To understand our [Ratio Deco] Deep Stop strategy, we must understand MAX DECO, and to understand that we start by looking at the dissolved gas theory, or “The Buhlmann Model.”
The Buhlmann Model is a theoretical dissolved gas model in which you have 16 half-time compartments. From the fastest compartment –five minute half times, to the slowest –240 minute half-times. These compartments load and unload inert gases in an exponential manner, or in a “half-life” theory, meaning that they load or unload 50% in the first of the six time segments. Then 50% of the remaining 50%, so essentially an additional 25% in the next time segment, and then 50% of the remaining 25%, so an additional 12.5%, and so on until the tissues in that particular time compartment reach saturation or desaturation in the 6 half=lives. In the Ratio Deco Strategy, we consider the tissues to be saturated or desaturated when they reach 97%, so we use five half-times instead of six.
The compartments are named after their “half-times”, so the fastest compartment is the five-minute compartment, and according to the Buhlmann Model it will take 30 minutes to saturate or desaturate that five minute compartment –25 minutes according to Ratio Deco implementation of five half times. Then the next fastest compartment is the 10-minute compartment, then the 15-minute compartment, and so on.
So for our purposes of creating the Proper Ascent Profile we need to initially determine our “Deep Stop” protocols. In order to do this we will look at the first five tissue compartments – 5-minute, 10 minute, 15-minute, 20-minute, and 30-minute compartments –these are considered as the fast tissue compartments, the ones we start to unload first. In the deep part of the deco these compartments will be used to determine deep stop depths and time to ensure they are allowed to off-gas properly.
For desaturation we will look at the fastest compartment first –the 5-minute compartment, and then the next fastest compartment and so on. For saturation purposes, we will look at the slowest compartment. The slowest of these fast tissue compartments is the 30-minute compartment, which will take 150 minutes to saturate and desaturate according to our Ratio Deco method. As a side note, this 30-minute compartment is what the US Navy dive tables are based on. So if we essentially decompress this compartment at 20’/6m while using oxygen, then this becomes the MAX DECO that we will do. . .
Now these five fast tissue half-time compartments will also have a “Max Stop” depth. This is considered to be the depth at which the compartment will first start to unload or desaturate. In other words, when you are on the bottom, you are saturating the compartments at different rates and they are reaching different levels of saturation. For example, after a 10 minute bottom time, the 5-minute compartment will have gone through two half-lives and will be saturated to 75% of ambient pressure. The 10 minute compartment will have gone through one half-life and therefore it will be at 50% saturation of ambient pressure, and the 15-minute compartment will be less than 50% of ambient pressure.
When you start your ascent, you will not start unloading these compartments immediately, as the ambient pressure will be greater than the dissolved gas pressure in each of the compartments. However, at some point during the ascent the ambient pressure becomes less than the dissolved pressure in a particular compartment, and that compartment starts to unload, or desaturate, or decompress. We call this the “Max Stop” depth of that compartment.
As you continue to ascend, the difference in pressure between the saturated gas in that tissue compartment and the ambient pressure is called the driving gradient. The greater the difference, the greater the gradient, and the more you off-gas.
However, at some point the gradient becomes too great and the inert gas no longer comes out of the tissues as molecules, but it starts to bubble. This point is called the Critical Tension or M-Value in Buhlmann’s Model.
The idea of the Buhlmann Model is that you can ascend all the way until you reach the M-Value, so you would ascend past the “Max Stop” depth, start off-gassing, and continue ascending, driving the gradient steeper, therefore maximizing the off-gas speed until you reached the M-value, and then you would stop, not crossing the M-value depth, and therefore not bubbling. Theoretically this maximizes the difference in pressure between the dissolved gas in the tissue and the ambient pressure, “driving the gradient” and maximizing the off-gas rate without bubbling.
But now we know this is not the case. Bubbles are formed much earlier in the ascent, and this is a shortcoming of Buhlmann's Model (it penalizes for deep stops for which current dual phase models like VPM and RGBM factor them in to reduce the driving gradient, and prevent bubble nuclei/seed growth). What is important is not to get confused between “Max Stop” depth and M-Value: Max Stop is when you first start to off-gas, and M-Value is when you first theoretically start to bubble.
So once again, in designing our Proper Ascent Profile for our Ratio Deco ascent profile, let’s figure out where we want to make our first stop. We already know that stopping at the M-Value line is not good, as we bubble much sooner than the M-Value line predicts, and therefore those bubbles grow in size and frequency requiring us to stop much longer and shallower –it is much harder to decompress from a bigger and more frequent bubbles than from a smaller and less frequent bubble. This harder decompression is what we colloquially call the “Bend and Treat” method.
Looking at the Buhlmann Model, we see the first compartment to saturate and desaturate is the 5-minute compartment. Using a rule of thumb derived from this compartment we can predict our “first stop” depth. That rule of thumb is 75% of the depth for the 5-minute comparment. The 10-minute compartment rule of thumb is 50% of the depth, and the 15-minute compartment is 25% of the depth.
