Why We Like Nitrox
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| Nitrox cylinders ready for sidemount diving Image: Rose Bennett |
Instructor and Repair Technician
fred@bubblesornot.com
What is PADI's most popular Specialty Course? Good guesses would include Deep, Boat, Underwater Videography and Project AWARE. It may surprise you that the Enriched Air Diver is number one.
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| Bob Bennett teaching a Sidemout Specialty course using Enriched Air. Image: Rose Bennett |
Terminology:
All divers know that air is comprised of 21% oxygen (O2) and 79% inert gases (78% nitrogen and 1% trace elements of helium, neon, argon, etc.). Any diving gas with > 21% and < 100% pure oxygen is considered enriched air nitrox (nitrogen and oxygen). It is abbreviated EAN or EANx. The number next to it is the oxygen content. EAN36 is 36% O2, 64% nitrogen (N2).
Brief History:
Swedish, English and French chemists discovered and wrote about oxygen in the 1770s and 1780s. The Frenchman Antoine-Laurent Lavoisier noted oxygen's corrosive characteristics when combining it with other elements, thus he named it oxygene from oxy and gene, the Greek words for acid former.
The benefits of using enriched air in diving have been well known for over 140 years. Mining and military interests drove 19th century inventors to develop self-contained underwater breathing apparatuses (SCUBA). The Englishman Henry Fleuss invented the first SCUBA unit in 1876. Fluess's invention, a rebreather initially designed to rescue mine workers, utilized 100% oxygen and a potash-soaked yarn to scrub the diver's exhaled carbon dioxide (CO2) from the breathing loop. The German firm Westfalia Maschinenfabrik developed SCUBA that mixed 45% O2 and 55% N2 in 1912.
Leap to 1979 when the National Oceanic and Atmospheric Administration (NOAA) presented its NOAA Diving Manual to the scientific community. It introduced NOAA Nitrox I which was EAN32. Shortly thereafter NOAA added Nitrox II, EAN36. Many dive shops now blend and store or "bank" one of these two blends.
Dick Rutkowski founded the International Association of Nitrox Divers (IAND, later IANTD) in 1985 and brought the benefits of enriched air to recreational (up to 40%) and technical divers (> EAN40) alike through standardized training and cylinder marking.
The advent of Nitrox-capable computers and Enriched Air training standards directly contributed to a boom in Nitrox certifications in the 1990s. Applying standards to training, cylinder marking, gas analysis and fill station operations boosted consumer confidence that Nitrox was safe to dive.
Twenty years ago Nitrox was viewed by many as an exotic breathing gas. Gases other than air were thought to be the province of scientists and explorers. Now in 2020 you'll find that recreational divers the world over breathe Nitrox for its myriad advantages.
Benefits
Our bodies don't metabolize nitrogen, thus it rapidly builds up in our bodies during diving. Upon ascent it comes out of solution. Nitrogen also becomes narcotic at depth. Nitrogen narcosis affects some divers as shallow as 80'. Limiting our bottom time, ascending slowly (30' per minute) and conducting safety stops help limit the risk of diving injuries.
What if we decreased N2 in our gas by increasing O2? We could extend our bottom time at a given depth or we could plan our dive using air tables to reduce risk of DCS, or a combination of the two (slightly longer bottom time and reduced DCS risk). Some divers report feeling more alert and less tired after a Nitrox dive compared to diving air.
The PPO2 Sweet Spot
The PPO2 Sweet Spot
Breathing gas below 0.16 PPO2 is hypoxic (not enough oxygen) and above 1.6 PO2 it is hyperoxic (too much oxygen). The risk in diving Nitrox is hyperoxia but we avoid that by planning our dive and diving our plan. That plan includes keeping one's PPO2 at 1.4 or less.
Value
BoN charges $7 for single cylinder air fills and $12 for Nitrox fills. Nitrox is definitely worth the additional cost. I'll demonstrate using the chart below.
BoN charges $7 for single cylinder air fills and $12 for Nitrox fills. Nitrox is definitely worth the additional cost. I'll demonstrate using the chart below.
