YABQ (Yet another Boyancy Question)

[tlj3071]

A couple of days ago I went to the LDS for some pool time. I have lost some weight and have a new Hollis HD100 BC, and decided I needed a weight check prior to a dive next month in Tortola. In fresh water with a full AL80, a 3mil full, and an empty BC, I needed 6lb to descend (I'm built more like a balloon than a fireplug ). So, I start figuring what I'll be apt to need in SW. My thinking goes something like this: 6lb in fresh + ~5lb to account for the air I'll use up (5.8 * 2500/3000 = 4.8), and then another ~6 lb to account for the ~2.5% density difference (225lb * 2.5% = 5.625). So the total should be ~17lb. However, the LDS owner (a friend and very experienced diver) says that I'm wrong. "Just take what you need in fresh water [6lb] and add 4lb for salt water." Now I'm an engineer, and I replied that it didn't seem like a constant solution to this linear problem was reasonable, but he insisted that based on his experience an additional 4lb would be correct. The last time I did an equivalent dive in more-or-less equivalent gear (rental jacket BC and weighing less), I needed 18lb after doing a weight-check following the first dive with an "empty" (500 psi) tank. I'm afraid that even assuming 10lb gets me down, I won't be able to hold a safety stop at the end of the dive.

So...what's the consensus? Closer to 10lb or closer to 17lb?

 

[jeangadbois]

I'm about 210 pounds if normal shape and I use 20lbs of weights with a 7mm properly weighted not sure if that helps.

 

[Rich5711]

I am terrible when it comes to math but I would go with 17lbs. Better to not have to worry about being too light. Just make sure you aren't drastically overweighted and the weights are placed to allow a nice horizontal profile keeping you from burning through your air with constant corrections.

 

[Kronek]

Much closer to 17. But it wouldn't hurt to take 10 on your belt, and have 10 pounds in various increments to test and see what you need.

 

[boulderjohn]

For some reason the math does not always work out the way you think it will. Some people, for example, believe firmly that you do not have to plan for the full weight of the air you will lose, if any at all. That is because when you first submerge, you have air trapped in places that you will lose during the dive, especially inside the wet suit and in little pockets in the BCD design. When I begin a dive in a wet suit, I am usually well aware of bubbles working their way out of the suit. Next, your wet suit compresses during a dive, so you don't need as much at depth as you did on the surface. When you ascend, it should puff up again, but it will not come all the way back for a while after the dive. I also find that you don't need to add quite as much for salt as you are planning.

When I am wearing a full 3 mm suit while instructing in a fresh water pool, I could get by with minimal weight--maybe 2-3 pounds--but I usually use a bit more. When I am diving in a 3 mm suit and a BCD in the ocean (which I haven't done in a while), I could get by with 8 pounds, but I usually used a bit more.

I would guess closer to 10 pounds than 18 pounds in your case; however, I suggest you experiment. Start a little on the heavy side and see if you can work your way down. Don't obsess about it, though. Yes, it is easier to dive properly weighted, but I don't think a couple of pounds makes all that much difference. When I am wearing weights on a dive, I am much more concerned about where they are then how much there is. I want the weights distributed so that my BCD helps me lie nice and horizontal in the water, and if that means a little extra weight to get that balance, I am fine with that.

 

[tlj3071]

Seems like a common problem in SCUBA -

For some reason the math does not always work out the way you think it will

. I didn't consider the suit compression. My last SW dive was in a 5mil to 118' (the Proteus - cool dive) and the suit compression was pretty noticable, but I didn't notice much on my last SW 3mil dive. I'll take your suggestion and start with 16lb and work down from there.

Thanks,

Tom

 

[fnfalman]

I'd add 4-lbs for the AL80 and 4-lbs for the saltwater (it's between 4-lbs and 6-lbs for saltwater depending on which body of saltwater).

I think that 17-lbs is way too heavy.

Let me put it this way: I'm 205-lbs at 5ft6 and on the fluffy side instead of fireplug side. In California ocean dives with a 7mm wetsuit and a steel tank, I dive with 14-lbs of lead. I find it hard to believe that somebody in a 3mm suit is going to need more weight than me unless that person has a lot more "natural buoyancy".

 

[spectrum]

What you really want to know as a baseline is what weight do you need to hover near the bottom of the pool after geting your gear saturated. Weight to get down can have a lot of irrelevant varriables in it.

Get in the pool as you did and muck around down there for 15 minutes. The get your weight so you hang there off the bottom being still with an empty BC. Allowing the time to saturate is very important since your suit and BC will have some stowaway buoyancy that will be gone at the end of the dive.

Then take the cylinder pressure and allow for the balance you hope to use on a full dive using the .08 math as you did above. Add that to your weight

Now take your total dry diver weight.. you, cylinder, all your gear and the weights calculated above and multiply by .025 or divide by 40 which is much easier mental math. Being a self described balloon I will guess that you are 225 and dress out at 280 wich suggests adding 7 pounds for the ocean. Add that to what you learned above for a typical ocean.

That will be enough and as you settle into your vacation your composure will certainly settle down and with you will need less lead. Considering all of that there is no single permanent easy answer. The most you can hope for is a working starting point. As Rick Murchison said, neutral buoyancy is a fleeting concept.

You are far better of with a few extra ponds going in than a few too little.

Pete

 

[tlj3071]

...I'd add 4-lbs for the AL80 and 4-lbs for the saltwater (it's between 4-lbs and 6-lbs for saltwater

I start with 6lb, and add 8/10 (as you suggest), I'm now at 14/16.

I think that 17-lbs is way too heavy.

So, I get the feeling from your answer that you think that 1-3lb is "way too heavy"?

Tom

 

[DivemasterDennis]

I agree with Boulder John. Lots of factors impact the weight you will need, and there is no substitute for an on site weight check if you have the opportunity to do it. But on the facts given, I'm firmly confident that 17 pounds is too much. But conditions and equipment on site will determine what is proper. 12 to 14 pounds should be ample, but again, variables on site will determine proper weighting.
DivemasterDennis