Diving and Decompression - A primer by Mark Ellyatt


The Decochek Optimiser program was designed to help experienced divers plan formal decompression stop dive-plans for both Open Circuit and Closed Circuit equipment exposures. To use this program effectively all users should realise that decompression diving is a risk filled undertaking and while often called a science simply involves too many variables to make it completely predictable or reliable (or even 'good for you').

Potential Hyperbaric injuries associated with scuba diving can come from a variety of sources - some more obvious than others. We can break these down into 3 main classifications: Physiological, Procedural and Dive Tables. Some will cause injury alone and others in conjunction with another. Accidents seeded during and after diving can only be minimised by attending to a series of possible aggravating/modifying situations. The following text is by no means exhaustive but the information contained, combined with ability and honed observation skills should help avoid injury.


The human body is not a machine and should not be thought of, or decompressed, as one. Dive plans are generally designed for a diver completely at rest. Exercise MUST be avoided in all phases of the dive (some light swimming movement during decompression may be desirable though).

Divers should maintain a steady temperature throughout the dive, as dramatic cooling or over heating must be avoided to avoid circulation change or discomfort. Extremity (hands in particular) comfort should be considered over the entire dive time. Cold hands and the subsequent inability to control buoyancy devices or drysuits can have dire consequences both during the ascent and throughout a dive in cold water. If runaway wing/suit inflators cannot be removed / rectified then consider shutting the feeding regulator valve down.

Recompression chamber studies show Dehydration to be present in almost all DCS cases presenting. Adequate hydration should be maintained throughout the diving season - simply drinking water before a dive will not give adequate hydration.

Divers decompressing for long periods while hanging from lines should practise regularly changing the hand / arm used, constant muscle tensing of one arm / hand will cause circulation differences possibly leading to DCS. After surfacing, divers should remain at rest - as much equipment as practical should be removed before exiting via ladder or walking ashore. Post dive exercise with heavy equipment on boats, especially in swell, but even without has caused DCS. Divers should of course not attempt to recover boat anchors or shot-weights after surfacing. Inappropriate use of Helium either as drysuit inflation and even breathing it in very cold water can lead to significant body temperature drop causing undesirable side effects and even DCS.

Amateur divers, just as in the general population are affected by a condition called PFO (Patent Foramen Ovale). This congenital Heart defect passes unnoticed at the surface except maybe as explanation for the occasional migraine. If sufferers descend underwater, venous blood that should get filtered for bubbles/gas seeds by the lungs maybe occasionally shunted to the Arterial circulation system and this can lead to severe DCS usually without apparent cause. As PFO’s are present in approximately 30% of the population it is indeed strange that no specific test is performed on potential scuba candidates before they go underwater. It is suggested that forceful Valsalva manoeuvres performed during the ascent may be the trigger mechanism for potential injury in divers both with and without PFO’s. Best to avoid forceful ear clearing during the ascent if at all possible especially during formal decompression stop dives.


Ascent and descent rates during the dive need close attention. Most dive plan generating software asks for a descent rate. If the user descends to the bottom faster than this rate then the decompression plan will be insufficient and may cause DCS. When planning dives it is advisable to use fast descent (ideally, unachievable, say 50metres per minute) speeds, if the option to use an immediate descent during dive planning is available this should be selected).

Exceeding ascent rates, even momentarily, is very likely the leading cause of diver injury. Always ascend near 10metres per minute or even slower in the formal decompression stop range. In general, the higher the fraction of Helium in the breathing case the closer the ascent must be to perfection. Lower fractions of Helium are more forgiving during momentary lapses of concentration and loss of buoyancy control. Divers should gain additional experience and buoyancy skills before using high fractions of helium.

Divers wishing to dive deeper than say 80metres for longer than 20minute bottom times should study the effects caused by counter diffusing gas mixtures (ICD). The Decochek program will warn of the risk of ICD if deemed present – users should check gas mixtures in question (highlighted) and modify them accordingly. Briefly, ICD can cause significant forms of DCS when inspired gas mixtures change from relatively low Equivalent Narcotic Depth (END’s) values to higher END’s. This END ‘jump’ is generally achieved when changing from Bottom Mix trimix mixtures to decompression mixes containing less or no helium. The use of decompression mixes containing appropriate Helium fractions is becoming more widespread. CCR users must check for open circuit Bail-out gas suitability at as many different depths as possible.

Dive Tables

Dozens of dive tables have been published but sadly many have not been validated outside of the inventor’s imagination. Some dive tables are designed for certain air diving exposures and maybe completely unsuitable for use while breathing mixtures containing Helium. Since the advent of technical diving, amateur divers have been finding out (sometimes painfully) for themselves what works and what does not. There is no formal database of dive plans that ended in injury and injurious dive plans are often repeated ad-nausea - indeed it would be a nigh-on impossible job to state the actual reason(s) for injury in any case.

Dive tables are often updated to reflect repeated injuries to the users. They are often reintroduced to include additional conservatism in effort to ultimately offer decompression plans advising ‘adequate’ decompression profiles. Users of any dive tables and or dive planning software must strive to obtain the latest version of the table of program to maximise the likelihood of success.

Dive tables employ an algorithm and a set of ‘rules’ in an attempt to generate decompression schedules. The algorithm often includes a system of compartments attempting to mimic the many differing tissues that make up the human body. Each of these theoretical tissues is loaded or unloaded by function of time, ambient pressure and inspired breathing mixtures. Simply speaking, at one end of the algorithm are a group of theoretical tissues served well by the body’s circulation system and affected relatively quickly by time and pressure change, these are generally called ‘Fast Tissues’. The other end of the algorithm includes tissues affected more slowly and are therefore called ‘Slow Tissues’. Fast tissues need to be controlled during the initial parts of the ascent, slow tissues control the final phase or time spent just below the surface and to some extent the post dive or surface-interval experience.

All dive tables attempt to decompress the whole body effectively but none tend to agree on how this is achieved. Some call for deeper stops based on anecdotal evidence and some offer seemingly very fast shallow stop decompressions, often in in direct contrast to empirically tested tables. It has been observed more recently that ‘slow tissue’ time can be minimised or reduced without ‘obvious’ side effect in amateur divers as their dive exposures are relatively short. However as advanced divers strive to get ever deeper or longer it has been observed in practise that short shallow stop time is not of benefit and the practise of shaving down shallow stop time is fool hardy and potentially dangerous. Diving injuries can be so minimal after decompression dives as to go unnoticed, that is until the cumulative damage is sufficient to produce significant injury.

The algorithm embedded in the Decochek Optimiser program is based on the AB-2 algorithm already tested extensively during deep and ultra-deep exposures. The program attempts to offer the user an ‘adequate’ decompression schedule. It doesn’t keep the diver underwater any longer than necessary and definitely does not offer the fashionably short but clearly inappropriate shallow stop times. Divers must not omit or shorten any of the stated decompression. The conservatism ‘slider’ in the program adds additional stop-time when necessary (cold water dives/exercise etc) by altering the algorithm's 'response curve', in an effort to further decompress the users ‘controlling’ tissues before the final act of surfacing.