Questions about Buoyancy control

[talon39]

I'm a fairly new diver and have some questions about bouyancy control. I am naturally negatively bouyant, so i requir about 50% of the weight i see others using. I was taught that with the proper amount of weight one should be able to float at eye level in the water with a normal breath of air and your BC completely deflated. This has always puzzled me a little. It seems that it would be difficult to decend if your still floating with your BC completely deflated. I assume you must exhale to get the descent started. Is this correct???
I am usually a bit negatively bouyant even when I use less weight so this has not been a problem, but I assume I should be using even less weight.

Where I have noticed a problem with Bouyancy is on ascents. I am dumping air out of my BC the whole way up and ascending slowly, but i seem to have a hard time stopping for the safety stop. Exactly how do you slow yourself down other than dumping air from the BC??? I guess i just need to start slowing down well before the safety stop depth. Without a line to grab on to I seem to have alot of trouble coming to a complete stop. Can I use breathing control to slow myself? i don't want to accidently hold my breath.

 

[gregor1234]

You aren't doing it correctly... you forgot one important step and are experiencing the consequences.

At the start of the dive with a full tank of air, use the procedure you describe (adjust to just float at eye-level) to get very near neutral bouyancy, THEN ADD 5 LBS.

Because the 80 cu ft of air in a AL80 tank weighs (just a little under) 5 LBS.

IF you start off neutral at the beginning of the dive, then as the 5 lb of air is lost from your tank as you breath, you become 5 lb bouyant so by the end of the dive you can no longer stay down (just like if you dropped a 5 lb weight off your belt). It sounds like this is exactly what you are experiencing.

So, first you get nearly neutral and then add 5 additional lbs to compensate for the air you'll be emptying from the tank. This way you begin the dive slightly "heavy" at the start but end up neutral at the end of the dive and able to hover motionless at the safety stop.

As an easy rule of thumb, the weight of air in various standard size tanks can be approximated by:
50 cu ft 3 lbs
70 cu ft 4 lb
80 cu ft 5 lbs
90 cu ft 6 lbs
100 cu ft 7 lbs

 

[Jim Lapenta]

You answered your own question. Start slowing well before you reach the stop. Perhaps around 20 ft or so. Also as you ascend you do indeed dump air form your bc to the point where your ascent is actually due to the slow kick you use to ascend. If you're ascending without kicking your too positive. The fastest change in air volume occurs in the first atmosphere of depth. The advice about the extra 5 lbs is also correct. While neutral buyoancy is the ultimate goal you would like to be a little negative on the ascent. Especially as a new diver. As you gain experience you will find that you don't need to be as negative at the start of the ascent to control it. Try it and see and let us know how it goes. Welcome to diving. Jim

 

[talon39]

Thanks for the advice. I remember the part about adding 5 pounds now. However as I said I'm generalling negatively bouyant at the start of the dive. I do not float at eye level as described. So I am probably about right. In fresh water with only 4 lbs of weight, i sink. I will atempt to get my weights right next time I dive. At this point it seems my best bet is to try to slow my ascent earlier. I guess it will just take time to get used to.

 

[partridge]

The method you mentioned is mainly used to estimate the amount of weight you need. Be sure to do it the other way to get it spot on. At your 15 foot safety stop, with about 500 psi in your tank, dump all air from your bc and breathe normally. If you float, you need more weight. If you sink you need less.

 

[spectrum]

As said above the 5 pounds will get you in the water but early in the dive you still have a lot of stowaway buoyancy with trapped air in your suit, BC fabric all over the place. At the end of the dive you're soaking wet and with your cylinder down near 500 PSI get neutral as you described. That will leave you golden for your safety stop and at the beginning of the dive with those 5 pounds of air in your cylinder you will get down with no more than a long deep exhale.

Remember to make small frequent BC adjustments on your way up or down and don't forget the time lag buoyancy changes require.

Pete

 

[toodive4]

The most common problem for new divers is not realizing how positively bouyant an empty aluminum tank is. An aluminum 80 may go from -4 lbs to +6 lbs. That can be a devastating swing of +10 at the end of the dive. The easiest way - and the way most of us do it is to switch to a steel tank. A steel 95 for example starts out at about -7 and ends up about -3. A +4 swing, however you still remain negative. This allows you to wear less weights (that other 6 lbs) somewhere else on your body and drops your weighting that 6 lbs completely.

Direct example... When I dive with an aluminum tank I have to wear 6lbs of lead with my 3 mil suit... to be sure I can stop at the end of the dive. With my steel tank I don't wear any weight at all. Hope this enlightens you...

Ken

I'm a fairly new diver and have some questions about bouyancy control. I am naturally negatively bouyant, so i requir about 50% of the weight i see others using. I was taught that with the proper amount of weight one should be able to float at eye level in the water with a normal breath of air and your BC completely deflated. This has always puzzled me a little. It seems that it would be difficult to decend if your still floating with your BC completely deflated. I assume you must exhale to get the descent started. Is this correct???
I am usually a bit negatively bouyant even when I use less weight so this has not been a problem, but I assume I should be using even less weight.

Where I have noticed a problem with Bouyancy is on ascents. I am dumping air out of my BC the whole way up and ascending slowly, but i seem to have a hard time stopping for the safety stop. Exactly how do you slow yourself down other than dumping air from the BC??? I guess i just need to start slowing down well before the safety stop depth. Without a line to grab on to I seem to have alot of trouble coming to a complete stop. Can I use breathing control to slow myself? i don't want to accidently hold my breath.

 

[DiveMaven]

I'd really recommend that if you have a good instructor take the Peak Performance Buoyancy specialty. We did it as part of our AOW and thought it was an excellent course. Our instructor helped us fine tune our weight for descent, the dive, and our ascent. He did such a good job on my fine tuning that I can hang perfectly in the water column at 18-20 feet for my safety stop, but as soon as I get to about 15 feet, I begin to slowly rise again with an empty BC. It's great to just hang out for my safety stop, then flick my fin once to slowly rise to the surface.

We did the PPB module first in our AOW sequence and continued to fine tune and work on buoyancy throughout the rest of our dives. It has proven to be invaluable to us.

 

[spectrum]

The most common problem for new divers is not realizing how positively bouyant an empty aluminum tank is. An aluminum 80 may go from -4 lbs to +6 lbs. That can be a devastating swing of +10 at the end of the dive. The easiest way - and the way most of us do it is to switch to a steel tank. A steel 95 for example starts out at about -7 and ends up about -3. A +4 swing, however you still remain negative. This allows you to wear less weights (that other 6 lbs) somewhere else on your body and drops your weighting that 6 lbs completely.

Direct example... When I dive with an aluminum tank I have to wear 6lbs of lead with my 3 mil suit... to be sure I can stop at the end of the dive. With my steel tank I don't wear any weight at all. Hope this enlightens you...

Ken

For a give number of cubic feet of air used the swing will be the same. Air is .08 LB/CF@1ATM regardless of what it came from. With a perfect fill and an actual capcity of 77.4 CF the most an AL80 can swing is 6.2 pounds and that's sucking it dry.

The advantage of a steel cylinder is that most or all remain neutral or heavier when empty/500LB and thus you don't need to wear weight just to keep the tank down at the end of the dive. Most steel cylnders also make a contribution to sinking your exposure protection. This 2 factors lead to wearing less actual ballast. Secondly the empty weight of the cylinder is often less for a given capacity making you even lighter on your feet and giving you less mass to propel through the water.

Together it's not uncommon to drop 7 pounds of ballast and be 12 pounds lighter overall. A nice improvement.

Pete

 

[StSomewhere]

talon, are you using aluminum or steel tanks?