"averaging" table NDLs and multi-level diving

[Reg Braithwaite]

Minimum Deco

The concept of N.D.L. derives from the idea that one does not “require” deco and therefore it is a No Deco Limit dive and the diver can return directly to the surface without any decompression stops. This concept makes some sense in the context of a dissolved gas theory. If the Buhlmann curve (maximum M-Value) is not reached before getting to the surface, then the diver is not required to make a deco stop at 10’/3m or deeper and is allowed to surface directly from depth. This type of model does not take into consideration the fact that a diver always experiences some bubbles in their system and that these need to be addressed, even after a short bottom time (for which the Buhlmann model would allow a direct ascent to the surface). The diver should make some decompression stops to address these bubbles and/or micro bubbles. These stops are termed Minimum Deco and should be conducted starting at 50% of max depth of the dive. The diver will start their ascent at the normal rate of 30’/10m per min until they reach this 50% mark, where they will slow their ascent rate to 10’/3m per min. A good way to practice this is to make a stop for 30 seconds at the stop depth and then spend 30 seconds ascending to the next stop. The diver should continue this slowed ascent rate (10’/3m per min) until they finish the 20’/6m stop. From 20’/6m you do a gradual 3min ascent to the surface to release the pressure slowly where it is changing the most. We use this concept both when conducting what is thought of as an traditional N.D.L. dive (See N.D.L. table below) and if we do a bailout (less than 5min bottom time) when we are deeper than 130’/39m

http://www.txfreak.de/ratiodeco.pdf

Ratio Deco is a particular technique for determining decompression on a dive using ratios that are easily computed, so much so that it is possible for a well-trained team to use the strategies to re-computer their dive plan "on the fly" to account for contingencies or changing circumstances.

The PDF I quoted above is rather old, and while it explains how to do ratio deco as practiced at that time, I didn't find it had a really great explanation of why ratio deco is supposed to work. I have since purchased a Ratio Deco online classroom which is really an online narrated power point. I found that much more informative.

(The PDF and the powerpoint are information. Absorbing and being able to regurgitate information is not the same thing as being qualified to conduct dives according to their principles.)

 

[lamont]

NDLs are determined by a specific M-value for a specific tissue compartment, and when using any table, a different tissue compartment and its M-value determine the NDL for a 30-meter dive than those which determine the NDL for a 15-meter dive, so "averaging" those NDL times (in whatever proportional way he does it -- he didn't elaborate) is like averaging apples and oranges. So in principle, it's wrong. Correct?

In physics in college there are problems where you often approximate an ideal pendulum ( F = - m * g * sin (theta) ) with a linear approximation. For small radian angles of theta, then sin (theta) = theta ( try graphing y= sin (x) and y = x between x = -pi to x = pi on a graphing calculator and look near x = 0 where y = x is a tangent to the y = sin (x) approximation ). This gives you the formula F = - m * g * x which is considerably easier to treat mathematically. That approximation is "wrong" in the sense that it doesn't work far from x = 0, but if the problem that you are solving is all around x = 0, you'll get results which are probably within the error of your measurements.

Depth averaging is similarly an approximation. It works best reasonably far from equilibrium where you are exponentially loading gas compartments fairly "rapidly" compared to the half-time of the compartment. A profile that it works on is one where you go descend to 110 fsw and follow a profile for 30 minutes that ends at 90 fsw and has an average depth of 100 fsw. For faster compartments (which tend to be controlling for that kind of dive) it works less well to do the reverse dive and go from 90 to 110 since fast compartments will be driven to more saturation by the deeper depth at the end. On a technical dive to an average depth of 100 fsw for 60 minutes over a range of 80-120 fsw that ends at 120 fsw, the faster compartments control the deeper stops and so 120 fsw will be a better depth for determining deep stop depth since that was the most recent forcing to those fast compartments, but time spent on O2 is going to be controlled by much slower compartments where depth averaging works as an approximation better.

Similarly, you can't depth average a multi-level dive that goes from an average depth of 100 fsw to a second phase at 60 fsw. You can still safely do that without a computer based on experience or other numerical approximation techniques (taking the dive time at 100 fsw, doubling it and subtracting from the NDL at 60 fsw has worked for me for the dives i do -- not a linear approximation since it assumes that the 3.00 atmospheres of driving gradient is worth twice a driving gradient of 2.00 atmospheres -- although these days i just know what works on those kinds of profiles).

The application of these techniques to recreational diving is fraught with a bit of peril because you need to understand the background of decompression, and it should be presented with a course where it explains that the techniques are approximation techniques around a range of typical diving profiles and that the approximations should be validated against dive planners like v-planner or other dive planners, and against experience. Recreational divers having little experience and little understanding in decompression science are likely to abuse the results -- either getting hurt by actually doing profiles that are outside of the ranges where the approximation works, or by running profiles on v-planner for those kinds of dives and concluding the approximation techniques don't work at all.

 

[Makhno]

Many thanks, lamont, for such a responsive and informative reply!