Andrea Zaferes
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Here is an article published in Alert Diver a couple of years ago. Enjoy
Two seconds per foot
by Andrea Zaferes, Lifeguard Systems
July 30, 2002
Below please find a letter for Alert Diver re: Haldane Revisited by Dr. Bennett
The issue of ascent rates must be looked at from many angles. Dr. Bennett provided a very useful and well put scientific angle in Haldane Revisited that all divers should be made privy too particularly dive leaders and those that write educational standards and procedures. I would like to address the diver skill angle of safety stops and ascent rates. The safety stop, as Dr. Bennett well explained is important for dropping fast-tissue tensions. It also is important for decreasing the risk of making rapid ascents in the last part of the ascent.
Can you relate to the following experience at any point in your dive career: one minute your gauge reads somewhere between 15-10 feet and then seconds later you are staring at the surface less than 2 feet away. Wow, how did that happen?
We tell divers that they should make safety stops and perform 30 ft/min (9 m/min) ascent rates but are we teaching them how and making sure that they can achieve them? We watch thousands of diver ascending annually with diver certifications ranging from entry level to course director and I can tell you that the answer to that question is a resounding no. The medical community is doing their part, but without diving educators and leaders to make sure research findings are put in practice then we all fail.
Let us look just at the term 30 feet per minute. What does that mean? Can you walk at a rate of 30 feet per minute accurately? How about a rate of 45 feet/min or 70 feet per minute? How can you measure that while doing it? Can you tell when you are driving 45, 60, 60 mph without looking at your speedometer? It is fine for scientists to say 30 feet per minute, but it is not fine for dive instructors and leaders. We need to give divers a usable term such as two seconds per foot. Ahhh, now they can ascend by counting two seconds for every foot their hands move upwards on the anchor line, or they can look at their gauges and count one-one-thousand, two-one-thousand for every decreasing foot on the gauge during free ascents.
Never take what anyone says for face value. Go out and try it. Take a group of divers. Put their hands behind their backs (to hide their watches) and have them walk a length of 60 feet at 30 feet/min and time them. Take another group and tell them two seconds a foot and see what happens. The divers in the latter group will go slower and be far closer to the rate we are looking for. Play with this in the water and see what happens. Also watch what happens when divers walk alone versus walking in a group. Peer pressure can be a powerful force.
Having usable, measurable ascent rate terminology arises, no pun intended, when divers do not have an ascent line. Any working dive guide will tell you that few divers can competently stop and hang at any depth, let alone shallow depths such as 15 feet. Why do so many divers end up popping up in the last 15 feet? One answer is because they let go of the ascent line and head for the boat after their safety stop. How many divers can perform a 2 second per foot ascent rate without a line? Not enough.
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. Thats 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 divers 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 cant you? Dont 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 dont take my word for it, go to a pool, overweight by 6 lbs and try it. Continuous kicking will cause a too fast ascent.
Two seconds per foot
by Andrea Zaferes, Lifeguard Systems
July 30, 2002
Below please find a letter for Alert Diver re: Haldane Revisited by Dr. Bennett
The issue of ascent rates must be looked at from many angles. Dr. Bennett provided a very useful and well put scientific angle in Haldane Revisited that all divers should be made privy too particularly dive leaders and those that write educational standards and procedures. I would like to address the diver skill angle of safety stops and ascent rates. The safety stop, as Dr. Bennett well explained is important for dropping fast-tissue tensions. It also is important for decreasing the risk of making rapid ascents in the last part of the ascent.
Can you relate to the following experience at any point in your dive career: one minute your gauge reads somewhere between 15-10 feet and then seconds later you are staring at the surface less than 2 feet away. Wow, how did that happen?
We tell divers that they should make safety stops and perform 30 ft/min (9 m/min) ascent rates but are we teaching them how and making sure that they can achieve them? We watch thousands of diver ascending annually with diver certifications ranging from entry level to course director and I can tell you that the answer to that question is a resounding no. The medical community is doing their part, but without diving educators and leaders to make sure research findings are put in practice then we all fail.
Let us look just at the term 30 feet per minute. What does that mean? Can you walk at a rate of 30 feet per minute accurately? How about a rate of 45 feet/min or 70 feet per minute? How can you measure that while doing it? Can you tell when you are driving 45, 60, 60 mph without looking at your speedometer? It is fine for scientists to say 30 feet per minute, but it is not fine for dive instructors and leaders. We need to give divers a usable term such as two seconds per foot. Ahhh, now they can ascend by counting two seconds for every foot their hands move upwards on the anchor line, or they can look at their gauges and count one-one-thousand, two-one-thousand for every decreasing foot on the gauge during free ascents.
Never take what anyone says for face value. Go out and try it. Take a group of divers. Put their hands behind their backs (to hide their watches) and have them walk a length of 60 feet at 30 feet/min and time them. Take another group and tell them two seconds a foot and see what happens. The divers in the latter group will go slower and be far closer to the rate we are looking for. Play with this in the water and see what happens. Also watch what happens when divers walk alone versus walking in a group. Peer pressure can be a powerful force.
Having usable, measurable ascent rate terminology arises, no pun intended, when divers do not have an ascent line. Any working dive guide will tell you that few divers can competently stop and hang at any depth, let alone shallow depths such as 15 feet. Why do so many divers end up popping up in the last 15 feet? One answer is because they let go of the ascent line and head for the boat after their safety stop. How many divers can perform a 2 second per foot ascent rate without a line? Not enough.
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. Thats 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 divers 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 cant you? Dont 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 dont take my word for it, go to a pool, overweight by 6 lbs and try it. Continuous kicking will cause a too fast ascent.
