We’re going to review only some aspects of this system, because this is a merely introduction. So, knowing this, lets get into it.

This system is based on a statistical approach to decompression schedule, this mean that you must used a set of standardized gases for a particular range of depth that were chosen for decompression consistency and easy to mix (keeping in mind the use of Nitrox 32 as top-up-mix).

For example if we applied this system to a NDL dive, there are a set of rules, if is not followed, the decompression generated will not be adjust to your dive, resulting in death or serious injury.

Normally a NDL Table is called Minimum DECO Table

This set of rules are:

Set point for gases Nitrox 32% or TMX 25/25:

• For 30 meters depth the NDL is 30 minutes.
  • For any 3 meters depth less below 30 meters we add +5 minutes of our set point up until you reach 18 meters, which is the second set point that have another rule.
• The 18 meters set point have a 60 min. NDL time, and for every 3 meters less depth you’ll add 20 min.  
• 6 meters stops and 3 meters stops can be combined to accommodate environmental conditions.
• There is always a minimum deco of 3 mins.
• On repetitive dive, if your rest time (surface interval) before the next dive is more 60 mins.  you will do the same ascent procedure to the surfaces; if it is less than 60 mins., you’ll double the shallow stops according the regular ascent procedure.

So a table of NDL must look like this:

12 meters	100 mins.
15 meters	80 mins.
18 meters	60 mins.
21 meters	45 mins.
24 meters	40 mins.
27 meters	35 mins.
30 meters	30 mins.

So the common rules that you will uses are: first you work with an average depth, not your maximum depth. Second, your ascent velocity until you reach your deep stop is 9 meters per minute. Third, your deep stop is located at 50% of your average depth (or if you’re deeper than the average depth at the moment, use the depth at you are currently). Then you’ll make a 1 minute stop here, and to make a proper ascent you’ll add stops from here, each stops consist in 1 min stop every 3 meters from your deep stop until you reach the surface (the ascent is included in the min stop).  The reality is that what you are doing here is a control ascent to the surface.

Remember that no matter what time or depth you did in the table parameters, you’ll always do at least 3 minutes stops for a safer ascent.

As you see in this small introduction, this procedure was made based on statistical analysis of different profiles with the same gases. So this required a set of rules that you must complied and follow. There are simple to use but practice is necessary, overall for the use of average depth, although today you can find computers that show you average depth at the moment.     

If you’re interested in this diving procedure you can visit UTD international, they have an internet course which is awesome.

*Although, by pure theory, there is not required stops and you should be able to ascend directly to the surfaces, hence NDL – the reality is that if you add some stops you’ll act more conservative and safer –

What you can find on the internet about this, an example:

The following information is take it from: http://www.hhssoftware.com/v-planner/decomyths.html#x80percent

80% / 65% ATA stops, pauses and slowed ascent rate.

Some tech divers like to use simple rule based systems to create their deco schedules. These are often called Ratio Deco, or Deco on the fly. They are comprised of a simple formula that can be used in-water but also require other specific settings such as mix and depths.

Initially, one of these simple rules was to create a first stop depth as “first stop = 80% or 65% of ATA depth”. This rule has since been relaxed slightly to “Adjust (slow) ascent rate at 80% of the ATA, to a rate of 30 seconds/10ft or 3m (20ft/6m per min). Then add 15 or 30 second pauses at each level.” Examined closely, the second method will give almost the same extra deep stops as the first.

Does this look right? The first stop distance gets smaller on shallow dives?

The 100 ft dive has the smallest gas load and requires the least amount of decompression. However, the x% first stop / slow and pause rule causes this 100ft dive to commence decompression stops almost at the bottom. The distance permitted from bottom to first stop / pause is not sufficient. The same can be said for the 200ft example, and all depths in between. The problem is the deco schedule commences too early in the ascent: see Adding deep stops

The 80% value is valid at 300ft only. At other depths, the 80% value goes out of scale, and gives an invalid first stop dimension.

The x% rule was originally designed for use with 270 to 300ft dives at WKPP but needs a correction to remain in context with any other depth. The efforts to extrapolate the rule base from the 300 ft dives to all other depths, have failed to address the change in scale required.

A better solution to the “simple rule” first stop:

There is an easier “simple rule” that approximates this first stop depth quite accurately.

1 Stop= Depth -2 ata

The first stop depth is related directly to the pressure change from the maximum dive depth (distance from bottom). The first stop location is primarily to limit and prevent bubbling quantities and sizes from growing large early on in the ascent.

For most dive parameters, the fastest tissue cell gas loads begin to stabilize after 10 or 15 minutes. The dimension of the first stop (distance from bottom) will be very similar across many decompression dives.

This approach uses the same first stop distance for all decompression dives. It corresponds nicely with many planning tools, bubble models and other common deep stop practices in use today.

Note: All these simple “one variable” based rule systems will fail at some point. Decompression stop placement is more complex than a single variable or constant can account for.

Warning: All these methods are approximations, offered for quick check purposes only, and need additional rules (bottom mix controls, depth ranges, deco gases required, etc.) . They are not to be used as substitutions for actual computed plan dimensions. Simple rules like these cannot account for all dive variables.