Ratio decompression
Encyclopedia
Ratio decompression is a technique for calculating decompression schedules for scuba divers
engaged in deep diving
without using dive tables, decompression software or a dive computer
. It is generally taught as part of the "DIR
" philosophy of diving promoted by organisations such Global Underwater Explorers
(GUE) and Unified Team Diving
(UTD) at the advanced technical diving
level. It is designed for decompression diving executed deeper than standard recreational diving
depth limits using trimix as a "bottom mix" breathing gas.
behind the off-gassing of nitrogen
or helium
absorbed by the body from breathing gases under pressure has never been definitively established, particularly in relation to the formation of bubbles in the body's tissues, and a number of different algorithm
s have been developed over the years, based largely on empirical data, to predict the rate of off-gassing to minimise the risk of decompression sickness
in divers. However, all diving algorithms are just models and do not describe the individual physiology of the individual diver accurately in all circumstances: divers have been known to suffer symptomatic decompression sickness whilst diving within the limits of dive tables or dive computer
s (sometimes referred to as an "undeserved hit"), and divers have exeeded No Decompression Limits but remained asymptomatic.
While Ratio Decompression is not a complete decompression model, its most resemble those of Bühlmann alogrithm
, and the Varying Permeability Model
algorithm, with emphasis on the use of deep stops and gradient factors.
Whereas a characteristic decompression profile, even based upon the more conservative algorithms, will normally result in a diver ascending relatively quickly through shorter deeps stops before spending a great deal of time at the shallower stops (resulting in a much sharper angle in the depth/time graph of the ascent profile), ratio deco will allow a diver dynamically take a total decompression obligation for a given dive and create a profile which makes better use the most effective parts of the decompression profile and comparatively less time at the less effective stops (resulting in a much softer curve in the depth/time graph of the ascent profile).
The starting point is to ascertain the correct ratio (from whence the technique gets its name) of the amount of total decompression time as a ratio to the total bottom time. This ratio is fixed solely by reference to depth. Although on traditional tables the amount of decompression would vary according to time at depth, the basis of the theory that most dives will operate within a range of normalcy which makes the use of fixed ratios permissible. Certain depths establish certain ratios; a 1:1 ratio occurs at approximately 150 feet (45.7 m); a 2:1 ratio occurs at approximately 220 feet (67.1 m). Between these depths, for each 10 feet (3 m) deeper or shallower than a fixed ratio depth, the diver will then add or subtract a specified number of minutes to their total decompression time. Accordingly, once the diver knows their planned depth and time, they can look up the most proximate ratio, calculate the difference in depths, and add or subtract the appropriate number of minutes from their total bottom time to give a total decompression time.
Unlike traditional dive tables (but on the same basis as dive computers), ratio deco is calculated by reference to average depth rather than maximum depth. The technique also requires that the dive be divided into 5 minute segments, and the total decompression time accumulated for each 5 minute segment be calculated. To add an element of conservatism, divers lump 5 minute segments into pairs, and use the deeper depth of the pair to calculate the amount of decompression time accumulated.
Once the diver has calculated the total required decompression time, they calculate the depth at which the deep stops commence. To do this, they calculate the absolute pressure (in atmospheres
absolute) at their maximum depth, and multiplying this figure by either 6 (for feet) or 2 (for meters), and then deducting that figure from the maximum depth, and rounding up to the next shallower increment of 10 feet (3 m). That is the depth at which the deep stops will commence, and is equivalent to a pressure of 80% of the pressure at maximum depth. The diver will then do standard deep stops at every 10 feet (3 m) until they reach the depth for the appropriate gas switch to their decompression gas. The diver is expected to do at least 3 minutes at the gas switch stop to acclimatise to the higher partial pressure
of oxygen (known as the "oxygen window in technical diving
"), and use this window to calculate the remaining stops.
After the gas switch is made, the actual decompression is performed. The total decompression is divided into two - half up to a depth of 30 feet (9 m), and half between 20 feet (6 m) and the surface. For the deeper half, the diver simply calculates the total number of stops, stopping every 10 feet (3 m), up to and including the last stop, and then divides the deep half of the decompression time equally between all of the stops. At 20 feet (6 m) the diver will then perform the second half of the total required decompression, and then ascend as slowly as possible to the surface (characteristically aiming for 3 foot (0.9144 m) per minute).
or IANTD
. Although the safety record of ratio deco appears to be good, it suffers from a number of limitations.
GUE has not been keen on the wider use of the technique, and has always stressed that ratio deco should form part of the wider DIR philosophy
espoused by the organisation. GUE has expressed concerns that divers trying to utilise the technique without proper training, or without employing DIR approach to skill development, dehydration and fitness leads to an unacceptably high risk of decompression sickness.
However, the technique has undoubted value in emergency situations where a dive plan is "blown" for one reason or another.
Nonetheless, given the limited amount of forensic research available on any decompression algorithm, it is difficult to see what further comment the book would have been in a position to make.
