Throughout our diving careers, we are admonished to ascend slowly from our dives. Ascending slowly is so important that most dive computers are programmed with ascent rate monitors that beep obnoxiously (and sometimes embarrassingly) the moment a diver exceeds the computer's designated maximum ascent rate. Dive training organizations repeat information related to safe ascent rates time and time again during scuba certification courses. PADI, with its love of acronyms, even came up with a sticker and T-shirt campaign for safe ascent rates - "Be a S.A.F.E. DIVEr - Slowly Ascend From Every Dive". After being exposed to all this hoopla about ascent rates, a student asked me an intelligent question, "What about descent rates?"
Is there a maximum safe descent rate for scuba diving? In short, no. Scuba divers can descend as fast as they please. Training organizations define maximum ascent rates for reasons that do not apply to descent rates. Maximum ascent rates are set to reduce the chance of the nitrogen gas dissolved in a diver's tissues expanding more rapidly than his body can eliminate it and forming bubbles, which lead to decompression sickness. Air trapped in a diver's lungs and ears will also expand on ascent, and a diver who ascends too quickly may risk over-expanding his lungs and middle ear if the air cannot escape quickly enough. A diver does not risk these injuries during descent.
"Is there a maximum safe descent rate for scuba diving? In short, no. Still, a diver shouldn't simply dump all the air from his buoyancy compensator and plunge straight to the bottom."
As a diver descends, the air in his body air spaces will compress, not expand. If a diver has dived recently, any nitrogen already in his system will compress as he descends according to Boyle's Law. Since decompression sickness is caused by by gas expansion (not compression) descending quickly does not lead to an increased risk of decompression sickness.
Although descending quickly does not increase a diver's likelihood of experiencing lung over-expansion and decompression sickness, a diver still shouldn't dump all the air out of his buoyancy compensator and plunge into the water. There is a limiting factor in determining a diver's descent rate - his ability to equalize his ears. The required frequency and the ease of ear equalization will vary from diver to diver. Some divers can sink quickly, equalizing only a few times, and arrive at the dive's planned depth within moments. Other divers need time to slowly work their way to the bottom. To avoid an ear barotrauma, a diver should descend slowly enough that he can control his descent, slowly enough be able to stop his descent if he has difficulty equalizing his ears.
Controlled descents are important for conservation as well as for dive safety. A diver who descends faster than his ability to maintain control of his depth may find himself crashing into the bottom or exceeding his planned maximum depth. In the former case, physical contact with a coral or other organisms may kill or injure them. Most underwater creatures have defense mechanisms to protect themselves from threats (such as a diver). An out-of-control diver may not only hurt organisms, he may injure himself. Exceeding a planned maximum depth may invalidate a dive plan, or in an extreme case, lead to decompression sickness.
Dive training organizations do not set a maximum descent rate because the speed at which a diver can safely descend varies from diver to diver. Factors which determine a safe descent rate for an individual include his ability to equalize his ears and his proficiency with controlling and arresting a quick descent in the case of an emergency. In most dive scenarios, a slow and controlled descent is preferable to a fast one. However, in some advanced dive plans - such as attempting to arrive at a shipwreck in a strong current, a quick descent may be necessary in order to reach the planned dive site.
Speak Up! How quickly do you descend when diving, and what determines your descent rate?
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