Don’t Trust Your Gas Blender – Analyze Every Tank

by Jon Kieren

Girl analyzing a nitrox tank

Photo Courtesy of Andy Phillips

People make mistakes, it’s human nature. I make them all the time. I’m sure that even after this article has been edited several times and published someone out there will find at least a couple of typos and call us out on it. A typo is one thing. However, a simple mistake in the blending process can result in a diver breathing a mix with significantly more or less oxygen than they had expected, causing serious injury or death. If we KNOW that people make simple mistakes so often, then why do so many nitrox divers today NOT analyze their gas before diving? There are two primary reasons: either they don’t understand why it’s so important (a topic that is covered in every nitrox course), or they have just become complacent. This article will discuss both scenarios and how to avoid them.

Why is it so important to analyze your breathing gas? Simply, it can kill you if it’s wrong. If the oxygen content is less than the diver had expected, they can end up with unexpected and unknown decompression obligations.

Example – You make a dive to 30 metres/100 feet assuming you’re breathing 32% nitrox. You spend 39 minutes on the bottom and surface with no decompression obligation. Unfortunately, the nitrox tank you were diving was accidentally filled with air (21% oxygen), and in reality you just blew off 26 minutes of decompression. A significant error that is almost sure to result in Decompression Sickness. This situation can be made significantly worse by conducting repeated dives.

What if the oxygen content is HIGHER than you expected? Should be better off then, right? As far as decompression obligations are concerned, yes. However, a far greater risk in diving nitrox is Oxygen Toxicity and can cause severe convulsions (not a good situation underwater).

Example – Using the same dive as above, assuming you were on 32% nitrox at 30 metres/100 feet, your partial pressure of oxygen (PO2) would be close to its upper limit at about 1.3 ata. If that nitrox mix was in fact a 50% nitrox mix, your PO2 would now be over 2.0 ata and would be considered extremely dangerous.

The examples above are not the only concerns of breathing the wrong gas at the wrong depth; a thorough nitrox course will cover the others, as well as how to avoid them. So if you have to be Nitrox certified to dive nitrox, and the risks and proper procedures for avoiding those risks are covered in the course, why do people still end up breathing the wrong gas? The simple answer is: complacency. Over time, divers become complacent with their gas analysis procedures and start to skip it altogether, which means they end up in the water with absolutely no idea what they are breathing. Pretty scary.

Normalization of deviance is a term used by astronaut Mike Mullane (*Mullane 2014) to describe the process of complacency in safety procedures. In brief, it explains how humans have the tendency to take shortcuts due to different factors including time, peer pressure, etc. Once this shortcut is taken and nothing bad happens, the brain will incorrectly assume that the shortcut is “safe”. This shortcut now becomes the norm, and we have completely eliminated a critical step in a procedure. This applies to diving at every level. How many times have you seen divers jump in the water without doing a proper predive check? It is taught and its importance stressed in every open water course, yet it gets skipped every day because so many divers have “gotten away with it” they assume it’s safe to dive without making predive checks and then eliminate it from their procedure. Unfortunately, it also results in emergencies from divers forgetting to turn on their air and inflate their BCDs.

The same happens to nitrox divers. Maybe one day they are in a rush and forget to analyze their gas at the fill station. They get to the dive site and realize that they forgot to analyze but now do not have access to an analyzer. They are left with two choices, either not dive today or dive without analyzing their gas. The diver has been getting fills from that fill station for years and has never gotten the wrong mix, so they decide to dive anyway and assume the fill is correct. Nothing bad happens, so they now believe this shortcut is safe. “If I get my fills from XZY Dive Center, I know that it will be correct and I do not need to analyze my gas”. They have eliminated the most critical step in diving nitrox, and this is now the norm.

We know people make mistakes, and that’s why we have safety procedures in diving. These procedures help us catch the little mistakes before they create catastrophic emergencies. When diving nitrox, analyze every tank before every dive without exception. It could save your life.

* Mullane, Mike. (March 2014). Stopping Normalization of Deviance.

3 replies
  1. John Lennon
    John Lennon says:

    I have just started using Nitrox and my fill site has not shown me the readings. I ask them to fill 32% Nitrox 1.4 that allows me to dive to 30mts. Me and my son always have a safety stop at 5mts for 3 mins regardless of what we use.Do you think we should have our own analyze ?

    • CapnGlen
      CapnGlen says:

      absolutely get your own analyzer. At the very least, borrow theirs to analyze before you leave the shop. If they wont loan you thier analyzer, dont go back… most shops will analyze in front of you or hand you the analyzer and have you analyze your own tank. This is the safest method. YOU are responsible for your own safety. Why trust a stranger?

  2. Tim St.Clair
    Tim St.Clair says:

    No argument with the message that divers should ideally always check their mix, certainly if planning repetitive dives over multiple days and/or intending to use the nitrox for extended max. bottom times, and/or accelerated decompression. Also, if a dive centre’s using partial-pressure or continuous-flow blending, end-user analysis of gas is absolutely a must, because even highly experienced gas-blenders can make mistakes.

    I would point out though, that many dive centres catering primarily to sport divers (at least in Europe and the Red Sea) are now using membrane compressor systems to bulk-fill their rental nitrox cylinders, and given their target market, they simply set the system to deliver a standard O2% of 32±1% (and membrane systems generally can’t do better than ~40% O2 anyway). This is perfect for sport divers’ needs, since it has a routine MOD of 32-35 m (pO2 = 1.4 ata) and an emergency MOD of 38-41 m (pO2 = 1.6 ata). For those divers who aren’t yet qualified to dive with (or don’t want to pay for) nitrox fills, the compressor operator simply disconnects the membrane system, and fills the rental tanks with air instead.

    The point is, a sport diver getting their gas from such a centre CAN be >99.999% certain of their gas fill, even without analysis: their tank must contain EITHER 31-33% nitrox OR 21% air. The diver could still dive (well) within no-stop limits AND avoid O2 toxicity, by using the most conservative limits for those two possibilities: setting their dive computer to 21% O2, but programming in a depth alarm of 30 m. Unless they were intending to do a square-profile to a greater target depth (unlikely in most of these settings), they wouldn’t have to cancel their dive.

    To get the full benefits of nitrox though, yes, analysis is essential.

    Also, this made me smile:
    “You make a dive to 30 metres/100 feet assuming you’re breathing 32% nitrox. You spend 39 minutes on the bottom and surface with no decompression obligation.” My 1st-gen Suunto Vytec (admittedly one of the more conservative algorithms on the market) won’t give me more than about 28 mins max. bottom time at 30 m diving 32%O2 on a first dive, with the absolute minimally conservative settings — so what tables/computer gives you 39 minutes?


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