Not Calibrating the Analyzer
(or Doing so Incorrectly)
The oxygen sensors built into or attached to gas analyzers are sensitive to a lot more than just the concentration of oxygen in your breathing mixture. Factors that can affect analyzer readings include:
- Ambient temperature.
- Humidity (or the lack thereof).
- Pressure/flow rate of the gas mixture being analyzed.
This helps explain why, even though your analyzer was reading properly the last time you used it, it can be wildly off the next time you take it out of the box. It also explains why calibrating the analyzer against a known reference gas every time you use it is so important.
Oxygen analyzers aren’t all that accurate to begin with. (I’m always amused to see divers label their oxygen content as being 32.3 or 49.4 — as if their deco software or dive computer would allow them to program it in tenths of a percent. Why not just round to the nearest whole number and be done with it? Whole numbers are easier to read.) In any event, why fail to do something that can increase your analyzer’s accuracy by at least a few percentage points?
As you should remember from your entry-level Nitrox course, calibrating an analyzer requires a reference gas whose oxygen content is unequivocal. This pretty much limits us to air or pure O2. For greatest accuracy, your reference gas should be as close to that of the mixture you will be analyzing as possible. That means air for mixes ≤EAN50, and pure O2 for EAN80 (assuming you are one of those rare birds who thinks it’s fun to spend several times the time and effort needed to just deco on pure O2).
The key to accurate calibration is keeping factors such as temperature, moisture content and flow rate the same when calibrating to your reference gas as they will be when analyzing your mix.
The best way to do this is with a closed system that employs a flow meter which plugs into your BC inflator, dry suit or similar hose (differences in flow rate can affect your analysis up to 0.2 percent or more).
Cheap analyzer systems frequently use a nipple that you are supposed to hold tightly against an open valve orifice. The problem with these is that the flow rate from your reference gas cylinder will never be precisely the same as it is from the cylinder whose mix you are analyzing. These analyzers are also more prone to contamination by ambient moisture and other factors. It is best to use a completely closed system with a separate flow meter whenever possible.
Good gas analysis also requires patience. It may take up to a minute or more before the numbers on your analyzer “settle down” and stop fluctuating when both calibrating and taking an O2 reading on your mix. Once they do, you should wait another 30 seconds or more to make sure these numbers don’t fluctuate further (this is tough to do with the kind of analyzer you are supposed to hold next to an open valve orifice).
It’s truly disappointing to see how many entry-level Nitrox students don’t learn this kind of disciplined approach to gas analysis as part of their course. Most Nitrox courses don’t even require that students actually make Nitrox dives — and the bulk of the academics can be mastered through self study. In fact, the only thing that legitimizes interaction with a live instructor is the opportunity to get hands-on experience analyzing gas the right way. When instructors blow this opportunity — well, their students might be better off just getting their Nitrox Diver card on line.
You can’t always blame the instructor, though. Not when at least one training agency puts the following in their entry-level Nitrox text:
“Oxygen analyzers are simple devices to use…In most cases, they require nothing more than to be turned on and allowed to stabilize while reading room air.”
I’m not making this one up. It really says that. Nice, moist “room air.” (Yeah, that’ll lead to a nice, accurate reading…)
Failure to label cylinders correctly »
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