Let's dive the Vandenburg artificial reef in Key Largo. This 523-foot Leviathan rests in 140' of water but the best parts of the ship are between 110' up to 50' of sea water. A dive on air gets you 16 minutes of bottom time (BT) while using EAN32 offers 25 minutes, 56% more BT! Using a Nitrox-capable computer and spiraling your way up the superstructure could extend your BT to 45 minutes or more.
Let's go shore diving on some reefs in Bonaire down to 60'. Are you diving air? Your no-stop BT limit will be 55 minutes. Dive EAN36 and your BT will be 115 minutes, a staggering 210% increase. Most divers would run low on gas before running out of BT.
Consider using Nitrox on multi-day, multi-dive vacations. Back off a few minutes on your no-stop limits and you'll reduce your risk of DCS while still enjoying more BT than breathing air.
Bottom Time Comparison for Air, 32% and 36% / PPO2 ATA
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Depth
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Air
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EAN32
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EAN36
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60
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55 minutes / 0.6
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90 minutes / 0.9
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115 minutes / 1.0
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80
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30 minutes / 0.72
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45 minutes / 1.1
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55 minutes / 1.23
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90
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25 minutes / 0.78
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35 minutes / 1.2
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40 minutes / 1.34
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100
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20 minutes / 0.85
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30 minutes / 1.3
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35 minutes / 1.45
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110
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16 minutes / 0.91
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25 minutes / 1.4
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29 minutes / 1.56
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Too much of a good thing = O2 Toxicity
Oxygen toxicity manifests itself in two ways: central nervous system (CNS) O2 toxicity and pulmonary O2 toxicity.
1. CNS O2 toxicity results from exposure to PPO2 higher than 1.6. PADI and industry standards use 1.4 PPO2 as the upper limit during diving, reserving up to a maximum of 1.6 PPO2 ATA only for emergency use. (Technical divers regularly breathe O2 at 1.6 ATA while decompressing at 20' / 6 meters, but this is beyond the scope of recreational diving.)
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| Billy and Stephen prepare for a dive to 110' breathing EANx32. Image: Rose Bennett |
2. Pulmonary O2 toxicity is a risk when breathing high PPO2 for extended periods of time, such as deep dives involving long decompression with various gases, including 100% oxygen. Recreational Nitrox divers are at little risk of pulmonary O2 toxicity as their oxygen exposure is significantly less than technical and commercial divers.
Certification
You can complete the academic portion online and then meet with your instructor for a practical discussion on Nitrox diving. The Enriched Air Diver course doesn't require open water dives but many divers dive Nitrox as part of another course such as the Advanced Open Water course, a stepping stone to earning the PADI Master Scuba Diver rating.
You can complete the academic portion online and then meet with your instructor for a practical discussion on Nitrox diving. The Enriched Air Diver course doesn't require open water dives but many divers dive Nitrox as part of another course such as the Advanced Open Water course, a stepping stone to earning the PADI Master Scuba Diver rating.
Divers 12 years of age and older may earn their Nitrox certification. Doesn't it make sense for the entire family to dive the same gas?
Fill Station Safety
Continuous blending systems are safer than partial pressure blending. Keeping your gear clean, away from hydrocarbons, annually servicing regulators, annual VIP for cylinders, and oxygen cleaning at every cylinder hydro are industry-standard safety precautions for using Nitrox.
The Diver's Role in Safe Nitrox Use
- Once your cylinder is labeled for EAN use, only fill that cylinder with Nitrox, preferably from the same dive shop that has an oxygen-rated fill station.
- Once your cylinder is labeled for EAN use, only fill that cylinder with Nitrox, preferably from the same dive shop that has an oxygen-rated fill station.
- Always analyze your Nitrox cylinder to confirm the contents, mark the EAN content and date on your cylinder, and sign the shop's Nitrox fill log.
- Confirm your buddy's gas mix and have her confirm yours. Accurately enter your EAN content into your computer. You can simplify dive planning and execution if all dive team members are breathing the same Nitrox blend.
