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The Greatest Pressure Change Is Near the Surface (Essential Concepts for Scuba Diving)

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.

This data suggests that divers should be vigilent about their ascent rates near the surface and ascend most slowly through the last fifteen feet of the dive to reduce the risk of decompression sickness.

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.

Table: Percentage of Pressure Change During Ascent

DEPTH CHANGE IN ATA PERCENT CHANGE
40-35 ft 2.21-2.06 ATA - 7%
35-30 ft 2.06-1.91 ATA - 8%
30-25 ft 1.91-1.76 ATA - 9%
25-20 ft 1.76-1.61 ATA - 9%
20-15 ft 1.61-1.45 ATA - 11%
15-10 ft 1.45-1.30 ATA - 12%
10-5 ft 1.30-1.15 ATA - 13%
5-0 ft 1.10-1.15 - 15%
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