All set for a two hour stage dive in Dan’s Cave, with 15 minutes of planned deco.

First and foremost let me state that you must receive formal training from one of the many dive organisations before attempting a dive with decompression. Without formal training you risk injuring yourself. Please get training before attempting anything you read in this post. I will not be held responsible for you attempting to use the methods contained below. The following post is for illustration purposes only.

To use the US Navy Air Tables you should be familiar with how to use tables (at least PADI’s RDP),  know how to calculate your Equivalent Air Depth if you are diving on a gas other than air, and how to calculate your CNS and OTU exposure. So if you are able to use the above methods then we can crack on with an example. As these involve a lot of calculations I won’t repeat them here, but I will give you all the answers.

Example

Plan a dive to 35 metres (115ft) for 50 minutes using 28% Nitrox. Then following a surface interval of 90 minutes plan a dive to 23 metres (75ft) for 50 minutes using 32% Nitrox. Calculate the decompression obligation (if any) for each dive, the % CNS and OTU Exposure after each dive, and the ending %CNS and OTU Exposure.

As we are using the US Navy Air Tables we need to convert our depths to equivalent air depths (EAD) due to the fact that we are diving Nitrox. If we weren’t diving Nitrox we wouldn’t need to perform this step.

Dive 1

We find that we have an EAD of 31 metres, looking at Table 9-6 we see that we either have 30.4m (100ft) or 33.1m (110 ft). As our EAD falls between these depths we choose the deepest one. The NDL for a 33.1m (110 ft) dive is 20 minutes. So clearly by spending 50 minutes there we have gone into deco. Looking at Table 9-8 for a depth of 33.1m (110 ft) it tells us that we have two deco stops that we have to make. Now we need to calculate the partial pressure of the gas we are using at 3 different depths. First for the depth of the dive, the second is at our 1st deco stop, and the third is at the 2nd deco stop. Once we have these partial pressures we can work out the %CNS and the OTU exposures. Then using Table 9-7 we can work out the Repetitive Dive Group for a 90 minute surface interval, which turns out to be I. Notice that the %CNS had halved after the 90 minute surface interval.

The table below shows all the solutions to the calculations.

 Dive 2

This follows a similar method as the previous dive however we need to include the Residual Nitrogen Time (RNT) in our calculation. So we now use Table 9-7 to calculate the RNT. We finished our surface interval with a Repetitive Dive Group of I and we are doing a dive to an EAD of 18 m (60ft) that gives us a RNT of 61 minutes. This must be added on to our dive time, giving us a total bottom time of 111 minutes. Looking at Table 9-8 for 18m (60ft) we see that 111 minutes isn’t there, so we use the next time, which is 120 minutes.  This gives us a deco obligation of 26 minutes at 3m (10ft).

So here we have completed the calculations for a series of two dives. Both dives had deco obligations, PADI’s RDP will not allow you to calculate any decompression obligations so this is why we used the US Navy Air Tables. Tables do exist from IANTD, TDI and other technical diving organisations and you are welcome to use them. Also decompression tables exist for different diving with different blends of nitrox. It is also possible to reduce your deco obligation by using higher percentages of O₂
but it must be noted that you really have to pay attention to your %CNS and OTU scores because you can quickly reach your limits if you are not careful.

Why should I plan my deco, can’t my dive computer take care of it all?

Yes your dive computer will take care of it, but how much deco will you have to do? This is the most important question as it leads to further questions such as: will you have enough air, and is your exposure protection sufficient? If your computer fails then you might have to just use a timing device and follow the plan that you had created. Obviously these calculations are worst case scenarios, we don’t spend 50 minutes at the bottom as we spend time descending and ascending. We might dive a multilevel profile. This is where the computer beats the tables as it will calculate on the fly your deco obligation and it will probably end up less than you have calculated…which can only be a good thing as you get out of the water earlier.