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| Cave divers preparing to descend using EAN32. Image: Bubbles or Not Diving |
- Keep Nitrox-dedicated cylinders clean. Remove hydrocarbon based materials (WD40, solvents, spare quart of oil) from your cylinder storage area (e.g. trunk, truck bed, garage).
- Buy some Viton O-rings for your Save-A-Dive kit. Viton is an oxygen-compatible material used in cylinders and regulators for up to 100% oxygen service.
- Understand that a Nitrox-dedicated cylinder must be cleaned for Oxygen service after passing a 5-year hydro test. (The cost of a hydrostatic test covers verification that a cylinder's expansion is within DoT standards. Ensuring a cylinder is safe for enriched air requires more work.)
- A cylinder may require O2 cleaning if a technician finds internal contamination during the annual visual inspection. This involves cleaning the inside of the cylinder with an O2-compatible cleaner, drying the cylinder upside down using warm air, then reinspecting for cleanliness using special lights that help detect contaminants.
- A cylinder may require O2 cleaning if a technician finds internal contamination during the annual visual inspection. This involves cleaning the inside of the cylinder with an O2-compatible cleaner, drying the cylinder upside down using warm air, then reinspecting for cleanliness using special lights that help detect contaminants.
Equipment Considerations
Cylinders and their valves must be O2 compatible, meaning cleaned and equipped with Viton O-rings and oxygen-compatible lubricant (e.g. Christo-Lube MCG-111). Most cylinder and regulator manufacturers now sell their equipment as nitrox ready for blends up to EAN40.
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| EOI sticker for O2 applications. Note that the level of O2 content dictates which star is punched. Image: Fred Stratton |
A cylinder meeting Nitrox cleanliness standards will have a large yellow and green sticker identifying it as such. The evidence of inspection (EOI) or VIP sticker will have one of two holes punched in it for its level of oxygen service: EAN blends up to 40% fill using a continuous blending system or EAN up to 100%.
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| Analyzing cylinder contents and marking maximum operating depth (MOD) is absolutely essential. Image: Fred Stratton |
Cylinders containing 100% O2 will have a large green and white sticker that also notes that the maximum operating depth (MOD) for breathing pure O2 underwater is 20'. This yields PPO2 of 1.6 ATA.
Divers aspiring to explore below 130' must consider that a full oxygen conversion for their equipment is required as courses such as DSAT's TecDiver program (from Tec 40 to Tec Trimix 65 to Tec CCR 100) involve decompression using oxygen for stage decompression and bailout gases, including up to 100% oxygen.
Blending Nitrox
There are three main methods for blending Nitrox.
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| BoN Diving now uses an advanced continuous blend Nitrox system and banks EAN32. Image: Nardi Compressors |
1. Partial pressure blending: The Nitrox blender fills an empty cylinder with pure O2 to a predetermined PSI, then tops it off with air to achieve the target EAN blend.
2. Nitrogen separating membrane (denitrogenated air or DNAx): A thin membrane filters out nitrogen. This system is expensive and sometimes produces inconsistent EAN blends.
3. Continuous blending system: An oxygen-rated compressor takes in oxygen from large O2 cylinders and blends it with compressed air. Redundant oxygen sensors continuously measure the blend so the blender can maintain consistency. Modern systems have advanced electronic controls enabling very precise blending.
Continuous blending is safer than partial pressure blending. BoN utilizes a Nardi 15 cfm compressor and an electronically controlled blending systems with redundant safety features (e.g. multiple oxygen sensors and system alerts).
Summary
- Nitrox has a safe track record among recreational divers when observing safe limits (up to EAN40 with maximum PPO2 of 1.4 ATA).
- Increased bottom time over dives on air gives divers better value.
- Extend your bottom time or increase your safety margin, or a combination of both.
- EAN is now widely available.
- The PADI Enriched Air Diver certification is internationally recognized.
- A Nitrox certification lays the foundation for technical diving classes such as Tec 40 which introduces decompression dives down to 130' and Tec 65 Trimix which explores our blue world down to 210' using myriad mixes and complex gas planning.
Coming in March
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