Scuba diving
Scuba diving is a form of underwater diving in which a diver uses a scuba set to breathe underwater....
engaged in deep diving
Deep diving
The meaning of the term deep diving is a form of technical diving. It is defined by the level of the diver's diver training, diving equipment, breathing gas, and surface support:...
without using dive tables, decompression software or a dive computer
Dive computer
A dive computer or decompression meter is a device used by a scuba diver to measure the time and depth of a dive so that a safe ascent profile can be calculated and displayed so that the diver can avoid decompression sickness.- Purpose :...
. It is generally taught as part of the "DIR
Doing It Right
Doing It Right is a holistic approach to scuba diving. According to the DIR approach fundamental skills, teamwork, environmental and situational awareness, and the use of a highly optimized and streamlined equipment configuration are the fundamentals of DIR diving...
" philosophy of diving promoted by organisations such Global Underwater Explorers
Global Underwater Explorers
Global Underwater Explorers is a scuba diving organization that provides education within recreational, technical and cave diving. It is a not-for-profit, membership organization, based in High Springs, Florida, United States....
(GUE) and Unified Team Diving
Unified Team Diving
Unified Team Diving is a SCUBA diving training agency founded in 2008 to incorporate DIR/Hogarthian principles into SCUBA education at all levels.-History:...
(UTD) at the advanced technical diving
Technical diving
Technical diving is a form of scuba diving that exceeds the scope of recreational diving...
level. It is designed for decompression diving executed deeper than standard recreational diving
Recreational diving
Recreational diving or sport diving is a type of diving that uses SCUBA equipment for the purpose of leisure and enjoyment. In some diving circles, the term "recreational diving" is used in contradistinction to "technical diving", a more demanding aspect of the sport which requires greater levels...
depth limits using trimix as a "bottom mix" breathing gas.
Theory
The physiologyPhysiology
Physiology is the science of the function of living systems. This includes how organisms, organ systems, organs, cells, and bio-molecules carry out the chemical or physical functions that exist in a living system. The highest honor awarded in physiology is the Nobel Prize in Physiology or...
behind the off-gassing of nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
or helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
absorbed by the body from breathing gases under pressure has never been definitively established, particularly in relation to the formation of bubbles in the body's tissues, and a number of different algorithm
Algorithm
In mathematics and computer science, an algorithm is an effective method expressed as a finite list of well-defined instructions for calculating a function. Algorithms are used for calculation, data processing, and automated reasoning...
s have been developed over the years, based largely on empirical data, to predict the rate of off-gassing to minimise the risk of decompression sickness
Decompression sickness
Decompression sickness describes a condition arising from dissolved gases coming out of solution into bubbles inside the body on depressurization...
in divers. However, all diving algorithms are just models and do not describe the individual physiology of the individual diver accurately in all circumstances: divers have been known to suffer symptomatic decompression sickness whilst diving within the limits of dive tables or dive computer
Dive computer
A dive computer or decompression meter is a device used by a scuba diver to measure the time and depth of a dive so that a safe ascent profile can be calculated and displayed so that the diver can avoid decompression sickness.- Purpose :...
s (sometimes referred to as an "undeserved hit"), and divers have exeeded No Decompression Limits but remained asymptomatic.
While Ratio Decompression is not a complete decompression model, its most resemble those of Bühlmann alogrithm
Bühlmann tables
The Bühlmann decompression algorithm is a mathematical model of the way in which inert gases enter and leave the body as the ambient pressure changes. It is used to create Bühlmann tables. These are decompression tables which allow divers to plan the depth and duration for dives and show...
, and the Varying Permeability Model
Varying Permeability Model
The Varying Permeability Model, Variable Permeability Model or VPM is an algorithm that is used to calculate the decompression stops needed for a particular dive profile. It was developed by D.E. Yount and others for use in professional diving and recreational diving...
algorithm, with emphasis on the use of deep stops and gradient factors.
Whereas a characteristic decompression profile, even based upon the more conservative algorithms, will normally result in a diver ascending relatively quickly through shorter deeps stops before spending a great deal of time at the shallower stops (resulting in a much sharper angle in the depth/time graph of the ascent profile), ratio deco will allow a diver dynamically take a total decompression obligation for a given dive and create a profile which makes better use the most effective parts of the decompression profile and comparatively less time at the less effective stops (resulting in a much softer curve in the depth/time graph of the ascent profile).
Methodology
The basis for calculating a decompression schedule using ratio decompression is actually relatively simple (and certainly much simpler than the extremely complicated algorithms used by dive computers). The following represents a slightly simplified summary of the process.The starting point is to ascertain the correct ratio (from whence the technique gets its name) of the amount of total decompression time as a ratio to the total bottom time. This ratio is fixed solely by reference to depth. Although on traditional tables the amount of decompression would vary according to time at depth, the basis of the theory that most dives will operate within a range of normalcy which makes the use of fixed ratios permissible. Certain depths establish certain ratios; a 1:1 ratio occurs at approximately 150 feet (45.7 m); a 2:1 ratio occurs at approximately 220 feet (67.1 m). Between these depths, for each 10 feet (3 m) deeper or shallower than a fixed ratio depth, the diver will then add or subtract a specified number of minutes to their total decompression time. Accordingly, once the diver knows their planned depth and time, they can look up the most proximate ratio, calculate the difference in depths, and add or subtract the appropriate number of minutes from their total bottom time to give a total decompression time.
