in the suit in the pool? If so, then your current weight is about right. I suspect you will sink like a stone, so start in the shallow end.

Also, if you are 210 lbs body weight, then you, your tank and your lead and misc gear are around 300 lbs total. You need about 2.5- 2.7% of 300 lbs extra for saltwater.

 

[TMHeimer]

I've never dived a 3mm suit. I dive a 7 mil farmer john wetsuit and use 42 pounds. Others say it is way too much, including a couple of instructors-- until they checked me out thoroughly and found that is exactly what I need (says one who also uses 38 in the same suit). So do the proper weight check and go with that-- the wt. check doesn't lie. Our instructors did it before the first checkout dive so that one was pretty good for buoyancy, and adjusted any problems after that.
Weight belt: I use a belt with pockets and put 2 pound weights in there (16 pounds in the belt). I use special dive suspenders so there is no discomfort at all and no busting a gut tightening it. I believe an even better device is a harness.

 

[Cdncoldwater]

My personal experience, add 2-4 pounds going from fresh to salt water; especially as a new diver. You can trim weight as you gain experience.

 

[boulderjohn]

Once you have your weight dialed in for freshwater, when you go to salt water with the same gear, add 3%. Now that I have said that, I am going to go with others who say you do not have your weight dialed in.

I am a little bigger than you, but not a lot. With a 3mmsuit in freshwater, I need maybe 4-6 pounds using a typical jacket BCD and an AL 80 tank. When I dived in the ocean with a typical BCD and a 3mm suit, I used 8 pounds.

Now, maybe you need more than I do--we are all different--but I have a hard time believing you need that much more.

 

[tursiops]

Many, if not most, new divers have trouble getting down and staying down at their correct weight. It's a combination of involuntary finning while descending in a vertical position and a failure to fully dump the air from the BCD.

In addition, if you are not relaxed and your muscles are tense, you'll need more weight to get down. Yet another new-diver problem.

 

[Seville]

I am using a 7mm wet suit not a 3mm.