Reg Braithwaite
Contributor
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.)