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Air Diving and Decompression 4-19 divers should remain at the surface for as long as possible between dives. 4.4 USING THE U.S. NAVY DIVE TABLES TO MAKE STAGED DECOMPRESSION DIVES There are also U.S. Navy decompression tables cover- ing dives that exceed the No-Decompression Limits (NDLs) and which require divers to stop at predetermined depths during ascent to decompress. Although the majority of dives conducted by NOAA divers do not involve staged decompression, it is important to understand how to do so, as a contingency procedure for unforeseen circumstances. 4.4.1 What Is Decompression? The word “decompression” has two different mean- ings in diving. The first is the dictionary definition, the sec- ond is the act of doing it in a controlled way. The dictionary definition of decompression is the reduc- tion of pressure or release from compression. In the context of a pressure vessel, this meaning is more or less obvious, reducing the pressure is decompressing the vessel. It might well be called depressurizing. In the context of a diver ascending, the ascent takes the diver to a place where the pressure is lower, and this too is decompressing. Decompres- sion is something that occurs on the ascent from every dive. However, although divers occasionally use the word as defined, they also use the word “decompression” to mean the release or reduction of pressure in a controlled or planned way to avoid bubble formation and decompres- sion sickness (DCS). The latter is an outcome of decom- pression when the pressure release is not done properly. So, it is in the best interest of the submerged diver to “decompress” in order to reach surface pressure. “Decom- pression” in this sense means the diver is required to follow a specific time, depth, and breathing gas profile. This pro- file, which may be called a decompression table or decom- pression schedule, is designed to allow a diver to ascend to the surface without incidence or symptoms. It may involve stops, or only require a specific ascent rate without stops. The process of ascending to the surface is decompres- sion in both senses. Ascending without stops is still decom- pressing. The important point is that every ascent is a decompression. Further, every dive of any consequence involves a certain decompression obligation. As far as div- ing is concerned, decompression can include: Ascents: Because the ambient pressure decreases dur- ing ascent, ascents are a form of decompression. Slow ascents generally result in the formation of fewer gas bub- bles in body tissues than faster ascents do. This is why the U.S. Navy Dive Tables require an ascent rate of no more than 30 feet per minute. Precautionary Decompression Stops: Commonly known as “safety stops,” these are stops made during ascent, even though the U.S. Navy Dive Tables do not require them. NOAA “no-stop” dive procedures recom- mends the divers make a safety stop of three to five min- utes in the range of 10 to 20 fsw, nominally 15 fsw, for all no-stop dives conducted 60 fsw or deeper, plus all repeti- tive dives. The safety stop has been shown experimentally to reduce the level of ultrasonically-detected bubbles, and should therefore, reduce the likelihood of decompression sickness. In any event, the safety stop also requires the diver to have good buoyancy control in order to slow the ascent before surfacing and this is an all-around safety fac- tor. Time spent at a safety stop is not part of bottom time or SIT and does not affect the divers repetitive group letter designation following the dive.