In this series, we will take a look at how to create a solid dive plan. First, let’s take a look at what is being taught in the traditional open water courses. When I went through my open water course, I was taught that the most important component of a dive plan is our No Decompression Limit, or NDL. The NDL was the factor that controlled how long we could stay under water. As I began my journey towards better diving, I learned that this was wrong. The controlling factor of how long we can stay under water is determined by how much gas we are carrying, not our NDL. This article will not discuss gas management. That will be discussed in another article. Instead, I want to discuss how NDL can be determined in another way. That is, we don’t need to rely on tables or a dive computer. We can calculate NDL times in our head.

Disclaimer
Before we get into how to calculate NDL in this manner, I must strongly caution everyone that if you do not have a simple understanding of decompression theory and cannot control your ascents properly, you need to continue to rely on your tables or dive computer until you gain more knowledge and skill.

To calculate our NDL in real time, we simply need to know our average depth or if you are not comfortable calculating average depth, you need to know your maximum depth. We call this the 120 rule. This isn’t a hard and fast rule. Decompression isn’t an exact science. Even if you follow your tables or dive computer, a diver can still get bent. So keep this in mind when reading this article. The 120 rule goes like this. If you take your average/max depth and subtract that from 120, the result is your NDL time. Let’s try an example of this. If your average depth is 100 feet, if you subtract 100 from 120, you’re left with 20. That is 20 minutes of NDL time.

Don’t believe me? Here is a picture of PADI’s Recreational Dive Planner (RDP). Click on it to see it in full size. This is the table that people learn when taking a PADI open water class. If you are not familiar with PADI’s table, you follow the depth row on the top to your desired depth. If you follow that column down, the very last number in the black box is the maximum NDL for that depth. If we follow the depth row to 100, the follow that column down to the very last number in the column. Low and behold, it is 20.

Want to give it a try with another depth? Let’s try an 80 foot dive. The 120 rule gives us a 40 minute NDL. The PADI table gives us only 30 minutes. So why is this different? Remember, decompression isn’t an exact science. If you were to compare the PADI table with the US Navy tables, they won’t be the same. The 120 rule is more aggressive on shallower dives, but when you use it for 100 to 110 foot depth, the 120 rule is more conservative. A 110 foot dive on the PADI table gives us a NDL of 16 minutes. The 120 rule gives us only 10 minutes.

What if we dive deeper than 120 feet? Well, the best practice is to avoid diving air deeper than 60 feet. We use Nitrox 32% for dives in the 60-100 foot range. When diving deeper, we like to use a little bit of helium in the mix to eliminate narcosis and carbon dioxide. So we don’t use the 120 rule past 120 feet.

So how does Nitrox throw this off? It really doesn’t. If we use a standard gas of 32% Nitrox, we simply convert our depth to an equivalent air depth (EAD). If you’ve taken a Nitrox class, you should have been taught the formula for this. But we can do this in our head since we are using a standard mix of 32%. Simply subtract 20% of our average/max depth from our average/max depth. So if my average/max depth is 100 feet. 80% of 100 feet is 20, so my EAD is 80 feet (100-20=80). If you look up the AED for 100 feet on your EAD table, you will see that it is 81 feet. Pretty darn close and since decompression isn’t an exact science, we can round off that 1 foot difference.

So we can now calculate how to determine our NDL time in our head. In part 2, we will discuss how to calculate multi-level dives and surface interval credit when using the 120 rule.

Dive Safe,
Duane Johnson
Precision Diving

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