Analyzing multiple nitrox tanks

[Capt Jim Wyatt]

Calibration must be done using a gas with a "known" O2 %. We know what the % of O2 is in atmospheric air, however temperature & humidity can alter the reading by as much as 1%. Also the analyzer has an inherent error of somewhere around 1%, so you could be off as much as 2% by using only atmospheric air without compensating for humidity and temperature.

Wave the analyzer around in atmospheric air to get a reading of somewhere around 21%. Then blow air slowly from what you think is a known source (scuba tank with air in it) into the analyzer, if the difference is less than 1% you can be certain that the tank does indeed have only air in it. AS you blow air into it (Flow rate should be 2 liters/minute) set the calibration knob to 20.9%. You can then have a very high degree of confidence that the instrument is calibrated correctly.

 

[lv2dive]

As a matter of fact I did that for a year... 2 dives a week, still here to tell the story . Occasionally I'd verify the guess. IMO analyzing serves mainly to catch a ****-up, eg it's been pumped on pure O2, or only air when you needed nitrox. Those mistakes will be > 10% off.

Anything that comes within a few % of my expectation is fine with me.

99/100 times, or more, one isn't going to have a problem not analyzing their tanks for O2 %age. In general, I don't think about the times things go well, I think about the times they don't. In other words, the exceptions that prove the rules. If I can establish a practice that takes a few moments to help stack the deck in my favor, I'm going to. Even if I never ended up needling it. Below are a few items I've posted before on ScubaBoard, but I think bear reiterating here:

• I had an experience where I was on a boat where the captain retrieved the tanks from the shop every morning. The boat HAD an analyzer but it suddenly stopped working. In this particular case, the captain (who I trusted) had analyzed and marked the tanks at the shop that morning - and I knew that all the shops in the area banked 30% - so I went ahead and dove the gas without personally analyzing... but I was not comfortable about the situation and bought an analyzer before my next dive.
• On another group trip, this time to Indonesia, I insisted on analyzing my Nitrox every day, myself. The US dive leader (a DM!) basically told me I was being silly, the gas came from a bank and it was always 32%. I said, I won't know that if I don't analyze it.
• Why are the last two stories relevant? They lead to this one. Fast forward a few years, I was analyzing my gas from a shop that also "banks their gas", and my tanks analyzed out at 36%... that day, the shop had been filling directly from the compressor when I got my fills, and someone made a mistake somewhere. The difference between 32% and 36% can be significant if one is planning on spending a fair amount of time at 100' - several of my dives in that cave were in the 2-4 hour range - the results could have disastrous had I not analyzed.

Can one get away without analyzing their gas? Sure, ten, a hundred or two thousand times. But that next time might be a killer.

 

[sealark]

I always just analyze with air from a tank that has just air in it. I check the meter every day I charge nitrox because as stated temperature and humidity make a difference. I adjust to 21% and tweak backwards until it just changes to 20%. Then I don't check it until the next time I charge. I built my meter from oxygen hackers handbook and get the analyzer element from Maxtec. I replace the element when it starts to take a lot of turns to calibrate usually about 3 to 4 years. I fill about 300 nitrox tanks a year.