Enriched Air / Nitrox for Diving in Mahahual—and Why Divers Should Use It?

Q: What is Enriched Air Nitrox?

Normal air contains about 21% oxygen and 79% nitrogen. Enriched Air Nitrox contains more oxygen and less nitrogen. Nitrox 32 is the most common recreational mix, although 28% and 36% are also regularly used. Divers normally shorten the name to Nitrox, so we will do the same. Breathing less nitrogen can give you more time before reaching your no-decompression limit. It can also help you stay farther from that limit if you choose not to use the extra time.

Q: What is a no-decompression limit?

Your no-decompression limit, or NDL, is the estimated time you can remain at your current depth and still ascend without making mandatory decompression stops. The deeper you go, the faster your body absorbs nitrogen and the shorter your NDL becomes. Your computer tracks your depth and time, then uses a mathematical formula to estimate the nitrogen in your body. It cannot measure it directly. Different computers use different formulas and settings, so two divers on the same dive may see different NDLs. Our bodies are different too, and factors such as cold, effort, health and previous dives can affect decompression stress. An NDL is therefore not a perfect line between “safe” and “unsafe.” Staying within it lowers the risk of decompression sickness, but it cannot remove that risk completely.

Q: What is maximum operating depth?

The maximum operating depth, or MOD, is the deepest depth at which a particular gas mix can be used without exceeding the planned oxygen limit. Every breathing gas has an MOD. Using the usual recreational diving oxygen limit, these examples show the MOD of each mix and the approximate NDL at that depth: - Nitrox 28: MOD 40 metres — approximately 12 minutes NDL, compared with 9 minutes on air - Nitrox 32: MOD approximately 34 metres — approximately 20 minutes NDL, compared with 14 minutes on air - Nitrox 36: MOD approximately 29 metres — approximately 37 minutes NDL, compared with 20 minutes on air MOD is a gas limit—not a target depth. The maximum depth for any dive is the shallowest limit set by your training, gas mixture, available gas, equipment, conditions, NDL or decompression plan, and the other risks of that particular dive. A gas may be suitable for a certain depth, but that does not mean the diver is trained, equipped or prepared to go there.

Sep 1

4 min read

Updated: Sep 6

Cartoon scuba diver in Mahahual checking a half-full cylinder while his dive computer shows two minutes of no-decompression time remaining

You have plenty of gas left, but your computer says it is time to leave the best part of the dive.

Why?

You are getting close to your no-decompression limit. This is one of the main reasons divers use Enriched Air Nitrox. For divers exploring Mahahual's reefs, Nitrox can provide longer no-decompression limits on depths from 20m to recreational limits and repetitive dives.

What is Enriched Air Nitrox?

Normal air contains about 21% oxygen and 79% nitrogen.

Enriched Air Nitrox contains more oxygen and less nitrogen. Nitrox 32 is the most common recreational mix, although 28% and 36% are also regularly used.

Divers normally shorten the name to Nitrox, so we will do the same.

Breathing less nitrogen can give you more time before reaching your no-decompression limit. It can also help you stay farther from that limit if you choose not to use the extra time.

What is a no-decompression limit?

Your no-decompression limit, or NDL, is the estimated time you can remain at your current depth and still ascend without making mandatory decompression stops.

The deeper you go, the faster your body absorbs nitrogen and the shorter your NDL becomes.

Your computer tracks your depth and time, then uses a mathematical formula to estimate the nitrogen in your body. It cannot measure it directly.

Different computers use different formulas and settings, so two divers on the same dive may see different NDLs. Our bodies are different too, and factors such as cold, effort, health and previous dives can affect decompression stress.

An NDL is therefore not a perfect line between “safe” and “unsafe.” Staying within it lowers the risk of decompression sickness, but it cannot remove that risk completely.

Two scuba divers at 24 metres with different dive computers showing 17 and 16 minutes of NDL, illustrating that no-decompression limits are calculated estimates

When is Nitrox most useful?

On a shallow dive, your gas supply will often limit the dive before your NDL does. In that situation, Nitrox may not make a noticeable difference.

It becomes more useful at moderate and deeper recreational depths, where your NDL is more likely to limit your time. Depending on the mixture, Nitrox can be useful from around 18 metres down to 40 metres.

Nitrox 32 and 36 work well for many dives between approximately 18 and 30 metres. Nitrox 28 can reduce nitrogen exposure on deeper recreational dives approaching 40 metres.

Many of Mahahual’s reef dives fall within this range. Nitrox can be particularly useful when making several dives in one day or diving over several days.

However, Nitrox does not allow you to dive deeper. More oxygen means less nitrogen, but it also gives the gas a shallower maximum operating depth.

What is maximum operating depth?

The maximum operating depth, or MOD, is the deepest depth at which a particular gas mix can be used without exceeding the planned oxygen limit.

Every breathing gas has an MOD. Using the usual recreational diving oxygen limit, these examples show the MOD of each mix and the approximate NDL at that depth:

Nitrox 28: MOD 40 metres — approximately 12 minutes NDL, compared with 9 minutes on air
Nitrox 32: MOD approximately 34 metres — approximately 20 minutes NDL, compared with 14 minutes on air
Nitrox 36: MOD approximately 29 metres — approximately 37 minutes NDL, compared with 20 minutes on air

MOD is a gas limit—not a target depth. The maximum depth for any dive is the shallowest limit set by your training, gas mixture, available gas, equipment, conditions, NDL or decompression plan, and the other risks of that particular dive. A gas may be suitable for a certain depth, but that does not mean the diver is trained, equipped or prepared to go there.

Nitrox MOD and NDL comparison for demonstration only: Nitrox 28 at 40m—12 minutes versus 9 on air; Nitrox 32 at 34m—20 versus 14; Nitrox 36 at 29m—37 versus 20.

These are approximate first-dive examples based on equivalent air depth and the PADI RDP model. They assume the whole dive is made at the stated depth with no previous dives affecting the available time.

They are for demonstration purposes only and must not be used to plan a dive. A dive computer may show different limits because it tracks changes in depth and uses its own algorithm and settings.

The key idea is simple: more oxygen can provide a longer NDL, but it also creates a shallower MOD.

This is why Nitrox divers analyse their cylinder, confirm the oxygen percentage and correctly set their computer before every dive.

Does Nitrox make diving safer?

Nitrox can provide an extra safety buffer, but only if you choose not to use all the additional time it gives you. Following the same dive plan as you would use on air means less nitrogen exposure and a more conservative profile.

If you use every extra minute, you are mainly using Nitrox to extend your dive time.

Nitrox cannot prevent decompression sickness. It is simply another tool for managing nitrogen exposure.

Learn to Use Nitrox This September

September Super Special for the PADI Enriched Air Nitrox Course with Andy in Mahahual, including eLearning, practical training and certification

The PADI Enriched Air / Nitrox Diver course teaches you how to analyse a cylinder, understand oxygen exposure, calculate MOD and correctly set your dive computer.

No training dives are required. The course combines PADI eLearning with a practical session.

As a September Super Special, you can complete the course with Andy in Mahahual for MX$2,600 / US$150, including eLearning, practical training and certification.

Combining the PADI Deep Diver and PADI Enriched Air / Nitrox certifications gives you the training to plan recreational dives to a maximum of 40 metres and choose an appropriate Nitrox mix to make better use of your available no-decompression time. If you are interested in planned decompression diving or exploring beyond recreational limits, the next step is PADI TecRec technical-diving training.

If you have questions or are unsure whether Nitrox would benefit the diving you plan to do, contact Andy. He will be happy to talk it through with you.