Analyzing multiple nitrox tanks

[mi000ke]

I recently got nitrox certified, and on my first nitrox dives I was the only one on the boat using it. Did my calibration with an air tank and then analyzed the nitrox as instructed. I dove yesterday, first time since getting my nitrox cert, and almost everyone on the dive was using nitrox and of course analyzing their gas.

In cases like that, is it OK to calibrate the analyzer with air just once before analyzing all of the nitrox tanks (assuming I watched it being calibrated or calibrated it myself), or should it be recalibrated before analyzing each tank ?

 

[gopbroek]

Short answer: No
Long answer: As O2 sensors age the output degrades with time and this is why there is a calibration knob incorporated and the calibration adjustment is made to a relatively know / stable source (ambient air). Over the relatively short time it takes to measure a group of bottles any drift or aging should be negligible.

From Wiki, 'Electro-galvanic fuel cell' article : Electro-galvanic fuel cells have a limited lifetime which is reduced by exposure to high concentrations of oxygen. The reaction between oxygen and lead at the anode consumes lead, which eventually results in the cell failing to sense high concentrations of oxygen. Typically, a cell used for diving applications will function correctly for 3 years if stored in a sealed bag of air but only for four months if stored in pure oxygen.

 

[Tug]

When I analyze a batch of cylinders, I only calibrate once at the beginning.

On a related note, even on my rebreather I only calibrate the cells every few months and don't worry about that.

 

[RainPilot]

I generally calibrate in the morning before the first tank. Be careful calibrating with a tank of "air". If for some reason it's not actually air you will have some errors going forward.

Just calibrate with the atmosphere it's normally fairly standard

 

[Hickdive]

Calibrate once per analysing session with atmospheric air. Apply a correction if necessary for temp/humidity (see card usually supplied with analyser)

Do not calibrate from a cylinder of "air" as there is no guarantee that it is actually air and you don't want to calibrate to 20.9% on a cylinder that is potentially not air. OTOH the atmosphere is always 20.9%.

 

[redacted]

If I get anomolus reading, I recalibrate before I accept it.

 

[mi000ke]

Do not calibrate from a cylinder of "air" as there is no guarantee that it is actually air and you don't want to calibrate to 20.9% on a cylinder that is potentially not air. OTOH the atmosphere is always 20.9%.

I thought you needed a light but sufficient flow of gas into the analyzer. How do you do that without using a tank of air under pressure?

 

[mi000ke]

never mind - found an older thread on calibration

 

[yle]

Interesting question, which seems pretty simple, and interesting responses. I'll share a recent story that will provide a different perspective.

I was in Cozumel a couple months ago. My party of four had asked for 8 nitrox tanks (2 each) for our dives on that particular day. I have my own O2 analyzer and while I usually don't bother bringing it with me when I travel, this time I did.

When we arrived at the dive shop, the tanks were there and ready to be loaded on the boat. The shop had already analyzed them and had labeled them all 32%. I pulled out my analyzer and started checking them for myself.

32... 32... 32... 21

Yup. One of the 8 tanks was air. How could this have possibly happened!?!? That's always the question immediately following an accident. Fortunately it didn't get to that point.

The shop guy was indignant and insisted something was wrong with my analyzer. I was equally indignant and told him something was wrong with him and his analyzer. And so began the pissing match.

He pulled his analyzer out (yes, his actual analyzer, not his...) and re-analyzed the tanks:

32... 32... 32... 32

He was ready to declare victory until I told him "recalibrate your analyzer and then measure that tank again."

He did. And his smug look disappeared. It read 21.

So what happened? I didn't tell him this, but just the week before I was in a shop at home and the same thing had happened to me: analyzing several nitrox tanks in a row, the readings were similar... just as I expected them to be. But some of my readings were slightly different than what the shop had measured. I pointed this out, and the shop guy told me that when analyzing multiple tanks in quick succession, residual gas from the previous tank (i.e. gas that gets trapped in the analyzer) can cause an inaccurate reading. He had me re-calibrate the analyzer between tanks (i.e. clear the residual gas from the previous tank) and remeasure. My readings then matched his.

The potential for mistake compounded by expectations: in the case of my Cozumel tanks, the shop guy expected every tank to be 32. So when he quickly moved his analyzer from one tank to the next, and he got 32 for each, he simply confirmed his expectations.

Two lessons here:

1. Be mindful of what your analyzer is really reading, i.e. the gas that is in contact with the sensor, and where it came from. If you have any doubt, re-calibrate.

2. Analyze your own gas. It's a standard rule that's hammered on in every nitrox class (or at least should be), but some places (like the shop I went to in Coz) just assume they are always right and encourage you to trust them (because, after all, you're on vacation!) But if you get bent from diving on 21 when you thought you had 32... the most you'll get from them is an apology.

 

[Charles2]

Do not rush when analyzing your tank. I have seen calibration knobs that are so loose that they seem to change readings when just being passed from one person to another. I have seem similar issues to the ones @yle posted. Although not always true, if you get significantly different readings from two tanks that were filled by the same shop (perhaps connected to the same manifold while filling), recalibrate. By all means, just take your time. If anything seems out of reason, recalibrate and reanalyze before you accept the analysis.