Decompression practice

Decompression models take into account variables such as depth and time of dive, breathing gasses, altitude, and equipment to develop appropriate procedures for safe ascent.

What is commonly known as no-decompression diving, or more accurately no-stop decompression, relies on limiting ascent rate for avoidance of excessive bubble formation.

Descent faster than the specified maximum will expose the diver to greater ingassing rate earlier in the dive, and the bottom time must be reduced accordingly.

Ascent rates slower than the recommended standard for the algorithm will generally be treated by a computer as part of a multilevel dive profile and the decompression requirement adjusted accordingly.

The NDL helps divers plan dives so that they can stay at a given depth for a limited time and then ascend without stopping while still avoiding an unacceptable risk of decompression sickness.

[16] Although the science of calculating these limits has been refined over the last century, there is still much that is unknown about how inert gases enter and leave the human body, and the NDL may vary between decompression models for identical initial conditions.

[16] The depth and duration of each stop is calculated to reduce the inert gas excess in the most critical tissues to a concentration which will allow further ascent without unacceptable risk.

This practice is based on empirical observations by technical divers such as Richard Pyle, who found that they were less fatigued if they made some additional stops for short periods at depths considerably deeper than those calculated with the currently published decompression algorithms.

[32] A study by Divers Alert Network in 2004 suggests that addition of a deep (c. 15 m) as well as a shallow (c. 6 m) safety stop to a theoretically no-stop ascent will significantly reduce decompression stress indicated by precordial doppler detected bubble (PDDB) levels.

This will result in a greater diffusion gradient for a given ambient pressure, and consequently accelerated decompression for a relatively low risk of bubble formation.

In open circuit scuba the upper limit for oxygen partial pressure is generally accepted as 1.6 bar,[40] equivalent to a depth of 6 msw (metres of sea water), but in-water and surface decompression at higher partial pressures is routinely used in surface supplied diving operation, both by the military and civilian contractors, as the consequences of CNS oxygen toxicity are considerably reduced when the diver has a secure breathing gas supply.

Rules for safe ascent are based on extension of the decompression model calculations to the desired altitude, but are generally simplified to a few fixed periods for a range of exposures.

[54] NASA astronauts train underwater to simulate the weightlessness and occasionally need to fly afterwards at cabin altitudes not exceeding 10,000 feet (3,000 meters).

NASA guidelines for EADs of 20–50 fsw (6–15 msw) with maximum dive durations of 100–400 minutes allow either air or oxygen to be breathed in the preflight surface intervals.

[53] A study by another military organization, the Special Operations Command also indicated that preflight oxygen might be an effective means for reducing DCS risk.

They also often lie outside the range of profiles with validated decompression schedules, and tend to use algorithms developed for other types of diving, often extrapolated to depths for which no formal testing has been done.

If this "surface interval" from 40 ft in the water to 50 fsw in the chamber exceeds 5 minutes, a penalty is incurred, as this indicates a higher risk of DCS symptoms developing, so longer decompression is required.

[65] The US Navy Heliox saturation decompression rates require a partial pressure of oxygen to be maintained at between 0.44 and 0.48  atm when possible, but not to exceed 23% by volume, to restrict the risk of fire.

[72][73][74] In-water recompression (IWR) is the emergency treatment of decompression sickness (DCS) by sending the diver back underwater to allow the gas bubbles in the tissues, which are causing the symptoms, to resolve.

It is used in military aviation before flights to high altitudes, and in spaceflight before extravehicular activity in space suits with a relatively low working internal pressure.

There is a wide range of choice: A critical aspect of successful decompression is that the depth and ascent rate of the diver must be monitored and sufficiently accurately controlled.

[89] Reducing the partial pressure of the inert gas component of the breathing mixture will accelerate decompression as the concentration gradient will be greater for a given depth.

A considerable amount of self-experimentation is done by technical divers, but conditions are generally not optimally recorded, and there are usually several unknowns, and no control group.

The vast majority of professional and recreational diving is done under low risk conditions and without recognised symptoms, but in spite of this there are occasionally unexplained incidences of decompression sickness.

The earlier tendency to blame the diver for not properly following the procedures has been shown to not only be counterproductive, but sometimes factually wrong, and it is now generally recognised that there is statistically a small but real risk of symptomatic decompression sickness for even highly conservative profiles.

This acceptance by the diving community that sometimes one is simply unlucky encourages more divers to report borderline cases, and the statistics gathered may provide more complete and precise indications of risk as they are analysed.

These practices were empirically developed by divers and supervisors to account for factors that they considered increased risk, such as hard work during the dive, or cold water.

With the development of computer programs to calculate decompression schedules for specified dive profiles, came the possibility of adjusting the allowed percentage of the maximum supersaturation (M-values).

The general tendency in dive computers intended for the recreational market is to provide one or two preset conservatism settings which have the effect of reducing allowed no-decompression limit in a way which is not transparent to the user.

[134][135] Technical, commercial, military and scientific divers may all be expected to do decompression dives in the normal course of their sport or occupation, and are specifically trained in appropriate procedures and equipment relevant to their level of certification.