|Technical and Recreational Divers Support|
Divers often wonder about different decompression models and which to use. A discussion on this topic is best done with discussing dive table and decompression history. This will not be a complete discussion by any means, but is done with the intent of presenting some changes in decompression methodology and how Departure came to being - which was named for departing from traditional methods to become more biologically relevant.
The place to start is with tissue half-times, or more appropriately called compartments. There are plenty of discussions as to how this works on the web, so the exact workings will not be discussed since this is designed as only an overview of models and not a full presentation. But the premise was that a half-time compartment would take up gas at a logarithmic rate, i.e. that it would take up 50% of the possible gas during one half-time period. This is the same theory of radioactive half-life decay, except in reverse where gas is being taken up by the body instead of being lost. This idea was proposed by J.S. Haldane and is referred to as Haldanian theory.
When first viewed, the idea of compartment half-times seemed to fit what was known about diving and time limits at the time. This led to dive tables and repetitive dive table development since surface intervals could be calculated due to now having a decompression model. The surface intervals were calculated by using the same half-time idea where gas taken up during a dive would be lost using a half-life decay or loss. So in 120 minutes, the "120 minute compartment" would lose 50% of the gas it had taken up from the dive. This is how the "original" U.S. Navy table was developed. The term "original" is used since it is the table most are familiar with even though it is not the true original.
The next notable "step" was by A.A. Bühlmann. Bühlmann did a lot of research including at altitude. The results of his research led to his half-time algorithm. However, the actual mechanics of his algorithm is fundamentally the same as Haldane as it is simply a half-time model. The only differences are that Bühlmann used more half-time compartments, different surfacing tensions (the amount of gas that is allowed to be taken up during a dive for a direct ascent to the surface), and a different "slope" for each compartment if decompression was needed (i.e. Bühlmann would keep a diver slightly deeper, but still "shallow"). But Bühlmann's gas uptake and elimination was just a simple half-time uptake and decay model. So does this work? Many would say it did and that it does. But modern research shows that gas uptake and elimination are not symmetrical, i.e. gas elimination during a surface interval is slower than the uptake. This is because of bubbles that occur after a dive. (To read more on this, refer to the ascending from a dive article). For an immediate and obvious comparison, all that is needed is to reference Bühlmann's flying after diving rules (developed from his model) and what is currently known. Bühlmann's table stated that after a single no-decompression stop dive, a surface of 5 hours was sufficient after any dive or series of dives (4 hours after a 60' dive for 50 minutes) where modern research shows this is far from acceptable (i.e. current research recommends a minimum surface interval of 18 hours before flying after repetitive dives). So why the difference and why does Bühlmann's suggestion not work? It is again because gas uptake and gas loss during a surface interval are not identical.
This flying after diving example and Bühlmann's surface interval gas elimination rate can now be thought of in terms of gas elimination during a decompression stop. In other words, if the gas elimination rate during surface intervals is not correct then this correlates to gas elimination during decompression stops. So the concept is when significant pressure is removed from a diver, gas elimination is slowed down and slower than the uptake. So why is Bühlmann so popular and why are other decompression programs "Bühlmann"? Unfortunately the answer may be as simple as Bühlmann published his algorithm (known as his ZH algorithm) and anyone with just a little bit of knowledge and create a dive/decompression program with just a spreadsheet or those with some computer knowledge can place it into a nicer looking software package. This is what happened with the original (and yes even modern) dive computers.
Original Dive Computers
While there were several attempts at making initial dive computers, perhaps the first one that entered the mainstream market was the EDGE by Orca in 1983. The EDGE simply used straight half-time modeling just as described above. Just as Bühlmann used standard half-time decay for his half-time compartments - which allowed theoretical flight after very short surface intervals, the EDGE's standard half-time decay rate also modeled rapid off-gassing - which would allow repetitive deep dives after very short surface intervals. This was then tested where several deep dives were done back to back with only a 1 hour surface interval between them. Such profiles were allowed and generated by the EDGE and standard half-time models. The tests were aborted after decompression sickness became rampant among the divers. So the verdict is clearly in, Haldanian half-time modeling does not work. Does the model simply need a slight adjustment or is something else needed? Since the purpose of this article is to discuss decompression stops and profiles, lets start with the stops.
Research had been showing that performing deeper stops to help combat bubbles (by keeping them under more pressure) during decompression had positive benefits. (For a discussion on bubbles and their role in gas elimination rates, refer to the ascending from a dive article.) This led to several adaptations or solutions to decompression. One was known as the Pyle stops (named after Richard Pyle) where deeper stops where inserted into a decompression profile. Another method used were Gradient Factors (created by Erik Baker). This in essence was a modification of Bühlmann (or any other half-time model) by changing the gas pressures (tensions) allowed upon surfacing as well as setting a deeper decompression point - which then recomputed the "slope" to the surface (where the slope would be determined by the deep point chosen and the surfacing tension chosen). These two set points became known as the high and the low gradient factor. This method was based on the knowledge that deeper stops were better as well as allowing a diver to choose lower tissue pressures upon surfacing for a more conservative dive.