Unlike traditional dive tables (but on the same basis as dive computers), ratio deco is calculated by reference to average depth rather than maximum depth. The technique also requires that the dive be divided into 5 minute segments, and the total decompression time accumulated for each 5 minute segment be calculated. To add an element of conservatism, divers lump 5 minute segments into pairs, and use the deeper depth of the pair to calculate the amount of decompression time accumulated.
Once the diver has calculated the total required decompression time, they calculate the depth at which the deep stops commence. To do this, they calculate the absolute pressure (in atmospheres
Atmosphere (unit)
The standard atmosphere is an international reference pressure defined as 101325 Pa and formerly used as unit of pressure. For practical purposes it has been replaced by the bar which is 105 Pa...
absolute) at their maximum depth, and multiplying this figure by either 6 (for feet) or 2 (for meters), and then deducting that figure from the maximum depth, and rounding up to the next shallower increment of 10 feet (3 m). That is the depth at which the deep stops will commence, and is equivalent to a pressure of 80% of the pressure at maximum depth. The diver will then do standard deep stops at every 10 feet (3 m) until they reach the depth for the appropriate gas switch to their decompression gas. The diver is expected to do at least 3 minutes at the gas switch stop to acclimatise to the higher partial pressure
Partial pressure
In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture....
of oxygen (known as the "oxygen window in technical diving
Oxygen window in technical diving
In diving, the oxygen window is the difference between the partial pressure of oxygen in arterial blood and the ppO2 in body tissues. It is caused by metabolic consumption of oxygen.The term "oxygen window" was first used by Albert R. Behnke in 1967...
"), and use this window to calculate the remaining stops.
After the gas switch is made, the actual decompression is performed. The total decompression is divided into two - half up to a depth of 30 feet (9 m), and half between 20 feet (6 m) and the surface. For the deeper half, the diver simply calculates the total number of stops, stopping every 10 feet (3 m), up to and including the last stop, and then divides the deep half of the decompression time equally between all of the stops. At 20 feet (6 m) the diver will then perform the second half of the total required decompression, and then ascend as slowly as possible to the surface (characteristically aiming for 3 foot (0.9144 m) per minute).
Limitations
Ratio decompression has never been adopted by more mainstream technical diver training agencies, such as TDITechnical Diving International
Technical Diving International is the largest technical diving certification agency in the world. As one of the first agencies to provide training in mixed gas diving and rebreathers, TDI is seen as an innovator of new diving techniques and programs which previously were not available to the...
or IANTD
International Association of Nitrox and Technical Divers
International Association of Nitrox and Technical Divers is a SCUBA diving organization concerned with certification and training in Enriched Air Nitrox diving, Technical diving and Free diving.- History :...
. Although the safety record of ratio deco appears to be good, it suffers from a number of limitations.
- The diver is generally limited to two potential bottom breathing mixes (specific mixes of Trimix). The technique does not work for deep air diving, or if a diver elects to use a non-optimal bottom mix.
- The variability suggests that increasingly greater risks are assumed at greater depths and for greater exposures; the modelling works much better when the decompression time to bottom time ratio is 1:1 or less.
- Whilst the mathematical computations are manageable, it involves a greater degree of task loadingTask loadingTask loading in Scuba diving is a term used to refer to a multiplicity of responsibilities leading to an increased risk failure on the part of the diver to undertake some key basic function which would normally be routine for safety underwater....
, that appears unnecessary given the ready availability of dive computers and dive planning software.
- It can result in the diver conducting more decompression than is necessary by adding several deep stops and an additional 6 minutes to surface after decompression time has elapsed. This criticism is probably unwarranted - almost all decompression software and computers result in a diver doing more decompression than, on average, is necessary; but because of lack of certainty over the physiology, a sizeable degree of conservatism is usually employed.
GUE has not been keen on the wider use of the technique, and has always stressed that ratio deco should form part of the wider DIR philosophy
Doing It Right
Doing It Right is a holistic approach to scuba diving. According to the DIR approach fundamental skills, teamwork, environmental and situational awareness, and the use of a highly optimized and streamlined equipment configuration are the fundamentals of DIR diving...
espoused by the organisation. GUE has expressed concerns that divers trying to utilise the technique without proper training, or without employing DIR approach to skill development, dehydration and fitness leads to an unacceptably high risk of decompression sickness.
However, the technique has undoubted value in emergency situations where a dive plan is "blown" for one reason or another.
Independent review
Although to date no independent forensic review of ratio decompression as a decompression algorithm has been conducted, in his book Deco for Divers, Mark Powell considers ratio decompression, and analyses it in slightly simplistic "flattening the curve" terms, illustrating it by way of comparison to certain more traditional models.Nonetheless, given the limited amount of forensic research available on any decompression algorithm, it is difficult to see what further comment the book would have been in a position to make.