Analox O2 EII - weird behavior

[Storker]

The other week, my Analox started acting up. Slow response, and eventually it wouldn't calibrate to 20.9% in air. I was pretty certain the sensor was gone (it's two and a half year since I bought the analyzer) and got ready to order a new cell. When I opened the analyzer to check the connector, I accidentally turned on the analyzer. It had no issues calibrating to 20.9%, in fact I could adjust it all the way up to >32% in air.Since everything seemed quite hunky dory, I canceled my plans of ordering a new sensor. And since the battery I have in it is a fairly fresh Li battery, I don't think low battery was the issue either.

Since I don't dive every week, i usually store the analyzer with the sensor saver in place. Is it possible that this can have something to do with the weird response? I've noticed before that it needs a little time to behave properly if it's been stored for some time with the sensor saver in place.

 

[guruboy]

Find some 100% O2 and see if you can measure that.

I've seen it suggested that you give it up to 24 hours to "acclimate" before use

 

[ScubaInChicago]

I'm pretty sure the instructions say to "wake up" the cell with o2 when new or sitting in an airtight environment. I would do a three point calibration to verify the sensor is good. Calibrate to air, then measure know nitrox, then 100% o2.

 

[Jack Hammer]

Mine acted similar a few years back. I put a new battery in it and the weirdness went away. Perhaps measure the battery voltage and calibrate, as mentioned, against known gasses.

 

[Storker]

Find some 100% O2 and see if you can measure that.

I've seen it suggested that you give it up to 24 hours to "acclimate" before use

Unfortunately, my only source for 100% O2 is our club's 50L, and its regulator is hooked up to our mixing system. I don't think it's a good idea to dismantle the system...

What would be the reason for the cell to need "acclimatization" or "waking up" after storage with the plug in place?

 

[kelemvor]

It sounds to me like you have a bad solder connection or other mechanical problem. I think a cold solder joint on the potentiometer used to make the adjustments might cause it. Disassembling the unit could have jostled the wires enough to give it (for now) a good connection again.

 

[ScubaInChicago]

What would be the reason for the cell to need "acclimatization" or "waking up" after storage with the plug in place?

The sensor works by oxidation of the element. If put in a sealed environment the oxidation is slowed. To jump start it again it is recommended to flush with o2.

 

[ScubaInChicago]

This link will help you understand your cell a little bit better:

https://www.analoxsensortechnology....2Analox_O2EII_RM-002-03_-_Calibration_Gas.pdf

 

[Storker]

The sensor works by oxidation of the element.

Yep. And in the process, the chemicals inside are consumed.

If put in a sealed environment the oxidation is slowed.

Well, that's the point of sealing it: To prolong the lifetime of the cell by reducing the rate of the reactions while the analyzer isn't in use. If it needs a long time to "wake up" again, my first guess was that it was due to diffusion in the cell being the rate limiting step. And if diffusion is rate limiting, I'd be a bit wary about trusting the cell's response when going from one O2 concentration to another. Thus my question of why.

This link will help you understand your cell a little bit better:

https://www.analoxsensortechnology....2Analox_O2EII_RM-002-03_-_Calibration_Gas.pdf

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. Especially if the cell isn't brand new and fresh. Using 100% O2 also goes against the basic principle that an instrument should be calibrated as close to the expected measuring range as practically possible (which we also can see from the graph on p.7). Air at 20.9% is a lot closer to 32% than what 100% is, so from that standpoint air is a much better calibration gas for a normal rec diver than what 100% is.

 

[guruboy]

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.

So while I don't use 100% before a dive (mainly due to practical reasons), I would certainly use it if available.