In this third installment of our series on addressing poor air consumption when scuba diving, we’ll examine neutral buoyancy and breathing.
Donald descended through the water column, pinching his nose to equalize his ears and looking toward the seabed. Diving on a wall with a depth of over 650 feet (200 m), he reached for his BCD’s inflator button at 65 feet (20 m) and put some air in his jacket. He didn’t start earlier because he liked the feeling of dropping like a parachutist from a plane. An incremental addition of air just wasn’t as exciting. Still, he had it under control and slowed to a near-stop at 82 feet (25 m), the maximum depth allowed in the pre-dive briefing.
Donald turned to look at the wall, which was still moving past his eyes. A quick glance at his computer showed his depth at 89 feet (27 m). So he kicked against the unintended extension of his descent, soon finding himself level with the group.
After 10 minutes, Donald noticed that his air pressure had dropped to 2,175 psi (150 bar). It was a relaxing dive with little current, but he noticed his breathing was slightly heavier than intended. He stopped finning for a moment and slowly dropped a little deeper — not far, but remaining still would mean he’d drop even further.
Poor air consumption when scuba diving: why?
When a diver breathes in and out, it represents a cycle. My own is about 15 seconds; eight seconds inhaling and seven seconds of exhaling. That’s four breaths per minute while swimming at a relaxed pace without a head-current.
Tilting upward and finning against negative buoyancy is one of the biggest air thieves in the sea. You have now introduced mild exertion. Not such that you’ll be exhausted or stressed, and you may not even realize it, but you will expend a little more energy, which will require a small increase in your breathing rate.
You are now taking six cycles per minute. Although that may not be noticeable, it’s 50 percent more than your buddy who takes four cycles. When she has 2,175 psi (150 bar), you will have 1,800 psi (125 bar). When she has 1,45 psi (100 bar), you will have only 725 psi (50 bar). And by the time her tank reads 725 psi, you’ll be back on the boat with a cup of tea wondering what went wrong.
Like an athlete, your rate of breathing is proportional to your rate of exertion. Imagine a mild walk turning into a light trot and then into a full-blown sprint; each level of exertion requires a faster intake of air. It’s impossible to run but breathe as if you’re walking. The same is true underwater; increase the workload and you increase the speed of your breath. But you know this, so why do you still get caught unaware?
Many divers are too engrossed with marine life to take stock and check if they’re truly neutrally buoyant. They will always spot a runaway descent due to excess weight at depth, but it’s the failure to make minor adjustments that mystifies divers. If you’re not sure what the problem is, then stop, face the reef, cross your arms and legs and wait. Feedback will be instant.
A slightly negative diver will slightly drop. But the answer isn’t to flap your arms like a chicken, attempting flight or kick your way back up, because it will just happen all over again. A diver has two perfectly functioning tools — his lungs and his BCD — to deal with this problem, yet many choose to ignore them.
Proper BCD Control
Breathing in fills the lungs and slows the fall. Of course lungs have a limit, and we shouldn’t fill them to full capacity because any subsequent upward movement may cause an expansion injury. Inside the left-hand pocket of many traditional BCDs there is a white label which, among other things, states the volume and lift capacity of the internal bladder. An adult male, size large BCD, for example, would have a volume of around 20 liters, over three times the capacity of the diver’s lungs. When the lungs reach their limit, the BCD must take over and a simple inflate will do the job. But keep in mind that proper buoyancy control is as fundamental to diving as the air in your tank.
Pumping several liters of air into your jacket on the way down means that it’s bursting to get out on the way back up. Many divers get caught out from 40 to 20 feet (12 and 6 m) where the pressure-differential is the greatest. What worked between 60 and 40 feet (18 and 12 m) will not slow your ascent in shallower water, where jacket expansion is exponentially greater.
If you’re rising too quickly, stop, vent, look at the escaping air, and check your depth to avoid runaway ascents. Incorrect body position can result in trapped air in your BCD; this would happen, for example, in a head-down position while trying to raise your shoulder-mounted deflator hose to dump air. If the trapped air near your waist is higher in the water column than the outlet from which you’re trying to dump it, then you’re violating the laws of physics by attempting to make air travel downwards. Either become vertical or pull the vent string for the dump valve down by your waist. Finning against positive buoyancy expends energy and consumes more air.
So, what about our diver, Donald? He breathes in and presses the inflator button of his BCD. He is moving neither up or down, hovering motionless in midwater next to the reef wall. Now it’s time to swim with the group. But he’s still feeling a little weighty around the middle and his heavy rubber fins make his feet sag a bit.
His air consumption has improved slightly, but what about his trim? If only he could sort that out he would be gliding effortlessly along with the rest of the group.
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.