Now that we know our first stop depth is at “Max Stop” depth, we need to figure out the time we need to spend at that first stop depth to maximize the off-gas efficiency. We know the maximum time should be five minutes, as this is the “half-life” of the compartment which maximizes the off-gas of that compartment depth. We also know that if we stayed longer than five minutes, we would still be off-gassing, but not nearly as efficiently as the first half-life time, and we may still be on-gassing some of the slower compartments. If we stayed for 25 minutes at the max stop depth –5 minute compartment X 5 “half-lives = 25 minutes –then theoretically that 5-minute compartment would be 97% equalized to the ambient pressure, however it would not be the most efficient use of that 25 minutes, and we would continue to be saturating the slower compartments at that ambient pressure.
But if we stayed for five minutes at max stop depth for that compartment then continued to ascend, lowering the ambient pressure, creating a new gradient and maximizing “new” 5-minute half-lives for each of the shallower stops, we would create a more efficient off-gassing of that compartment. This would occur at each of the depths shallower than that first stop depth and would maximize efficiency until we reach the 10-minute compartment “Max Stop” depth at 50% of the bottom depth.
So we create a strategy in which we stop at the “max stop” depth, stay for five minutes and then ascend, stopping for five minutes and then ascend, stopping for five minutes at each 10’/3m depth above that until we reached the “max stop” depth of the following compartment –the 10 minute compartment. Theoretically we will have maximized both our 25-minute total deco time frame of off-gas for the 5-minute compartment, and have done the least on-gas of the slower compartments, still without compromising or bubbling. So now we are ready to tackle the 10-minute compartment.
So, how do we determine how much time for each of these stops is needed if you have not reached the max bottom time or max saturation of the slowest of the fast tissue compartments (which is the 30-minute compartment)? Remember, we would need to have a 150 minute bottom time (or more) to reach a 97% saturation, or 5 half-lives of that 30-minute compartment. So we consider a bottom time of 150 minutes or more as full saturation of the five “fast tissue” compartments. In order to decompress them you would not need to do any more than 5 minute stops from 75% of your depth to decompress the fastest of the fast tissue compartments, and then 10 minutes for each stop for the 10-minute compartment from its max stop depth, and so on. This then becomes MAX DECO Strategy.
But what if we don’t do a 150 minute bottom time dive, but something far less? Then we do not need to do five minutes each stop for the 5-minute compartment because it’s not fully saturated. So, we could essentially break that five minutes down into individual minutes—such as 1 or 2; 3 or 4, or 5 minutes. This time includes the ascent to the next 10’/3m stop. So based on this 150-minute bottom time and 5-minute deco per stops, we could simply divide the bottom time of 150 by the deco time of 5 minutes to get a requisite minute value per stop strategy, per amount of Bottom Time.
So, 150 divided-by 5 = 30 minute Bottom Time for each minute of deep stop. In other words, we have now figured out that for every 30 minutes of bottom time over NDL, you will need to conduct at least one minute of deep stop deco, starting at the 75% of max depth or average depth depending if you are deeper or shallower than the average depth.
Referring to the attached table link below (see page 6 – 7), you can see that if bottom time is up to 30 minutes more than NDL, then do one minute per stop. If BT is between 30 minutes and 60 minutes over NDL, then do two minutes per stop, and if BT is more than 60 and less than 90 minutes, then do three minutes per stop. Remember that this time is for each stop, starting at the 5-minute compartment “Max stop” depth and then for each 10’/3m above until you reach the 10-minute compartment “max stop” depth. . .
Now to get stop time for each depth above the 10-minute compartment “max stop” depth or 50% of your max depth or average depth, you would take the max BT of 150 minutes and divide it by 10 minutes (10 minute half-times), 150/10 = 15 minutes, or each 15 minutes of additional bottom time would require one additional minute of stop time at each of the 10’/3m deco stops above the max stop depth for the 10-minute compartment. Basically you would do one minute per 15-minute bottom time.
Keep in mind that this is for all bottom times even if you are within NDL. So that means to create a proper ascent profile when recreational diving and doing bottom times that keep you within NDL, you simply do one minute stops (including the ascent time) from 50% of your recreational dive depth. So for example, if depth is 80’/24m and your bottom time = 25 minutes, you are within NDL. You would do one minute for each stop depth (10’/3m) starting at 50% of your depth until you reached the surface.
So a proper NDL ascent profile would be to leave 80’/34m and ascend at a rate of 33’/10m per minute until you reach 50% of the depth. Then at 40’/12m, it is one minute. Then ascend to 30’/9m (counting the travel time as part of the 30’/9m stop) and do one minute, then up to 20’/6m and another minute, then up to 10’/3m to do your final minute before surfacing. This is a Proper NDL Ascent Profile and is always conducted regardless of how much time you spent on the bottom as long as it is LESS THAN NDL.
However, if your bottom time was more than NDL, for example 25 minutes over NDL, and you’re doing a decompression dive, your stop times would be one minute for each 10’/3m starting at 75% of your bottom depth and then three minutes per 10’/3m stop starting at 50% of your depth.
As a side note, if your bottom time was less than 15 minutes over NDL, then you would do nothing for 75% stop depth and start 1-minute stops at 50% of depth. There is no need for the 75% stop depth and start 1-minute stops at 50% of depth. There is no need for the 75% stops in this case, as the tissue loading is so minimal.
Take a look at the rule of thumb tables (WKPP applied deep stop theory) on p. 6-7 of the attached file below:
