Cal Coast Divers: Dive Theory Pages

Currently there is no universally accepted mechanism for Decompression Sickness (DCS) in scuba divers. While the physiological mechanisms are not clearly understood, many models and algorithms have been developed to minimize the likelihood of divers’ susceptibility to DCS. Many diving tables have been created in an effort to prevent super saturation conditions in the body, while inherent limits in bottom time eventually led to more accurate methods of tracking nitrogen in the body. Eventually dive computer engineers incorporated practiced dive table theory with the ability of a computer to sample and model nitrogen uptake and off gassing. Dive computers offer many advantages over their predecessor dive tables and as such have become the norm for divers over the past fifteen years; however their nearly universal use may come at the future expense of diver safety.

Everytime a diver enters the water, that diver accepts the risk of taking a decompression hit (DCS) regardless of what type of nitrogen tracking system they use. The mechanisms of nitrogen loading and off gassing are complex. Now closing in on one hundred years of experimentation in the field of nitrogen loading and elimination, the known factors which contribute to DCS remain complex, while a clear explanation to what happens at the atomic and cellular level still eludes scientists to this day. However despite the large gaps in knowledge governing DCS, many reliable models have been developed to track the loading and elimination of nitrogen, although no model to date can guarantee a diver using that model will not develop DCS. The original models to reduce the risk of developing DCS were proposed by John Scott Haldane. Since Haldane’s original model, several models have taken center stage in reducing susceptibility to DCS. Today dive computers lead the way for the vast majority of divers for tracking nitrogen levels during and in between dives. While these dive computers offer many advantages and safety features, many within the industry have begun to emphasize the use of dive computers at the expense of understanding the principles on which make the computers practical. Overtime the push towards learning dive tables during entry level courses may be completely abandoned; perhaps at the expense of safety in favor of the ease of using a dive computer. In order to discuss these future implications, a greater understanding of dive theory is needed.

Everytime A Diver descends down the Water column, changes in atmospheric pressures expose divers to the threat of Decompression Sickness (DCS), otherwise known as the bends, or caissons disease. DCS occurs when nitrogen dissolved within the body turns into a gas and inhibits proper oxygenation of the areas in which the bubble is located. In essence the bubble becomes trapped in the vascular system and inhibits blood flow to that area. As blood transports oxygen throughout the body, a blockage within the vascular system can have minor to life threatening results as organs within the body slowly deteriorate without the oxygen. The principles governing how nitrogen is loaded into the body, and overtime develops into DCS are complex and are not clearly understood.

At sea level, for the sake of simplicity, air is approximately 79% nitrogen, in the form of N₂, and 21% oxygen, in the form of O₂. These and the other trace elements of air are inspired and transferred to the lungs upon inhalation. Roughly five percent of the oxygen inspired is taken up by the lungs to be metabolized by the body. Once in the bloodstream the cardiovascular system delivers the oxygen throughout the body where it is metabolized or used up in order to carry out important physical processes. Nitrogen in air, which is also taken up during inhalation, is not utilized during human metabolic processes. As such, nitrogen is considered to be an inert gas.

Depth fsw (feet – meters)	Pressure	Volume.
0’ – 0m	1 ATA	Full
33’ – 10m	2 ATA	1/2 Full
66’ – 20m	3 ATA	1/3 Full
99’ – 30 m	4 ATA	1/4 Full
132’ – 40m	5 ATA	1/5 Full

Because they are not metabolized, inert gasses such as nitrogen and the other trace gasses present in the air we breathe begin to saturate the body. Saturation is said to occur when the ambient pressure of a gas within a liquid equals the pressure being exerted on the liquid. As a diver descends, the amount of ambient pressure exerted upon the diver increases as a function of depth. The relationship between volume and depth is shown in Table 1. As can be shown, 33 feet salt water (fsw) is the equivalent to one atmosphere of air at sea level. Therefore as a diver descends to a depth of 33 fsw, an additional atmosphere of pressure is exerted on the diver. Similarly, as a diver descends the volume of air required to fill the lungs stays the same, however due to changes in pressure, the amount of air required to fill the lungs increases. For example a diver breathing air from a compressed cylinder at a depth of 33 fsw requires two times the volume of air as at the surface to fill the diver’s lungs.

