Analox O2 EII - weird behavior

[Storker]

The graph on page 7 actually shows why using 100% to calibrate is a good thing (reveals a failure mode).

You need at least two points to verify the linearity of the sensor. The greater the % the earlier you can detect the failure mode.

Only if you regularly use at least two calibration gases. Let's get a raise of hands here, who always uses two different gases to check their analyzer's calibration (at least three if TMX)?

If you use just one calibration gas, your sole issue is linearity and correct response factor in the region between your calibration and your measurement. I could hardly care less if my analyzer hits a ceiling at 80% as long as I'm not using it to analyze anything above 40%. I'm a lot more concerned about linearity and correct response factor between 20.9% and 40%.

 

[Jack Hammer]

I already have a pretty decent idea about how an O2 cell works, but thanks anyway. Because the graph on p.7 shows pretty clearly why 100% O2 might not be the best calibration gas.

I think you may have misunderstood the document. That is showing how calibrating with 100% can show you your sensor is beginning to fail...



As to calibrating...

...the best cal gas to use is 100% oxygen, which is also readily available. This will mean that you are proving the ability of the sensor to operate over the entire range.

 

[guruboy]

So what's your great plan for making sure the ceiling isn't at 25% using only one gas?

People often refer to setting the analyzer to 21% on air as "calibrating" to air prior to analyzing gas. A more accurate term would be "zero the offset" (like you would when weighing something). Calibration actually verifies that the sensor is working properly, not that you can make it display 21%.

Your original issue was that the sensor acted funny and gave strange readings. In that situation, it would be smart to verify linearity across the full range.

 

[Storker]

I think you may have misunderstood the document.

I haven't. Chemical analysis is one of the things I have a good reason to believe I know something about.

That is showing how calibrating with 100% can show you your sensor is beginning to fail...

Emphasis mine. If you only need to analyze below 40%, a ceiling at 80% is irrelevant. But I agree that it's an indication that it soon might be a good idea to start looking for a fresh cell.

 

[Storker]

So what's your great plan for making sure the ceiling isn't at 25% using only one gas?

Analyzing a couple of tanks that I've already analyzed with my club's analyzer (which had a fresh cell installed only a few weeks ago).

And also verifying that my analyzer is within +/- 0.5-1% from the FO2 estimated from the numbers on my club's nitrox mixer's display.

Your original issue was that the sensor acted funny and gave strange readings.

No, my original issue was that the analyzer first showed every indication that the cell was toast, but then acted as if everything was OK.

 

[guruboy]

Analyzing a couple of tanks that I've already analyzed with my club's analyzer (which had a fresh cell installed only a few weeks ago).

And also verifying that my analyzer is within +/- 0.5-1% from the FO2 estimated from the numbers on my club's nitrox mixer's display.

That doesn't guarantee you anything when analyzing an unknown gas.

No, my original issue was that the analyzer first showed every indication that the cell was toast, but then acted as if everything was OK.

. . .

You don't consider that a "the sensor acted funny and gave strange readings" scenario?

 

[Storker]

That doesn't guarantee you anything when analyzing an unknown gas.

I never analyze a truly unknown gas. I blend myself and have a pretty decent idea about the expected FO2. Anything outside +/-2 percentage points away from the expected fo2 is a red flag

 

[guruboy]

Good luck to you

 

[Jack Hammer]

No, my original issue was that the analyzer first showed every indication that the cell was toast, but then acted as if everything was OK.

If you open the tank valve too much, allowing too much flow, you can get wonky readings. It's important to have a decent seal between the tank and analyzer and very slow flow.

 

[ScubaInChicago]

I never analyze a truly unknown gas. I blend myself and have a pretty decent idea about the expected FO2. Anything outside +/-2 percentage points away from the expected fo2 is a red flag

I'm sure Carlos Fonseca would agree with you, maybe you could ask him when you see him.