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From: "Andrea Zaferes" Subject: ascents part 2
Date: Tue, 30 Jul 2002 22:10:13 -0400
Reply-To: PublicSafety@wateroperations.com
One of the main reasons for this is overweighting which starts in their first dive classes and is allowed to continue throughout their diving life. We teach already certified divers and we take an average of 6 lbs off these students as the first step towards developing neutral buoyancy. In the last two years we have seen a worsening overweighting problem and hear more often that divers were taught to ascend by inflating their BCD. Sends shivers down our spines. In the last year, for example, I have had at least thirty public safety diver students tell me that "we are rescue divers so we were taught to put lots of weight on so we can sink straight to the bottom." We are seeing an increase in divers who are overweighting themselves by ten or more lbs. That's scarier than a freshwater eel bumping you in zero visibility water.

What does overweighting have to do with ascent rates and safety stops? It is simple, every pound of lead is equal to about a pint of buoyancy. If you are 6 lbs over-weighted on the surface then you have to have 6 pints of air in your BCD just to be neutral at the surface. By the time you reach ten feet that air is compressing and you are dropping and you respond by adding more air into your BCD, after possibly unconsciously taking in and holding a large inhalation. By the time you reach 2 ATA (33 fsw) you have to have 12 pints of surface air in your BCD to compensate, and 18 pints at 3 ATA (66 fsw). This is in addition to the air you have to add for suit compression caused by increased pressure at depth.

With so much gas in their BCDs think about what happens every time divers rise or fall a little in the water column. If they rise six inches the air expands causing them to become positively buoyant. They respond by dumping air out of their BCDs, causing them to sink too far and fast, so they reflexively take in a large breath and hit the power inflator. My mentor Walt "Butch" Hendrick aptly named this the Great Compensation Chase. The more air you have in your BCD, the faster and further you will rise and fall with each change in body position. Hovering and slow free ascent rates become a significant challenge, and in shallow water can be a near impossibility for the average diver. In order to ascend at a 2 sec/ft rate and hover it is necessary to remain neutrally buoyant continuously. Let us see why this is so.

Place yourself in an over-weighted diver's fins at a 40 foot bottom. You may have fifteen or so extra surface equivalent pints of air in your BCD as you begin your free ascent. You raise your power inflator above your head and hit the exhaust button. You raise your other arm to protect your head with your hand, just like you were taught. You begin kicking. You keep on kicking. After a few seconds of noticing significantly decreased visibility you realize that your fin tips are kicking up the silt off the bottom. Many of you can relate to this experience at least once can't you? Don't worry you are, sadly, not alone.

Think about the obvious laws of physics. If you are negatively buoyant in a vertical position and are not kicking then where are you going? Down. No two blennies about it. So we all agree that if you are negatively buoyant you must kick continuously to make an ascent and the moment you stop kicking you will descend. The next question then is, is it possible to make a 2 sec/ft free ascent while kicking continuously? I am going to say not likely, but don't take my word for it, go to a pool, overweight by 6 lbs and try it. Continuous kicking will cause a too fast ascent.

Now why do so many divers make negatively buoyant ascents? The reason is they were allowed to overweight and they were taught ascent procedures that made them negatively buoyant even if they were weighted neutrally. Try this. Weight yourself neutrally. To do this, at the surface vent all the air out of your BCD, cross your legs, stop moving, breathe normally, let your arms hang down naturally, and you should be hanging with your scalp at the water line. Descend by gently crossing your arms across your chest and tensing your arm muscles, or by gently raising an arm out of the water and making a slightly longer than normal exhalation. Once you are hanging vertically at 8-10 feet gently, slowly raise your arms above your head and see what happens. Lo and behold you will sink. Try it again and this time hold your gauge in one hand and your power inflator in the other. You should sink earlier and a little faster. Putting weight above your center of gravity, over your head, will make you negatively buoyant.

So think back to how you were taught to ascend? If you were neutral, then just the act of raising one, or worse, two arms above your head will make you negatively buoyant. Next we were taught to exhaust the air from our BCD at the start and during our ascent to prevent a rapid rise from BCD air volume expanding with decreased depth. If you had many pints of air to compensate for overweighting then what has to happen if you vent that air? You have to become negatively buoyant at a rate of 1 lb per pint of air lost. Are divers taught how to vent just the right enough air to remain neutral? Or are they taught to raise up both arms and vent? Answer honestly. It is no wonder that more divers are reverting to adding air back to their BCD to ascend - they literally cannot get off the bottom without kicking.

Andrea Zaferes
Lifeguard Systems & RIPTIDE
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