In this multi-part series on addressing poor air consumption when scuba diving, we’ll revisit Donald and his attempts to become a better diver.
The dive guide hands Donald his weight belt and tells him that it’s about right for his height and wetsuit thickness. He puts it on without further question, gears up and enters the water. During his buoyancy check, Donald appears a little heavy but not dangerously so. The instructor tells him not to worry. As the tank gets lighter on the dive, he says, the extra weight will compensate.
Donald deflates his BCD and sinks heavily. His wetsuit compresses with depth, further increasing the speed of his descent. Not wishing to hit the sandy bottom, he inflates his jacket and slows to a stop. He is neutrally buoyant, but his legs are tilting downwards as a result of the many weights around his middle.
He spends the dive finning in an upright angle, resulting in heavy resistance and an increased workload. Donald’s jacket accumulates big pockets of air around his shoulders, lifting the upper parts of the BCD uncomfortably around his ears.
The jacket is awkward and difficult to control. The air inside has become harder to steer and guide toward the outlet vent. Donald is not that experienced and struggles with it for the entire dive. His buoyancy corrections are exaggerated and largely ineffective. The tank does indeed become lighter, but not to any great extent. Donald finishes the dive after only 25 minutes. Four other students have to surface with him.
“Great dive, everybody,” beams the instructor. “Did you see that amazing blue-spotted ray under the table coral?” Donald mutters something and manages a half smile, feeling a little guilty at shortening everyone’s dive. He honestly tried his hardest but could not reduce his air consumption.
Diving with the Right Weight
A proper buoyancy check takes a solid two minutes, including making weight adjustments. Five students will take at least 10 minutes. This is a long time for an impatient instructor — much better to slap a few pounds on their belts and tell them to press the inflator button at depth.
Proper weighting makes for maximum control and comfort underwater. A diver who suffers from too much or too little weight will be trying to correct the situation for the entire dive. Subsequently, he or she will consume more air. A few minutes spent before the dive and a greater understanding of how weighting works will transform your diving forever.
The first step in choosing the correct amount of weight is…guesswork. Although charts list body weight, thermal protection and other buoyancy-changing variables, it’s probably quicker to just get in the water and figure it out by trial and error.
Local dive centers generally have a good idea because their variables may not change too much. They use the same tanks and the same type of rental wetsuits. The seawater in the area doesn’t change in salinity. So when you walk up to the counter, the divemaster manning the reception desk has probably sized you up before you even got there. It is likely they’ll be accurate to within just a pound or two.
During our open-water course, we learn that we should float on the surface in a feet-down position while wearing a mask and keeping a regulator in our mouth. To establish correct weighting, take a full breath and hold it for a few seconds while the contents of your BCD empty. With weights around your middle, feet still and an empty jacket, you should remain at the surface only via the air in your lungs. The manual says that if the water reaches eye-level or around the forehead, you’re weighted correctly.
Of course to get down and begin a dive, you must breathe out, empty your lungs and sink below the surface. This is where the problems can begin. The trouble with this well-meaning advice is that it stops way too soon. I have conducted these eye-level buoyancy checks with many groups of students only to find on the actual descent that many of them can’t get down at all. The shortcut is to add more weight, as if you were putting a foot on the diver’s head.
Elsewhere in our manual, we learn that you’ll need about two pounds (one kilo) of lead to offset a liter of air. The average adult male has around six liters of air in his lungs at full capacity. If he breathes out before descending, then together with proper weighting, he’ll drop comfortably below the surface. If the diver continues exhaling until his lungs are empty, he’ll develop a lung squeeze with increasing depth pressure. Conversely, if he barely drops a few inches and then takes a great inhalation, filling his lungs to the brim, then he’ll rise back to the surface. This is when many divers tell their guides they need more weight, not realizing that they’re about to burden themselves with unnecessary lead.
Manuals rarely share advice about lung volume and breathing techniques on a descent when you’re still close to the surface. Here, you want to avoid two things — a squeeze from empty lungs and adding extra weights to offset over-inhaling. By deflating and exhaling you begin your descent. If you take very small inhalations between the surface and 10 feet (3 m), you’ll avoid a squeeze. Below that depth, physics takes over and allows you to swim freely, breathing in and out at full volume.
How does this work?
The neoprene in wetsuits contains multitudes of tiny gas bubbles, trapped together to provide insulation underwater. If you throw a dry wetsuit into a swimming pool, about 90 percent of it will float on top of the surface. At 10 feet (3 m), the same suit will compress and squash the many bubbles, which reduces the positively buoyant characteristics you had on the surface. This effectively gives you back a few pounds of the lead it took to get down. You can also breathe fully without risk of rising to the surface in very shallow water.
But what about the end of the dive when your tank is lighter?
We carry out safety stops at depths between 20 to 15 feet (6 to 4.5 m). Here, our suits are still sufficiently compressed to keep us down. They are also saturated and heavy with water, which means a wayward ascent is unlikely. As long as your BCD is empty and you’re properly weighted, you’ll be fine right down to 725 psi (50 bar) in your tank. Deeper parts of the dive are much easier to control, as the heavier pressure keeps your suit and jacket in check.
A diver who is underweighted from the outset might still descend, but will suffer when their tank becomes lighter. Quite often, the diver turns upside down and fins until the deeper water compensates for the lack of lead. After a while they take short, shallow breaths to avoid filling their lungs and rising upwards. This is dangerous, however, because the diver no longer gets a full intake of air to properly oxygenate the circulatory system during the all-important off-gassing process. The extra exertion of trying to stay down will also increase air consumption and shorten the dive. This of course is the opposite of the intended effect of carrying less weight.
Returning to Donald, although he’s got his weights right now, that alone won’t lead to lower air consumption. Next on his list of issues to address are BCD control and proper trim.
By John Kean
John Kean is the author of four books. He holds the PADI Master Instructor rating, along with TDI’s Advanced Trimix Instructor qualification. Since 1997 he has amassed over 7,000 dives, trained over 2,000 students and project-managed several world record deep dive events. His book is available here.
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