Analox analyzer

[tbone1004]

@tarponchik please read the Analox manual on calibration
https://www.analoxsensortechnology....2Analox_O2EII_RM-002-03_-_Calibration_Gas.pdf


it states specifically that you should calibrate with a gas at a higher fO2 than what you are trying to analyze.
They also recommend calibrating with 100% O2, and doing linearity checks with 50% O2

So since I just found that, and they agree with everything that I've written, are you now saying that the manufacturer of the analyzer in question doesn't understand calibration and how the oxygen cells function?

 

[stuartv]

Sisyphus? Is that you??

 

[tarponchik]

This isn't necessarily true. With the Analox analyzer it is, but at least with the DiveNav COOTwo there's a two-point calibration method using both atmospheric air and pure oxygen. This takes both points into account to calibrate the linearity and slope.


The way that these sensors work is there will be zero voltage produced by the galvanic cell in the presence of zero oxygen. Let's pretend for a moment that that's not the case. You said above O2 is useless for calibration and is only good for "checking" as you can only adjust your 20.9% point (which isn't true, even with the Analox analyzer). If that is true, then using pure N2 would be the same. Without a more in-depth calibration ability than the analox analyzer gives you, there's no benefit to checking at anywhere other than a known-point and verifying at oxygen.

1. I am talking only about analyzers I know and have used.
2. You misunderstood, in fact, I said that 2 refs calibration would better than one from the start.

 

[tarponchik]

Sisyphus? Is that you??

Get lost, finally. You have nothing to say.

 

[tbone1004]

1. I am talking only about analyzers I know and have used.
2. You misunderstood, in fact, I said that 2 refs calibration would better than one from the start.

2 calibration points are ideal, but 0% O2 is not a valid reference point because all it confirms is that the sensor is not in the presence of any oxygen. O2 is the fuel required to generate voltage in the sensor, it is not possible for the sensor to generate voltage without O2 present, so your comment about the sensor generating voltage in the absence of O2 is like saying your car will still run if it is not connected to a gas tank.

If it is not possible to calibrate a sensor from solely from a data point, it has no validity in a multi-point calibration. It may be good as a verification, but is not valid as a calibration point. Calibration of an O2 sensor is as simple as telling the electronics what "multiple" to use to turn mV's into fO2. Since any multiple of 0, will always yield 0, calibrating from 0 is not possible.

 

[dberry]

Sensors die because every once in a while some O2 molecules bind irreversibly to the surface and it gradually gets oxidized.

We've used teledyne sensors in my labs for decades (not the same sensor, we replace them!) These are the same sensors in many O2 analyzers for SCUBA. They are electrochemical cells with a semi-permeable membrane inside containing a hydroxide solution. They die because the solution dries up slowly (diffusion of water through the membrane.) The more you use the sensor, the faster it goes. The gas you pass over the membrane has very little effect on the rate of evaporation (unless the gas is really humid.) I'm not aware of any component to which O2 could bind irreversibly and make a difference. Pure O2, 32%, He - almost no difference in sensor lifetime, AFAIK.

Perhaps Analox sensors work on a different principle, but I doubt they are anything other than similar electrochemical cells in a slightly different housing than the standard Teledyne sensors.

And yes, the best thing you can do to prolong the life of a sensor is to cap it. Ziplock, or for those of us with OxyCheq El Cheapo analyzers, buy the screw-on aluminum cap with an O-ring seal. The electrochemical cells DO dry out just sitting open to ambient air without the power on.

 

[tarponchik]

@tarponchik please read the Analox manual on calibration
https://www.analoxsensortechnology....2Analox_O2EII_RM-002-03_-_Calibration_Gas.pdf


it states specifically that you should calibrate with a gas at a higher fO2 than what you are trying to analyze.
They also recommend calibrating with 100% O2, and doing linearity checks with 50% O2

So since I just found that, and they agree with everything that I've written, are you now saying that the manufacturer of the analyzer in question doesn't understand calibration and how the oxygen cells function?

They do not agree with everything you said. For example, they agree with me on this one: "However there are also safety issues to be considered as pure oxygen can increase the risk of fire in many situations."

Also, they agree with both of us that sensor output becomes non-linear at high O2 and this results in error (the yellow line on their graph proves this). They suggest to use a 2nd calibration gas like 50% O2 to get around this (which you did not). Then they immediately switch into selling mode: "However, you’d now be carrying around two calibration gases and associated accessories, when all you really need to do is to change your oxygen sensor more frequently." Bingo! Buy more and support the economy!

So their answer why using pure O2 is better is "because you are more likely to notice that your sensor is too old." I won't argue against this one, maybe you will. However, because of the non-linearity near 100% issue, your measurements will only gradually become less accurate and you will deal with sort of creeping error.

BTW if you look at their graph, zero is the most reliable point. All 3 lines meet there, no deviations or non-linearity is seen.

 

[RainPilot]

 

[tarponchik]

We've used teledyne sensors in my labs for decades (not the same sensor, we replace them!) These are the same sensors in many O2 analyzers for SCUBA. They are electrochemical cells with a semi-permeable membrane inside containing a hydroxide solution. They die because the solution dries up slowly (diffusion of water through the membrane.) The more you use the sensor, the faster it goes. The gas you pass over the membrane has very little effect on the rate of evaporation (unless the gas is really humid.) I'm not aware of any component to which O2 could bind irreversibly and make a difference. Pure O2, 32%, He - almost no difference in sensor lifetime, AFAIK.

Perhaps Analox sensors work on a different principle, but I doubt they are anything other than similar electrochemical cells in a slightly different housing than the standard Teledyne sensors.

And yes, the best thing you can do to prolong the life of a sensor is to cap it. Ziplock, or for those of us with OxyCheq El Cheapo analyzers, buy the screw-on aluminum cap with an O-ring seal. The electrochemical cells DO dry out just sitting open to ambient air without the power on.

To quote, "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." But if you are concerned about the alkaline electrolyte, why CO2 isn't the main problem?

 

[stuartv]

Get lost, finally. You have nothing to say.

I certainly having nothing more to add to the many fine explanations tbone has wasted his time typing....