Neutral Buoyancy

1st Question: I read in the adventures in Diving manual (Peak Performance) that to start out for my weight 180 lbs and a 5mm wet suit at 10% my body weight (18lbs). My last dive I was using 14lbs and was sinking like a rock. I have used 12lbs and was able to stay neutrally buoyant without air in my BC. This leads to my second question... How do you descend feet first if you are neutrally buoyant without air in the BC without adding weight? This is the reason I went with the 14lbs. To stop myself from sinking I have to add air to my BC is this the right way?

If you are properly weighted with a mostly empty tank and your BCD empty then you will float at eye level with a neutral lung volume (ie lung +/- 1/2 full).

To descend you simply exhale.

R..

If you are comfortable with it and can naturally attain proper buoyancy- yes. One other thing to consider- I have a problem at the end of my dives where the tank- now with less air- tends to make me rise when I don't want to. So I wear an extra 2 pounds just to stop that from happening. So that means I am a bit over weighted at the start of my dives but that is OK with me- I just do what you did and slightly inflate the BC. Towards the end of the dive- I adjust accordingly. You should wear the amount of weight that works for you.

shmuggy:

You should wear the amount of weight that works for you.

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Correct. And 2lbs for maintaining good trim is nothing. That's not what people talk about when they talk about divers being "overweighted".

Another point is that, no matter how long you dive, you should occasionally do a buoyancy check. Even (or especially) after you think you have it figured out. Things like comfort and even the age of your suit can make a difference to how much weight you need so it's good to check it once or twice per season.

R..

Thanks, I was a little confused with all of it. I check my buoyancy every time I dive just try to perfect it. When I was using the 12Lbs I just dove in and swam to the bottom. I'll keep the 14 and work with it just wanted to make sure. Again thanks.

Keep in mind that the bouyancy check is best done at the end of the dive, when your tank is nearly empty. At the beginning of the dive, you need enough weight not only to descend, but to allow you to stay at your desired depth as your tank empties and becomes more bouyant. Also, your bouyancy will change as your depth changes due to the compression/expension of your neprene.
Bottom line: they're guidlines, not rules. Start with the suggestions, but dive with what works for YOU, not what works for a textbook.

shmuggy:

You should wear the amount of weight that works for you.

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That's sound advice, but the fact remains that there are untold numbers of divers out there who are grossly over-weighted, but think that "it works for them". They just ignore the knock-ons, such as; bad trim, yo-yo buoyancy and massive air consumption.

@ Molina67: To save re-typing answers to every buoyancy question asked here, I've put a tonne of info into this article series:

Scuba Buoyancy Masterclass 1of9 - Buoyancy Control for Scuba Divers -Scuba Tech Philippines

Thanks for the link

There are two different schools of thought on weighting with a thicker wetsuit. The goal of both is the same -- that the diver can easily hold a shallow stop at the end of the dive, with the tank down to a minimum amount of gas (usually 500 psi).

The first school advocates adjusting your weight until you are neutral on the surface with a full tank, and then adding a pound for every 13 cubic feet of gas that you are going to use during the dive. (So, if you are diving an Al80, you would add five pounds.) If you arrange yourself this way, you will begin the dive five pounds NEGATIVE with an empty BC, and you will easily sink if you deflate the bladder. This is not being "overweighted" -- it is being negative, which is totally different.

The second camp advises to weight yourself NEUTRAL with a full tank and a normal breathing pattern. Using this strategy, you have to exhale and hold it, and perhaps swim down a bit to descend in the shallows, until suit compression reduces your buoyancy. The thought behind this is that wetsuits often have a bit of air trapped in them at the beginning of the dive, and do not rebound at the end to their full expansion, so you will end the dive a little more negative than you were at the beginning. With thick neoprene, this can be enough so that you can hold a shallow stop, despite having exhausted several pounds of air into the water.

New divers often confuse "negative" with "overweighted". It is not unreasonable -- and in fact, is common practice -- to start the dive negative to a degree determined by the capacity of your tank. That is one of the reasons we use BCs! If you are sinking, it doesn't mean you are necessarily overweighted. It means you haven't COMPENSATED adequately with your "buoyancy compensator". Overweighted mean you are still significantly negative at the END of the dive, when the tank is nearly empty. This is what you want to avoid.

I would just like to add that no "rule of thumb" is right. Every person's weighting needs are different based upon lung size, gender, body density (ie % of fat), as well as diving conditions and equipment variables. I believe it is better to be slightly negative than underweighted, but overweighting is one of the biggest problems in achieving peak buoyancy control. I want to encourage everyone to note all variables that impact weighting in their dive log, make a "catalog"of them, and then proper weighting is easier. My personal chart shows that I wear 11 pounds in a 7/5 full mil wet suit with hood gloves and boots in fresh water with my "fresh water" gear, only 9 with just the 5 mil and no vest under, and that I wear 13 pounds in a full 3 mill in the ocean with my "warm water" gear, and that I add 1 pound if doing photos. That's oversimplified as there are variables for tank type and size, etc. I note my body weight too, as holiday gain may raise the need for an extra pound, at least on initial dives. Anyway, thanks for caring about your buoyancy. The essence of being a good and environmentally responsible diver is buoyancy control.
DivemasterDennis