What Is the Maximum Safe Ascent Rate for Scuba Diving?:
How fast of an ascent is too fast? The answer varies among scuba certification organizations. Some organizations list a maximum ascent rate of 30 feet/ 9 meters per minute, while others allow a faster ascent rate. For example, my old PADI dive tables (based on the US Navy Dive Tables) allow a maximum ascent rate of 60 feet/ 18 meters per minute.
A safe rule of thumb, (and the most commonly published maximum ascent rate at this time) is:
Never exceed an ascent rate of 30 feet/ 9 meters per a minute.
How to Monitor Your Ascent Rate When Scuba Diving:
The easiest way for a diver to monitor his ascent rate is to use a dive computer. Almost all dive computers have ascent rate alarms which will beep or vibrate when the diver exceeds the computer's programmed maximum ascent rate. The moment the computer alerts the diver that he is ascending too quickly, the diver should take steps to slow his ascent.
However, not all divers use dive computers. A diver without a computer may use a timing device (such as a dive watch) in combination with his depth gauge to monitor the time he takes to ascend a predetermined number of feet. For example, a diver may use his timing device to check that he doesn't ascend more than 15 feet in 30 seconds.
Every diver should carry a timing device underwater. However, in a worst case scenario, a diver may gauge his ascent rate by watching bubbles around him rise towards the surface. He should ascend more slowly than the smallest bubbles he can see. One trick is to look for tiny, champagne-sized bubbles and be certain to ascend more slowly than these bubbles. Another method of estimating an ascent rate is to ascend along a fixed anchor line or ascent line. However, these are rough approximations and divers would do much better to carry a dive computer or timing device.
Why Is Ascending Slowly Important When Scuba Diving?:
Quick ascents can lead to decompression illness. During a dive, a diver's body absorbs nitrogen gas. The nitrogen gas compresses due to water pressure following Boyle's Law, and slowly saturates his body tissues. If a diver ascends too quickly, the nitrogen gas in his body will expand at such a rate that he is unable to eliminate it efficiently, and the nitrogen will form small bubbles in his tissues. This is known as decompression sickness, and can be very painful, lead to tissue death, and even be life threatening.
In a worst-case scenario, a diver who ascends quite rapidly may rupture small structures in his lungs known as alveoli. In this case, bubbles may enter his arterial circulation and travel through his body, eventually lodging in blood vessels and blocking blood flow. This sort of decompression illness is called an arterial gas embolism (AGE), and is very dangerous. A bubble may lodge in an artery feeding the spinal column, in the brain, or in a host of other areas, causing loss or impediment of function.
Maintaining a slow ascent rate greatly reduces the risk of all forms of decompression illness.
Additional Safety Precautions -- Safety Stops and Deep Stops:
In addition to slow ascents, scuba diving training organizations also recommend making a safety stop at 15 feet/ 5 meters for 3-5 minutes. A safety stop allows a diver's body to eliminate additional nitrogen from the body before his final ascent.
When making deep dives (let's say 70 feet or deeper for the sake of argument) studies have also shown that a diver who makes a deep stop based on his dive profile (for example a 50-foot stop on a dive with a maximum depth of 80 feet) as well as a safety stop will have significantly less nitrogen in his body upon surfacing than a diver who does not.
A Diver's Alert Network (DAN) study, measured the amount of nitrogen remaining in a diver's system after a series of ascent profiles. Without getting too technical, the study measured the nitrogen saturation of tissues that become quickly filled with nitrogen, such as the spinal column. DAN ran a series of tests on divers who ascended at a rate of 30 feet/minute from repetitive dives to 80 feet . The results were fascinating:
• A diver who ascended at a rate of 30 feet/minute without stops surfaced with his "fast saturation tissues" 60% saturated.
• If the same diver made a safety stop of 5 minutes at 18 feet, these fast saturation tissues decreased to only 35% saturation.
• If the same diver made an additional deep stop of 5 minutes at 48 feet, he surfaced with his fast saturation tissues further decreased to only 25% saturation.
Making deep stops and safety stops, even on dives within the no-decompression limits (dives that do not require decompression stops), will significantly reduce the amount of nitrogen in a diver's body upon surfacing. The less nitrogen in his system, the lower the risk of decompression sickness. Making deep and safety stops makes sense!
The Final Ascent Should Be the Slowest:
The Take Home-Message About Ascent Rates and Scuba Diving:
Divers should slowly ascend from all dives to avoid decompression sickness and AGE. Mastering a slow ascent requires good buoyancy control and a method of monitoring the ascent rate (such as a dive computer or timing device and depth gauge). In addition, making a safety stop at 15 feet for a minimum of 3 minutes during every ascent, and deep stops when appropriate, will further reduce the amount of nitrogen in a diver's body upon ascent, which reduces his risk of decompression sickness.
Further reading and source: Diver's Alert Network (DAN) Article, "Haldane Revisited: DAN Looks at Safe Ascents" by Dr. Peter Bennett, Alert Diver Magazine, 2002. Read article.