Based upon these modifications, it is time for a diver to start thinking about decompression models. It was and is not uncommon in the decompression field for a band-aid approach to be taken. In other words, if a problem is discovered what to do about it? Many will simply try to tweak the current approach such as saying that another half-time compartment is needed, that slower ones are needed, that the surfacing pressures were wrong, etc. In other words, a band-aid would be place on the current model instead of embracing the idea that the model itself is just fundamentally flawed. So this leads to the discussion of alternative models - such as bubble models that acknowledge bubbles are occurring.
VPM and RGBM
In addition to his gradient factor contribution, Erik Baker also started working with Eric Maiken and David Yount on Yount's Varying Permeability Model (VPM) - a model that accounts for bubbles. Another model that surfaced was Weinke's Reduced Gradient Bubble Model (RGBM). The discussion between those advocating for one model over the other can get very heated including the relationship between RGBM and VPM, if any. But this does not matter for the purpose of this article. Instead it is sufficient to present that both are bubble models (unlike Haldanian/Bühlmann models).
Since divers are ultimately responsible for how they perform their own decompression - divers need information on how to pick their own diving parameters. While many divers will start advocating for one model, many have done so without investigating what the models actually mean. Simply knowing a bubble model is desired and hearing the accolades the developer has place on the model is not enough. The simple method is simply to see the profiles produced - which is the same method of looking at known flying after diving requirements compared to what the math of a model calculates.
So what does VPM or RGBM actually calculate. Some decompression profile comparisons as well as time limits are given on the comparison page where it is seen that VPM can give no-stop times of 99 minutes at 50', 322 minutes at 40 feet, 1291 minutes at 35' and unlimited time at 30' - which are far outside the realm of modern diving tables and most divers would never consider doing such dives assuming they had enough gas to so do. So when analyzing a model, if it has an issue at one portion of a dive (i.e. at shallow depths - which also affects shallow stop times), then the validity is the same when extending it deeper. It should also be pointed out that these times are from the VPM-B model (a conservative modification of the VPM model). So the bubble models too were generating profiles that were questionable.
Decompression improvements and evolution come from creative ideas, but one must be cautious before becoming the experiment. The bubble models were touting that they allow a diver to actually decompress less in some instances, but have led to problems. A great article on how decompression software has been placed on the market where divers became the guinea pigs can be found on divernet. In general the article talks about decompression programs and how they started becoming more conservative after divers were bending - and these were the bubble programs. In another article an experienced technical diver describes his serious problems with a program. So simply because an idea may sound good it does not mean that it should be immediately implemented.
After considering the above, a diver can understand the premise upon which Departure was designed. And just as other software should be analyzed, Departure is no exception. To start this process, it should be noted that Departure was the first available software on the market with a bubble model and it was done years prior to others and has been doing so since 1997. Because of this it was the first to generate deep stops and gain its benefits. In fact, its creator has been advocating and lecturing on deep stops since 1987 including before the Undersea Hyperbaric and Medical Society. But Departure's bubble model stopped there and did not extrapolate this out to the idea that decompression could actually be shortened (like other bubble models). Instead, it made biological relevant adjustments to what has been shown to work and where improvements were needed. While Departure has always had deep stops, it has also always kept the diver in the "moderate" decompression depths on the way to the surface (i.e. 40 feet) a little deeper than other models further making use of a bubble dynamics. Other models later made adjustments to their programs that would do the same thing - such as VPM being adjusted to the VPM-B model as well as extending the total stops - all of which Departure's model has always predicted. Departure also allows the diver to decide the surfacing tensions the diver desires as well as how to ascend through the stops. While a diver can make Departure more aggressive similar to other models, it lets the diver decide what to do and does not automatically do it for the diver. This is what sets Departure apart - and hence its name.
While deep stops have been discussed and in general are a part of any bubble type decompression model, not all programs initially start the first stops deeper. An example is if a diver is only slightly into a decompression obligation, then the first stop could only be at 20' or 10'. Departure instead starts the stops significantly deeper even it is just for a minute. The fact that a diver may only be slightly into decompression does not equate to the first stop being shallow since it is just "minimal" decompression. Instead, significant pressure must be kept on the diver (i.e. deeper stops) using bubble concepts until the diver off-gasses enough to surface. Therefore it is not uncommon for Departure to generate more time at a 20' or 30' stop than a 10' stop - which is again based on the bubble concept and unlike other bubble models. Recent research has also shown such an approach reduces bubbles once surfacing even during dives that do not require decompression stops. While Departure has always started decompression stops deeper, Departure has also taken advantage of this newer research and has incorporated "deeper stops" as an option into its program including simply using them for safety stops.
Making a decision
As a diver choosing to enter into mandatory decompression dives, you must also take responsibility for your choices including how to model your decompression obligations. When properly used as a tool combined with your own personal knowledge, software is an excellent way to analyze your profiles. To assist you in this step, Departure has a free basic version that you can download.