While the density of gas at depth changes, another physical law helps describe the absorption of gas in a liquid. Henry’s Law, which was formulated from the experimental conclusions of English chemist William Henry states “the amount of gas that will dissolve into a liquid at a given temperature is directly proportional to the partial pressure of that gas” (Encyclopedia of Recreational Diving). “Nitrogen or other inert gas under pressure enters the body through the lungs and changes from gaseous to dissolved form to enter the blood. Greater pressure with increasing depth dissolves more gas” (Hyperbaric Medical Review) until the amount of the gas in the body reaches equilibrium with the surrounding pressure. In essence, the body uptakes an inert gas until the pressure of the gas within the blood equals the surrounding pressure. Once in the body, the nitrogen seeks equilibrium within the blood and tissues of the body. “The driving force for inert gas uptake is the difference between high nitrogen partial pressure in the lung and low dissolved nitrogen tension in the rest of the body” (Hyperbaric Medical Review).

Since oxygen is metabolized by the body, it does not significantly contribute to DCS; however the inert gas nitrogen is not metabolized, and collects within the body until the pressure of nitrogen within the body equals the ambient pressure of nitrogen breathed in (Hyperbaric Medical Review). The saturation of nitrogen in the body in itself does not pose a threat until a rapid decrease in pressure causes the saturated body to supersaturate. Supersaturation occurs when the partial pressure of nitrogen in the body becomes greater than that of the ambient pressure.

When the partial pressure of nitrogen in the body exceeds the partial pressure of nitrogen being inhaled, the nitrogen within the body “transfers from tissues to blood, then from blood to lungs for exhalation” (Hyperbaric Medical Review). This process is known as off gassing or elimination. If the body is not able to off gas the nitrogen quickly enough, and the “supersaturation becomes too great, decompression may occur” (Hyperbaric Medical Review). How quickly nitrogen is loaded or eliminated from the body depends upon the gradient or difference between the surrounding pressure of nitrogen being inspired, and the partial pressure of nitrogen currently present in the body. In essence, the larger the gradient from high to low concentration, the greater the net movement of nitrogen to the area of lower concentration. Overtime the movement of inert gas will continue until equilibrium with the amount of inspired nitrogen has occurred. To complicate the issue of nitrogen loading and off gassing, different parts of the body will load and eliminate nitrogen at different rates based upon their densities. For example “inert gas elimination appears to be controlled by faster neurological tissues after short deep dives, as neurological symptoms (particularly CNS) are relatively common. Limb pain predominates after slow saturation decompression, on the other hand, implicating the slowly exchanging tissues” (Hyperbaric Medical Review).

Nitrogen loading and elimination of the body is also governed by other factors, while the susceptibility to DCS is increased when certain physiological characteristics exist. “Major determinants of risk of DCS are depth, time at depth, ascent rate, and multiple dives.” “Individual factors that may predispose [a diver] to DCS include fatigue, dehydration, smoking, carbon dioxide retention, and possibly poor physical conditioning” (Hyperbaric Medical Review). Dehydration causes the blood to thicken which inhibits blood circulation and in turn reduces nitrogen elimination. There also may be a correlation between poor physical conditioning and DCS risk.

Basic physical laws, such as Henry’s Law, do not explain the formation of nitrogen bubbles within the body at the lower degree of changes in partial pressure experienced by recreational divers. Nitrogen bubble adhesion to surface proteins on cell layers is believed to occur and may contribute to nitrogen bubble formation. Vortices within the blood stream, and reduced pressure pockets between muscles at work are also thought to be possible contributors to nitrogen bubble formation. However, because of the complex nature of DCS there is no guarantee that a diver following accepted diving practices will not be at risk for DCS.

