The air is the big, free tank that is always 20.9.
Analyzing your own nitrox tanks
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mrtechbench
Contributor
Analox o2eii came with a temp and humidity chart for calibration. @GJC posted it in another thread. I never tried comparing results if set to 20.9 vs whatever table said to use.
lizardland
Registered
A gas supply company will sell you calibration gases or medical grade air. Why not use that?
Having a random cyinder of air for calibration is not foolproof, especially in a dive shop where you have absolutely no knowledge of its provenance. Someone will screw it up eventually. I know I am not the only person that has worked somewhere where the bank had to be dumped because someone, somehow, managed to fill it with it with something they shouldn't have. Dive shops are not famous for employing well motivated geniuses.
"Waving the analyser in the air" has worked for what is now two generations of nitrox divers without significant failures. Sensors have an error of around +/- 2-4% depending on the manufacturer. Analysers are just simple voltmeters and have their own inherent error as well. How much difference do you think calibrating using air in a cylinder instead of atmospheric air actually makes in the grand scheme of things? Analysing nitrox is not scientific sampling despite how much people fiddle with the analyser to get it to read 20.9. It is a measure of "okayness" with a level of uncertainty that would horrify most scientists or engineers even when done with best practice.
Maybe there is an argument for it in climates at the extremes of temperature and humidity. For most mortals, waving it in the air has worked OK for at east 30 years.
lizardland
Registered
I need to get round to uploading my design to GitHub.
If you want simple electronics then Mark Munro's rebreather monitor design is really easy, really robust. If you have a look at his webpage and scroll down to the single display and click on the schematic and parts list. I've made dozens of these for people over the years. In fact, when I opened up my Analox analyser I was a bit shocked to see that it is exactly the same design inside. Details of PPO2 Displays
If you want fancy then I recommend some kind of Arduino-based controller, an ADS1115 board as the analog to digital converter (they have a good enough level of precision for analysers) and a display.
I use the Lilygo T-Display S3 which is an integrated ESP32 with an OLED display in my rebreather controller & ppO2 monitoring projects. Sensor connected to the ADS1115. You need to add an external EEPROM to store the calibration data (not necessary with Arduino as they have on board memory). Whole thing runs off a single 14500 battery, will do about 7-8 hours on a single charge and charges from USB.
If you find a reasonable priced CO sensor then let me know. I've been thinking about doing a CO checker for my own diving use.
Johnoly
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Had a Sunday only idiot worker who was not supposed to be touching the compressor, only the banks. I'm filling on the busy monday and the customer tanks I'm filling are 3 % off where my calcs say it should be. I've got a floor full of tanks and I'm wasting time testing each bank bottle and figured out what he did. 4 wks later the owner caught him on camera pocketing cash for gear sales. From then on before I started my day filling, I took the time to test all the banks including the air banks.
Easily quantifiable from the chart below:
The problem is moisture in a nice, warm atmosphere, like much tropical diving. Errors of 1% are likely and 2% is quite possible.
If you "calibrate" by waving the analyzer in the air on a tropical dive boat then you set the analyzer on (say) 20.9 but it may actually be only 19.9, or even 18.9. So you will overestimate your tank's O2 percentage, by that 1 or 2%. That can make a difference if you are diving near your NDL....you don't actaully have as much NDL as you thought.
I just spent 10 days diving in the Solomon Islands. Tanks routinely measured 31.7-32.4%, but the analyzers were calibrated to air on the dive deck. Super warm and moist. I routinely set my computer at 31% AND stayed away from my NDL. In retrospect, I should have used 30%.
elgringoperdido
Contributor
One should not do that. I suggest setting the computer at the lower end. Here's a quote from page 10 of NAUI Nitrox Diver (2004 edition):
"Another very common use of nitrox in diving is simply as a safety margin. Divers who choose to dive conservatively will often breathe nitrox but continue to use air dive tables or an air computer. Over a series of dives, they will absorb significantly less nitrogen than their tables indicate, effectively lowering their risk of decompression sickness."
The idea with maintaining a conservative profile, say, diving to 102 feet instead of 110 feet, is to keep the partial pressure of oxygen less than 1.4 atmospheres. To achieve that one assumes a higher ppO2. This is why I said I assume something like 34% if I think it's between 29 and 34%.
The idea with setting the dive computer (or using a dive table) at the lower ppO2 is to assume a greater mole fraction of nitrogen gas (and correspondingly greater ppN2 in the mix). This means that the computer gives you a shorter NDL than it would if you were using a higher ppO2 setting. I'd suggest setting the computer at 30 or even 29% in that case. You could even let it stay in "air" mode, as the naui manual suggests, but I think that's probably overkill.
I can only remember a few places like that. We stayed at a resort in Roatan for a week where they loaded the boats with nitrox tanks and passed around a couple of beat-up old analyzers which gave different readings. Also, at Buddy Dive Resort just north of Kralenkijk, Bonaire they offer "free" nitrox. Unlimited shore diving but you can only take two tanks per diver at a time. I saw lots of rule-breaking there. I went with a group of about 35 people with the local dive shop. Of those 35, maybe 20 were nitrox certified, but all of them were sucking down that nitrox. None of the people at the resort ever asked for anyone's nitrox credentials. And I noticed that many of the divers never bothered to analyze their mixtures. I analyzed mine each time using their analyzers and found some slight discrepancy depending upon which analyzer I used, although never more than a couple of percentage points.
To be clear: for the depth profile estimate more liberally the ppO2. For determining dive time the computer or dive table should be used with a conservative estimate of the ppO2. Refer to your nitrox training manuals for a refresher, if necessary. I'd guess that they would have a comment similar to the paragraph I posted above, regardless of the agency.
One other thing you could do is purchase your own high-quality analyzer and calibrate it each time. I have never had my own but I have found several that get good reviews for less than $200. I so rarely use nitrox that I just never seriously considered buying one. The NJ wreck dives offer too many opportunities to exceed the recommended depth on EAN that I'd rather not use it there. Mainly I use it when I'm in a situation where I'm making 3 or 4 dives every day for a week straight, typically in tropical waters and usually from a boat where all the other divers are using it in order to have shorter surface intervals. There are also people who swear that they feel better and experience less fatigue with EAN than with regular air, and maybe they really do. To be honest, I'm not feeling it.
I'm surprised that NAUI would offer implicit endorsement of "Kentucky windage".
Has that carried over in later editions of the same manual?
But that is not what you said in the quote above. You said there you used the higher end (34).
And you said:
Which suggests you ARE more concerned about O2 toxicity than you are DCS.
Or, maybe you don't know what you mean or do.
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