No Decompression Limit
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The No Decompression Limit (NDL) or no stop time, is the interval that a scuba diver may theoretically spend at a given depth without having to perform decompression stops. When divers breathe gas under pressure, inert gases, such as nitrogen and helium, dissolve into their body tissues. As the diver ascends from a dive the ambient pressure reduces and the dissolved inert gases in their tissues come out of solution. If this process occurs too quickly decompression sickness can result. The No decompression limit helps divers plan dives so that they can stay at a given depth and ascend without stopping without significant risk of decompression sickness.
No decompression limits are typically printed on dive tables in a grid that can be used to plan scuba dives. There are many different tables available as well as software programs and calculators, which will calculate no decompression limits. Dive tables allow scuba divers to plan multiple dives by allowing the diver to calculate the residual inert gases remaining in their body after Surface Interval Time, a period of time at the surface during which excess nitrogen leaves the body.
It should be noted that the NDLs are theoretical. Although there is a science to calculating no decompression limits there is still much that is unknown about how inert gas enter and leave the human body. In addition, every individual's body is unique and may absorb and release inert gases at different rates. For this reason dive tables typically have a degree of safety built into their recommendations. Divers can and do suffer decompression sickness while remaining inside NDLs.
[edit] History of No Decompression Limits
Dr. John Scott Haldane created the first tables for scuba diving based on his research for the British Royal Navy. Based on his understanding of Henry's Law and through experimentation with goats, Haldane developed the first dive table which was used by the Royal Navy to prevent decompression sickness in divers. In 1912, Chief Gunner George D. Stillson of the United States Navy created a program to test and refine Haldane's tables. This program ultimately led to the first publication of the United States Navy Diving Manual and the establishment of a Navy Diving School in Newport, Rhode Island. Diver training programs were later cut at the end of World War I.
In 1925 a United States submarine, the USS S-51, was sunk after a collision with a passenger liner. Public interest in raising the submarine sparked renewed interest in diving and, in 1927, the Naval School, Diving and Salvage was re-established at the Washington Navy Yard. At this time the US Navy moved their Experimental Diving Unit (EDU) to the same naval yard. In the following years, the Experimental Diving Unit developed the US Navy Air Decompression Tables which have become the accepted world standard for diving with compressed air.
[edit] Dive Computers
Dive computers are designed to be worn by a diver during a dive. These computers contain, at a minimum, a depth gauge and a timer. They use an algorithm to calculate the no decompression limit for the diver throughout the dive. Because dive computers can accurately measure the time spent at various depths they can provide the diver with a more accurate idea of the nitrogen load in their body. When planning a dive using dive tables, divers assume that the entire dive is spent at whatever the maximum depth will be. Using a computer, the diver is credited for the time they spend at lesser depths, typically during the descent and ascent.
Dive computers also provide a measure of safety for divers that accidentally dive a different profile than originally planned. If the diver exceeds a no decompression limit, the dive becomes a decompression dive and the diver must spend time at shallower depths to allow excess inert gases to leave the body. Many dive computers can provide some decompression stop information in the event that decompression limits are exceeded.
