Duane Johnson - Technical Diving Instructor

The biggest selling point dive stores make for dive computers is that they calculate your NDL every 1 second. When doing multi-level dives, this is a benefit over tables as dive tables only have one depth. Unless you are using the PADI wheel dive planner. However, we can use average depth and compare that with the results of the 120 rule to determine our NDL for a multi-level dive.

Disclaimer
Calculating average depth is not that difficult of a task and can be done rather quickly in your head. However, it does take some discipline. You MUST check your depth gauge at regular intervals. You MUST be able to remember your previous depth and NDL calculations. If you do not have this simple discipline, the you should stick with your dive computers or tables.

First off, when we look at how averages are calculated, there is some form of interval of measurement. When we talk about average depth in scuba diving. That interval is time. We must check our depth gauge at regular intervals to calculate the average depth of all those intervals. I regularly check my gauges every 5 minutes. I do this for every type of dive, from a shallow 30 foot dive to a 200 foot dive on trimix. For deeper dives, I will check my gauges more frequently. But 5 minutes is the longest I will let it go. So every 5 minutes, I’m checking my depth, bottom time, and tank pressure. We’ll talk about gas supply in a later article.

So let’s walk through a simple example so you can see how it works. I’m on a dive to 100 feet on air (yeah, yeah, I know. Humor me!). After the first 5 minutes I check my gauges, I’m at 100 feet. So my average depth from the surface to 100 feet is 50 feet. Now we are taking into consideration that we did a descent of about 50 feet per minute. If the descent is faster, then we can pad some more depth to the average depth to account for being at depth a little longer. So, after the first 5 minutes, my average depth is 50 feet. In 5 more minutes, 10 minutes into the dive, I check my gauges and I’m still at 100 feet. My average depth is now the average between 50 feet and 100. This results in an average depth of 75 feet after the first 10 minutes.

Let’s stop at this point and plug average depth into the 120 rule. After the first 10 minutes of the dive, I have an average depth of 75 feet. Using the 120 rule, I know I have a total of 45 minutes of NDL time. However, I’ve used up 10 minutes so far. Thus, I have 35 minutes remaining on my NDL.

By now I hope the light bulb has gone on above your head. The question that should be on the tip of your tongue is “But if you stay at 100 feet for more time, won’t your average depth increase?” Ding, ding, ding!!! Absolutely. So let’s continue our example and plug average depth into the 120 rule at each interval. At 15 minutes into the dive, my depth is 90 feet. So the average depth is the average between 75 and 90. This is going to be 82. But lets round up, for conservatism, to make the numbers more manageable. Let’s call our average depth 85 feet. Plug that into the 120 rule, we get a total of 35 minutes total NDL time, subtract our our 15 minutes we’ve been on the dive already, we have 20 minutes of NDL time remaining.

Now let’s ascend to our next level in our multi-level dive. At 15 minutes into the dive, we ascend to 60 feet to check out part of the reef there. Assuming we ascend at a rate of 30 feet per minute. This may be slow, but let’s do it for the sake of argument. To go from 90 feet to 60 feet, our travel time is 1 minute. Then we will be at 60 feet for 4 minutes before checking our gauges again. At 20 minutes into the dive, we calculate our average depth between 90 feet and 60 feet. This gives us 70 feet average depth and 30 minutes of NDL time remaining (120-70=50-20 minutes of the dive already=30 NDL time remaining). So by moving up to the next level of our multi-level dive, we just gained 10 more minutes of NDL time.

As you can see, the more time we spend at the deepest part of the dive, our running average starts to get deeper and deeper. Then when we move shallower, our total average depth is weighted more towards our deeper section of the dive because we spent more time there.

At this time in our example, we are most likely coming close to hitting our gas reserves and need to ascend. Hopefully, you have seen how we can calculate average depth and apply it to the 120 rule. We can do this for square and multi-level profiles. Where average depth gets tricky is in saw tooth type profiles. However, those profiles are mostly found in cave environments and we’ll save those calculations for your cave class.

In the next article, we’ll discuss ascent techniques.

Dive Safe,
Duane Johnson
Precision Diving