In order to reduce the susceptibility of a diver at depth to developing DCS, John Scott Haldane was commissioned to “investigate the causes of, and seek a remedy for, DCS” (PADI Divemaster Manual). During his research Haldane pressed goats in pressure chambers to determine the governing factors of nitrogen gas loading and elimination. From his experiments Haldane was able to create a mathematical model to explain the different rates of nitrogen loading and elimination. Since it is not known exactly how each tissue responds to these changes in partial pressures of nitrogen, Haldane proposed theoretical tissue compartments which are not meant to replicate what is occurring in any specific tissue, but are modeled to correlate with the theoretical loading and elimination of nitrogen data collected.

Haldane’s original model contained five theoretical tissue compartments. Nitrogen loading of these five compartments needed to reflect the fact that nitrogen will load at different rates based upon the gradient of partial pressures experienced by the tissue as well as the density and holding power of the tissue. As a result Haldane came up with halftimes. According to the PADI Divemaster Manual, “a halftime is the time, in minutes, for a particular compartment to go halfway from its beginning tissue pressure to equilibrium (saturation) at depth. The compartment dissolves in or out half the remaining nitrogen for each halftime, creating an exponential progression so that a compartment is 50 percent equilibrated after one halftime, 75 percent after two, 87.5 percent after three and so on. For practical purposes, after six halftimes the compartment is considered 100 percent equilibrated with the new depth (98.4 percent actually).”

Subsequent to the development of Haldane’s Model, further research was conducted to investigate the relationship between bottom time, ascent rates, and nitrogen bubble formation. From this research came the development of new tables. While fundamentally the same as Haldane’s model, new decompression dive tables would utilize more theoretical tissue compartments and tissue M values which better modeled the latest research.

As diving tables became more widespread it became important to give credit to divers doing no-decompression diving credit for time spent off gassing at the surface. The US Navy tables would be among the first to do this. In addition to allowing for elimination credit, the Navy table also added one 120 minute theoretical compartment to Haldane’s existing five compartments. The addition of this compartment was used to account for research which indicated the need for a longer compartment than Haldane’s 75 minute compartment. Further, in order to give credit for off gassing during surface intervals, a compartment needed to be used to govern the elimination of nitrogen during the surface interval.

Since all compartments would theoretically release nitrogen at the rate they absorbed it, the complexity of calculating the governing compartment for an infinite amount of dive depths and time limits highlighted the importance of using one compartment to model nitrogen elimination. “To solve this, the Navy designed its surface credit table based on the worst-case scenario, which for them was a repetitive dive preceded by a decompression dive. They therefore based repetitive credit on the slowest compartment, the 120 minute halftime.” “From a model perspective, the calculations assumed that all compartments washout (release nitrogen) at the 120 minute rate at the surface.” Future dive tables would expand upon the success of the Navy’s no decompression tables, while taking further steps to extend bottom time.

Bottom time was extended once again as the no-decompression tables underwent a third change. When the dive tables were adapted to the needs of recreational divers, additional compartments were added and the controlling compartment governing off gassing was modified for these new tables. Because these tables were designed to allow divers to do a dive without making a mandatory safety stop the “worse-case scenario” became overly conservative. As a result the 60 minute compartment became the controlling compartment, and the amount of time required to off gas during a surface interval was effectively cut in half for any dive.

Special rules were also developed to be used in conjunction with the no decompression dive tables. Dive tables are designed to calculate the amount of nitrogen a diver will absorb at one constant depth during a period of time called the bottom time. Bottom time begins when the diver first makes their descent and ends when the diver begins their ascent. Most dive planners, such as the Recreational Dive Planner (RDP) introduced by PADI, do not contain every possible depth up to the recreational diving limit of 130 feet. Instead these tables begin at 35 feet, then from 40 feet and on increase by ten. Similarly not all times are present up to the no decompression limits. To compensate for this divers are encouraged to round up to the next greatest depth and time to calculate their current pressure group. Divers also must calculate their total bottom time based upon the maximum depth they reached on their dive. Perhaps this characteristic of the dive table, above all else, helped usher in the age of diving computers.

