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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Decompression theory | 16/17 | https://en.wikipedia.org/wiki/Decompression_theory | reference | science, encyclopedia | 2026-05-05T10:06:49.112339+00:00 | kb-cron |
==== Effects of inert gas component changes ==== Gas switching during decompression on open circuit is done primarily to increase the partial pressure of oxygen to increase the oxygen window effect, while keeping below acute toxicity levels. It is well established both in theory and practice, that a higher oxygen partial pressure facilitates a more rapid and effective elimination of inert gas, both in the dissolved state and as bubbles. In closed circuit rebreather diving the oxygen partial pressure throughout the dive is maintained at a relatively high but tolerable level to reduce the ongassing as well as to accelerate offgassing of the diluent gas. Changes from helium-based diluents to nitrogen during ascent are desirable for reducing the use of expensive helium, but have other implications. It is unlikely that changes to nitrogen based decompression gas will accelerate decompression in typical technical bounce dive profiles, but there is some evidence that decompressing on helium-oxygen mixtures is more likely to result in neurological DCS, while nitrogen based decompression is more likely to produce other symptoms if DCS occurs. However, switching from helium rich to nitrogen rich decompression gas is implicated in inner ear DCS, connected with counter-diffusion effects. This risk can be reduced by sufficient initial decompression, using high oxygen partial pressure and making the helium to nitrogen switch relatively shallow.
==== Altitude exposure, altitude diving and flying after diving ====
The USAF conducted experiments on human subjects in 1982 to validate schedules for air diving no-decompression limits before immediate excursions to altitude and for altitude diving allowing immediate flying after the dive to an altitude of 8,500 feet (2,600 m). Another test series in 2004 was made to validate predictions of a bubble-model for altitude decompression using previously untested exposure profiles. Parameters included exertion, altitudes from 18,000 to 35,000 feet (5,500 to 10,700 m), prebreathe time and exposure time, but these exposures did not include recent dives. Experiments with an endpoint of DCS symptoms using profiles near the no-decompression exposure limits for recreational diving were carried out to determine how DCS occurrence during or after flight relates to the length of pre-flight surface interval (PFSI). The dives and PFSI were followed by a four-hour exposure at 75 kPa, equivalent to the maximum permitted commercial aircraft cabin altitude of 8,000 feet (2,400 m). DCS incidence decreased as surface interval increased, with no incidence for a 17 hour surface interval. Repetitive dives profiles usually needed longer surface intervals than single dives to minimise incidence. These tests have helped inform recommendations on time to fly. In-flight transthoracic echocardiography has shown that there is a low but non-zero probability of decompression sickness in commercial pressurised aircraft after a 24 hour pre-flight surface interval following a week of multiple repetitive recreational dives, indicated by detection of venous gas bubbles in a significant number of the divers tested.
== Current research == Research on decompression continues. Data is not generally available on the specifics, however Divers Alert Network (DAN) has an ongoing citizen science based programme run by DAN (Europe) which gathers data from volunteer recreational divers for analysis by DAN research staff and other researchers. This research is funded by subscription fees of DAN Europe members. The Diving Safety Laboratory is a database to which members can upload dive profiles from a wide range of dive computers converted to a standard format and other data about the dive. Data on hundreds of thousands of real dives is analysed to investigate aspects of diving safety. The large amounts of data gathered is used for probabilistic analysis of decompression risk. The data donors can get immediate feedback in the form of a simple risk analysis of their dive profiles rated as one of three nominal levels of risk (high, medium and low) based on comparison with Bühlmann ZH16c M-values computed for the same profile. Listed projects (not all directly related to decompression) include:
Gathering data on vascular gas bubbles and analysis of the data Identification of optimised ascent profile Investigating the causes of unexplained diving incidents Stress in recreational diving Correlation between patent foramen ovale (PFO) and risk of decompression illness Diving with asthma and diabetes and managing the associated risk Physiology and pathophysiology of breath-hold Hypothermia and diving Headache and diving Blood changes associated with diving Decompression risk of air travel after diving Physiological effects of rebreather diving Effects of decompression stress on endothelial stem cells and blood cells Early decompression stress biomarkers The effects of normobaric oxygen on blood and in DCI first aid