While working on a reference article on pressure-depth relationships, I was reminded of divers I had a few years ago. This couple has a creative way of finishing a dive. After maintaining a reasonably well-controlled safety stop, they both reached for their inflator hoses and pffffffffttt held down their inflate buttons and shot to the surface from 15 feet. When I chatted with them about this dangerous habit, they told me that their open water instructor taught them to ascend this way.
Why does this scare me so much? For the sake of simplicity, let's consider just one of the reasons that a fast ascent during any part of a dive is dangerous. Increased pressure underwater causes a diver's body tissues to absorb more nitrogen gas than they would normally contain at the surface. If a diver ascends slowly, this nitrogen gas expands bit by bit and the excess nitrogen is safely metabolized and released.
However, the body can only metabolize nitrogen so quickly. The faster a diver ascends, the faster nitrogen expands and must be removed from his tissues. If a diver goes through too great of pressure change too quickly, his body can not metabolize all of the expanding nitrogen and the excess nitrogen forms bubbles in his tissues and blood. These nitrogen bubbles can cause decompression sickness (DCS) by blocking blood flow to various parts of the body, causing strokes, paralysis, and other life threatening problems. Rapid pressure changes are one of the most common causes of DCS.
What many divers do not realize is that the greatest pressure change during ascents and descents occurs close to the surface. If a diver descends from the surface (1 atmosphere of pressure, or ATA) to 33 feet (2 ATA) the pressure doubles. If he descends from 33 feet (2 ATA) to 66 feet (3 ATA), the pressure only increases by half. The closer a diver is to the surface, the more rapidly the pressure is changing.
Most divers are familiar with the effects of the increased rate of pressure change near the surface. For example, divers must equalize their ears and adjust their buoyancy more frequently the closer they are to the surface.
Related to decompression sickness, this data suggests that divers ascend most slowly through the last 15 feet of the dive.
The divers in this story thought that once the safety stop was completed, the dive was over and they could simply pop up to the surface. Nothing could be further from the truth. While a safety stop allows divers to gain control of their ascent and eliminate as much extra nitrogen as possible from their tissues, it is not the end of the dive. The most critical part of a diver's ascent is after his safety stop.
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Comments
It would be interesting to know which instructor taught this way…. I would say something to him/her.
Once had a newbie ask me why I deflated my wing while going up… at least she didn’t press on her inflator button while doing the same
Makes me wondering if the instructor told them to inflate their BC from the safety stop to rocket to the surface or if they missed understood the instructor when they were told to inflate their BC once on the surface.
People are hearing so much information when going thru their OW course that it is hard for them to understand everything about scuba diving 100%.
I seem to learn something new about diving everyday. But during their OW check the instructor should have caught them using the inflator elevator.
I am an instructor and I often see other students/divers using this method to ascend to the surface. Probably in part to being lazy or just fooling around. I always make it a strong point to make sure my students understand why you do not use your bcd as an elevator. I also advise students when they are doing their safety stop and when they are done to look up and if there are other divers waiting to get on the boat, stay where you are and continue off gas. Then as the line up for the boat dwindles then start a gradual ascent 30′ per minute or less to the boat.
And they surely don’t have the time to see that boat coming in right over their heads….
I’m confused. I completely understand that the pressure change is greater the closer you get to the surface. But in the example from 0 to 5 feet, would the change only be .15 ATA? At the surface your at 1 at 5 feet you’d be at 1.15 (1 + 5/33) the change is .15 not 1.15. Am I missing something? Thanks for the great articles.