Because a diver must calculate their nitrogen level using their maximum depth reached during the dive, the actual diver profile may vary significantly on the side of conservancy from the nitrogen loading model used. For instance a diver may spend one minute at a depth of 60 feet and the rest of their time at 18 feet. According to dive table practice the entire dive must be planned as if it were conducted entirely at 60 feet. This practice greatly limits the amount of bottom time a diver may experience at depth. As a result the dive computers ability to constantly calculate and load compartments based upon their deepest depths within a preprogrammed time frame allows for more accurate dive profiles.

Dive computers have also been equipped with ascent rate monitors. This feature simplifies a diver’s sometimes complex task of ensuring they ascend at a rate no greater than 60 feet per minute. Some dive computers may also be adapted to monitor an ascent rate much slower than this as new research being conducted by DAN indicates slower ascent rates and longer safety stops reduce the likelihood of developing DCS (How Fast is Too Fast? -- DAN's Ascent Rate Study).

Diving computers are also relatively easy to use and hard to forget as the are typically attached to the first stage of a diver’s regulator set up. Ease of use in most computers means a diver can plan a dive including maximum bottom time allowed at a depth they choose with minimal effort. Many computers will even allow the diver to simulate the dive they planned to determine if a safety stop is recommended. In some algorithms available today, if a diver chooses not to do a safety stop, less time underwater may be allotted to that diver than would be if they had done a safety stop. In other words computers are now being equipped with tools to compensate for poor diving habits. The same holds true for fast ascent rates. In fact if a diver chooses to push the no decompression limits, and violates certain dive rules, the computer may lock out the diver for a 24 hour period. As divers are required to use their most conservative method of tracking their bottom time, this allows a diver time to reflect upon their diving habits for one day before being allowed to get back in the water.

Other advents in diving, such as the use of enriched air (air which contains oxygen partial pressures typically higher than 21 percent and nitrogen levels less that 79 percent) have been incorporated into the general features of many higher end dive computers. This allows divers to further extend their bottom time, and easily model their computer to better reflect the actually amount of oxygen they have.

Dive tables are first introduced in the open water certification course. Over the years, PADI, the Professional Association of Dive Instructors, has removed this important chapter, as it related to the RDP, from the Open Water Manual. This step may indicate a move away from emphasizing the importance of dive table problem solving, a step which may indicate PADI’s acknowledgement of the prevalent use of the dive computer at the expense of older dive tables like the RDP. The danger of this is evident when divers forget the importance of proper diving practices. After all, the dive computers of today are quite similar in theory to the basic tables developed nearly one hundred years ago. Should divers overlook this important relationship it may have difficult repercussions in the future.

Despite the growing number of divers using dive computers, DAN statistics indicate two thirds of the divers in their study who developed DCS “had followed no-decompression guidelines and appear to have been diving within recommended safety limits. Almost three out of four divers in the study were using dive computers.” So while dive computers have become common place and despite the fact the diving injuries have decreased in numbers since the adoption of the dive computer, people are still developing DCS. Apparently there is still room for improvement.

However, as our understanding of what actually takes place in the body increases, dive computers promise the ability to change to reflect new insight into this largely mysterious aspect of diving. No dive table or computer to date can guarantee that a diver will be immune to developing DCS even if they follow all of the recommendations of the table they use. However, computers which serve as electronic dive tables may be modified in the future to allow a user to download a model they feel best reflects their diving and limits their susceptibility to developing DCS. Dive tables may also be adapted for even greater ease of use and interactive capabilities.

2. Hyperbaric Medical Review. 2000. Bookspan, J. Undersea and Hyperbaric Medical Society, Maryland. ISBN 0-930406-18-4

3. National Oceanic and Atmospheric Administration Diving Manual. 2000. Fourth Edition. US Department of Commerce

4. Diving Physiology in Plain English. 1999. Bookspan, J. Undersea and Hyperbaric Medical Society, Maryland. Third edition. ISBN 0-930406-13-3

Abstract

Introduction

The Physiology and Physics of Diving

Decompression Theory

Diving Models

The Advantage of Diving Computers

The Future of Dive Computers

References