Departure's Decompression Software  



Departure is the feature program in the software collection and calculates decompression schedules to 1000 feet (305 meters). Features and Benefits
Departure was designed to give divers flexibility, options and control when planning dives. Departure's name comes from its innovative decompression modeling. It's decompression models depart from conventional halftime modeling and generates a more biologically relevant decompression profile. Departure was the first commercially available software that contained a bubble algorithm and calculated deep stops. It has been known for some time that deeper stops may be beneficial to a diver by keeping more pressure on the diver. By keeping more pressure on a diver during a deeper stop, bubbling may be minimized and gases may remain in solution. This promotes more efficient nitrogen/inert gas elimination by maintaining a driving force pressure gradient as compared to the slower process of bubble absorption. This may also help with counterdiffusion problems. Several methods have been used to calculate deeper stops from ruleofthumb guidelines to BandAid approach models. The term BandAid approach is used as some ideas and models try to "fix" models that have already been shown to have problems by merely modifying the models. One such method is a reduced gradient approach. A reduced gradient approach starts the stops deeper, but can finish the stops at the same final shallow depth as without the deep stop modification. In other words, there is no additional pressure on a diver during the final long stop. While this has been a simplistic explanation, it makes the point. Departure uses a different approach. Instead of
working towards a standard final shallow stop by way of some added deeper stops, Departure
uses an algorithm that actually calculates all the stops. While the
decompression profile calculated depends upon the decompression model chosen (which is
discussed below), it is possible that the depth of the final stop will be at 20 feet (6
meters) or even deeper. It is also possible that a decompression profile will
calculate more time for a 20 foot stop than a 15 or 10 foot stop (which is not the type of
profile divers are accustomed to). In this example, the profile shows that it
is more efficient to decompress at 20 feet than at a shallower depth. As can be
seen, the algorithm that calculates Departure's deeper stops is unique. This
algorithm is the foundation of Departure's Minimized Bubble decompression model.
To understand this program, it is helpful to understand how Departure was designed and why it is unique to the market. Some of our competitors have simply taken other people's numbers such as Buhlmann and put them into a software package with a lot of bells and whistles. We're the opposite in that we are not programmers, but divers with background in science such as biology and physics. This has allowed us to update models currently on the market. The biggest question we have been asked is which model we use and the most popular question is whether we use Buhlmann. The answer to this question is both a yes and no. Buhlmann used a halftime model, as does Departure. Therefore we are implementing the same model. Buhlmann also used a nonratio method of extraction to altitude. We have both a ratio extrapolation for those that want it and a nonratio extrapolation. Therefore we also have this feature of Buhlmann, but our extrapolation method is completely different and results in more conservative altitude times than the Buhlmann model. The diver also has a choice of time limits and can pick limits extremely close to Buhlmann's. When this is done, the diver has essentially chosen a Buhlmann model, except that Departure uses more halftime compartments than Buhlmann over a greater range. Departure also uses the same method Buhlmann employed for picking the halftime compartments of other gases (using the square roots of molecular weights  which was hypothesized prior to Buhlmann's usage). Where we differ is that Buhlmann did not calculate deep stops as his model did not take that into account, but we do as we have developed our own algorithm for this purpose. Our time limits are also more conservative than Buhlmann's at the "shallow" depths as we have developed our own time limit model (which is a combination of three different decompression models). Our time limit model calculates "shallow" depth time limits and their deviation to an infinite time as the minimum bends depth is approached (more on this later). Our time limit model does not allow or predict as much as a deviation as Buhlmann or others and therefore our time limits are more conservative at the shallow depths. This also translates into more conservative "M values" or nitrogen tissue tension for the "slower" compartments. In addition to Haldanian/Buhlmann type models, we have our proprietary Minimized Bubble model. This model was actually influenced by Brian Hill's thermodynamic model. Dr. Hill's work is what I started basing my models on over 12 years ago when I started generating my own models and tables. More on the decompression models later. The last comment I wish to make on the models at this time is that they may seem more conservative than you are used to. This is because we have attempted to take a conservative approach and because all our models are based upon halftime models. By using halftime models we stay within accepted parameters of decompression modeling and create more conservative profiles at the same time. As previously mentioned, Departure uses a halftime model that is laid over with other theories. A halftime model was chosen because of its wide acceptance and usage in the diving community. The halftime model is then laid over with other theories (such as the Minimized Bubble Model) to make it more biologically relevant. Twentythree halftime compartments are used ranging from a 2 minute compartment to a 720 minute compartment for nitrogen. The models also take into consideration inherent unsaturation due to oxygen metabolism (i.e. an oxygen window). Departure allows the diver to choose
different ascent options with a unique and proprietary decompression model and
not the same model with different outgassing elimination rates.
