The 5 main trends in decompression

Decompression theory has a reputation of being complicated and hard to learn.

In my experience, it doesn’t have to be!

We can reduce the complexity if we just keep in mind these 5 main trends of decompression theory.

*Note: this article isn’t meant to be a thorough understanding of decompression theory. The main goal is to simplify decompression theory in layman’s terms.


Trend 1 – The deeper you go, the more nitrogen you take up because of the higher pressure of the gas you breathe.

Whenever you scuba dive, you are breathing gas at ambient pressure.

This means that you are breathing more molecules of nitrogen for every breath that you take when deeper than sea level.

The deeper you go, the more molecules of gas you take in, even though the volume of gas in each breath doesn’t change.

This extra nitrogen is absorbed into your body and eventually, will need to be released when you ascend to the surface.


Trend 2 – The longer you stay, the more nitrogen will get absorbed into your tissues.

If you spend longer in the water at depth, then the more breaths you will take and more nitrogen will be absorbed into the body.


Trend 3 – If the gas you use has a lower nitrogen content, then there will be less nitrogen available to be absorbed into your tissues.

Let’s compare two gases, one with a lower nitrogen fraction than the other.

Which gas will cause more nitrogen absorption into the body given that all else is the same (number of breaths taken, depth and duration of dive)?

I think the answer would be fairly obvious.

This basic idea is the reason why Enriched Air Nitrox (EAN) gas mixes exist.

A commonly available EAN mix is 32% oxygen and 68% nitrogen.

This provides an 11% reduction in nitrogen content as compared to normal air.

Choosing an Enriched Air Nitrox mix is always a good idea for reducing your decompression obligation.


Trend 4 – The slower you ascend, the more excess nitrogen will leave your body.

A less commonly known trend would be how fast you surface from each dive.

By ascending slower, you’re allowing time for the nitrogen in your tissues to leave your body.

A faster ascent rate also causes any gas bubbles in your body to expand.

It will also cause the release of excess nitrogen as bubbles instead of being transferred back to the lungs for exhalation.

By slowing down our ascent rate, we can control the size of these bubbles and at the same time allow nitrogen to work its way out of our tissues.


Trend 5 – The longer the surface interval, the less nitrogen you will have before your next dive.

Once on the surface immediately after a dive, there will always be some excess nitrogen in your tissues. This is the concept of residual nitrogen.

The more residual nitrogen you have in your system, the more conservative you should be on your subsequent dive.

Thus, the longer you stay on the surface before your next dive, the more time the excess nitrogen has to leave your body.

The less nitrogen in your system, the better it is for your decompression obligation on your next dive.


Putting it all together

So what does this mean for divers?

How can we navigate the complexities of decompression while staying safe?


The 5 main trends in decompression

The 5 main trends in decompression


General rules

Once you understand these trends, then how do we put everything together?

I consider these 5 trends as adjustable variables in my overall decompression obligation.

I can tweak or adjust one or more while leaving the others in place, allowing me to ‘adjust’ my decompression obligation based on the dives I’m planning to do.


Planning a day of diving

Keep the number of dives per day to 3 (roughly 9 am, 1 pm, 4 pm, with a break for lunch).

You can add on a dawn dive and a night dive as these would extend the beginning and end of your diving day.

Let’s look at how this relates to a decompression table.

Let’s look at a PADI RDP table and trace a line that follows a surface interval of 1 hour and 2 hours.

The PADI RDP table showing a 1 hour and 2 hour surface interval line

The PADI RDP table showing a 1 hour and 2 hour surface interval line

We can notice that we end up with a pressure group of J and C respectively if we have a starting pressure group of Z.

With a more realistic pressure group of between G and M, you would end up with a pressure group between A to E after a 1 or 2-hour surface interval respectively.

With a 2-hour surface interval between each dive, 3-day dives are all that you have time for anyway.


What about if you happen to miss a safety stop?

If you do happen to miss a safety stop, then change up one of the other parameters on your next dive.

You could do a longer surface interval, do a slower ascent, stay shallower, or dive for a shorter time.

You can still change the depth and length of your dive if you are on a fixed dive schedule.


Always ascend slowly

For ascents, I would always take my time to ascend, limited by the gas that you have remaining.

However, I only do this after I’m shallower than 6m. Any deeper than that and you would still be taking on nitrogen into your system.

If there’s a nice shallow reef with things to look at while you are at your safety stop, and you also have plenty of gas left, then why not stay and enjoy the view?

Personally, I would ascend slowly (9 m per min) to half of my depth, then move at 3 m per min up to 6m.

Once at 6m, provided the team has gas, I’d take my time and enjoy the sights until everyone’s computers have cleared or we get bored!

We’ve been known to hang around at 6 m for 15 to 20 mins!


And always dive Nitrox!

As for gas choice, if you ask me, there’s no reason to dive air if there’s nitrox available.

If I can reduce my decompression obligation just by changing the gas I dive with, it’s a no-brainer.

There’s so much benefit in diving nitrox, and these days it doesn’t cost much either.


There you have it!

Some simple and easy to remember rules for planning a day of diving while keeping yourself safe.

Now, get out there and know your decompression obligation!

Leon Boey is a GUE, HSA and PADI instructor based in Singapore and Bali. He runs the dive education centre Livingseas in Singapore, Bali and Jakarta. Diving since 2005, he first fell in love with wrecks. He enjoys all sorts of diving, and loves being surrounded by fish.

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