It is designed to calculate deeper stops and keep the diver under more water
pressure. The idea behind this is to attempt to minimize bubbles during
decompression stops and thus increase the driving force for nitrogen/inert gas
elimination during a stop. On the conservative end of the ascent
options, the
model is similar to a zero supersaturation
model. The concept behind Departure was to develop a program that allows divers to choose how they want the decompression profile calculated such as choosing their time limits and the decompression model used. The first step in developing this program was to pick its time limits. Departure starts showing its uniqueness at this first step. While time limits are fairly consistent in the range of 50 feet and deeper, the time limits for the "shallow" depths are not. The reason for this inconsistency is the way in which these time limits are modeled or not modeled. Nostop time limits follow a pattern. If these time limits are graphed on loglog coordinate paper, the time limits appear to be a straight line. However, the time limits deviate off this straight line at the shallow depths. The reason for this deviation is the time limits start approaching infinity as the depths get shallower and approaches the "minimum bends depth." The minimum bends depth is the depth at which there is no time limit  the depth at which a diver may "saturate" and immediately surface. Modeling this deviation and time limits is where Departure starts departing from how time limits in the past have been calculated. Many time limits at the shallow depths have been calculated with educated guesses to take into account the shallow depth deviations. Instead of using educated guesses, Departure uses a proprietary model to calculate time limits. Combining three different decompression models into one developed this model. This new model actually calculates and models time limits at all depths including the shallow depths and their deviation. This model calculates time limits when the following is plugged in: 1) the desired minimum bends depth, 2) the desired model slope on loglog coordinates, and 3) a desired time limit at a known depth. By incorporating this model into Departure, flexibility is given to divers by allowing divers to pick their own time limits. While Departure allows divers to choose their time limits, this model results in time limits that maybe more conservative at the shallow depths than divers are used to. This is because this model does not show as much of a deviation at shallow depths as others allow. In fact, comparison of time limits at shallow depths is a good way to start evaluating tables, programs and models. While a brief safety stop can add a margin of conservatism after a deeper dive, a brief safety stop after a long shallow dive may not yield the same level of conservatism. Therefore, when safety stops are performed, the time limits at the shallow depths may be more critical than the time limits at the deeper depths (within reason). This also shows the importance of not deviating in too radical a manner at the shallow depths as too much of a deviation could result in a decompression profile that is not long enough. Many divers look at only the time limits at the deeper depths when evaluating tables and will notice a difference of 1 to 2 minutes at 130 feet, but do not notice a difference of 60 minutes or greater at 35 feet – which may actually be the more critical measurement. The reason the time limits used are so important is they are a product of the decompression model used and show the conservatism level of the model itself. So don’t forget to critically evaluate the shallow time limits for two reasons 1) because a safety stop may not be as beneficial after a long shallow dive as compared to a deep short dive, and 2) because time limits at the shallow depths may be calculated by educated guesses. Departure’s time limits are flexible and allow divers to choose 100 time settings from a sliding scale. Examples of these time limits are found at the end of this article along with some time limits from other sources. Now that Departure’s time limits have been designed, the next step in Departure’s development was the designing of its decompression models. Departure was developed around its unique "Minimized Bubble" model. It has been known for some time that deeper stops may be beneficial to divers. By keeping more pressure on divers during a deeper stop, bubbling may be minimized and gases may remain in solution. This promotes more efficient gas elimination by maintaining a driving force pressure gradient as compared to the slower process of algorithm that bubble absorption. This may also help with counter diffusion problems. The Minimized Bubble model uses an algorithm that calculates deeper stops. Departure also has two other models. One is a standard halftime model except that the first stop is calculated according to the Minimized Bubble model. The last model is entitled the "In Between Compromise" model. This model is just what it is called. It was developed for divers that may not want to use the Minimized Bubble model, but who also may not want to use a pure halftime model. For those divers at altitude, Departure allows divers to choose between different altitude decompression and offgassing models. Keeping with the theme of Departure, altitude divers can choose between standard models as well as unique proprietary models designed specifically for Departure. In addition to using these different decompression models, Departure also allows divers to choose how they want to model the outgassing during decompression stops. Divers can choose an exponential loss model or a modified linear model. Additionally, divers can choose a percentage to slow down the gas elimination such as by 50%. Departure’s decompression models are based upon the widely recognized halftime modeling. By using a halftime model as a foundation and overlaying them with other models/factors, the result is a unique model that stays within the accepted parameters of halftime modeling. Departure was not just designed to be different; it was also designed to be user friendly. While, Departure may not have all the bells and whistles of some other programs, it does have a straightforward display that provides simple important information at divers fingertips. One way this is accomplished is by always displaying the decompression profile so a diver can experiment with changing parameters and then immediately see the results. Departure has many other unique features, but this article was designed to give only a brief background on its development. We invite you to experience Departure for yourself and experience it on your own. But until then, here are some sample time limits from different sources. 

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