Analyzer Sensor Died

[rob.mwpropane]

That depends on how high a ppo2 you expect to read with the cells. If you take the rebreather out of the equation then you won't be analyzing anything higher than 100% o2. I built the pressure pot cell checker specifically for checking my rebreather cells but have found that it helps to get an idea on cell health. I use my El Cheapo style analyzer for blending nitrox through my compressor so it's good to know that the cells are linear and not severely current limited when doing so.

I have 100% but usually mix 70% for local diving.

I thought you could test mv of the sensor? Why pressure pot?

 

[sprockjohnson]

I have 100% but usually mix 70% for local diving.

I thought you could test mv of the sensor? Why pressure pot?

You do test mv in air and o2 but the pressure pot allows you to bring the ppo2 up higher to check for linearity and current limiting at higher levels of o2. Like I said not as relevant outside the rebreather but still helps to gauge the health of the cell. That being said, they still can just die from one day to the next.

 

[rob.mwpropane]

You do test mv in air and o2 but the pressure pot allows you to bring the ppo2 up higher to check for linearity and current limiting at higher levels of o2. Like I said not as relevant outside the rebreather but still helps to gauge the health of the cell. That being said, they still can just die from one day to the next.

Thanks!

 

[stuartv]

The Analox II is very easy to inadvertently knock it out of calibration. That little knob is very sensitive and way too easy to turn a smidge by accident.

Thus, you should calibrate every time you take it out of the box or after setting it down for any amount of time.

If you're analyzing mixes that are 30-ish % or less, calibrating in ambient air is generally okay. But, you DO run the risk of encountering a current limited cell that, for example, reads 21 in air, but only reads (for example) 25 when it's exposed to 30%.

If you're analyzing mixes like 70%, you should calibrate with O2 and then confirm that it reads very close to 21 in ambient air. Or calibrate in air (ambient or tank) and then confirm it reads 100 when you expose it to pure O2.

If you only calibrate to O2 and the cell is current-limited, it could tell you that you have 100% in a cylinder that is really only 70% (for example). Or it could tell you some other complete wrong value for a mix.

Using a second point to verify calibration is really important. Not doing it won't bite you very often. But, it CAN and when it does, it could be really bad.

Example:

Your cell puts out 10mV in Air.

If it's healthy, it will put out 4.76 times that or 47.6 mV in 100% O2. (because 100% / 21% = 4.76)

If it's current limited, it might still put out 10mV in air, but only put out 40mV in O2. I.e. it is current limited to 40mV (and yes, I know volts are not current - I didn't create the terminology).

If you only do a single point calibration (in O2) with no verification in air, then the analyzer will think 40mV means 100%.

Then, if you feed it a gas that produces 20mV, it will tell you that you have 50% (because you told it 40mV is 100%). But, in reality, you only have 42%. 10mV is 21% (air), so double that (20mV) means double the FO2 - i.e. 42%.

But, if you had done that calibration in O2 and then verified it with Air, you see the analyzer telling you that it's reading 10mV (in Air) and that means you're analyzing 25% Nitrox. That's because you told it during calibration that 40mV is 100%. Therefore, 10mV is 25%.

A reading of 25% in Air is your clue that your sensor is hosed. It could be current limited, or it could just be really non-linear. Either way, it's time to just junk it and put in a new sensor.

Similarly, instead of calibration in O2 and verification in Air, you can do the opposite.

Calibrate in Air with 10mV means your O2 should produce 47.6 mV. If the cell is limited to 40mV, then the analyzer will tell you that your 100% O2 is actually (4 * 21%=) 84%. So, you still get the clue that your sensor needs to be replaced.


With that Analox II, calibrate AND verify every time!

 

[rob.mwpropane]

The Analox II is very easy to inadvertently knock it out of calibration. That little knob is very sensitive and way too easy to turn a smidge by accident.

Thus, you should calibrate every time you take it out of the box or after setting it down for any amount of time.

If you're analyzing mixes that are 30-ish % or less, calibrating in ambient air is generally okay. But, you DO run the risk of encountering a current limited cell that, for example, reads 21 in air, but only reads (for example) 25 when it's exposed to 30%.

If you're analyzing mixes like 70%, you should calibrate with O2 and then confirm that it reads very close to 21 in ambient air. Or calibrate in air (ambient or tank) and then confirm it reads 100 when you expose it to pure O2.

If you only calibrate to O2 and the cell is current-limited, it could tell you that you have 100% in a cylinder that is really only 70% (for example). Or it could tell you some other complete wrong value for a mix.

Using a second point to verify calibration is really important. Not doing it won't bite you very often. But, it CAN and when it does, it could be really bad.

Example:

Your cell puts out 10mV in Air.

If it's healthy, it will put out 4.76 times that or 47.6 mV in 100% O2. (because 100% / 21% = 4.76)

If it's current limited, it might still put out 10mV in air, but only put out 40mV in O2. I.e. it is current limited to 40mV (and yes, I know volts are not current - I didn't create the terminology).

If you only do a single point calibration (in O2) with no verification in air, then the analyzer will think 40mV means 100%.

Then, if you feed it a gas that produces 20mV, it will tell you that you have 50% (because you told it 40mV is 100%). But, in reality, you only have 42%. 10mV is 21% (air), so double that (20mV) means double the FO2 - i.e. 42%.

But, if you had done that calibration in O2 and then verified it with Air, you see the analyzer telling you that it's reading 10mV (in Air) and that means you're analyzing 25% Nitrox. That's because you told it during calibration that 40mV is 100%. Therefore, 10mV is 25%.

A reading of 25% in Air is your clue that your sensor is hosed. It could be current limited, or it could just be really non-linear. Either way, it's time to just junk it and put in a new sensor.

Similarly, instead of calibration in O2 and verification in Air, you can do the opposite.

Calibrate in Air with 10mV means your O2 should produce 47.6 mV. If the cell is limited to 40mV, then the analyzer will tell you that your 100% O2 is actually (4 * 21%=) 84%. So, you still get the clue that your sensor needs to be replaced.


With that Analox II, calibrate AND verify every time!

Thanks Stuart. That little knob is finicky. I had always thought a bad sensor would drift a little, I just didn't expect a reading of 8 when it was supposed to be 21.

I have plenty of O2 atthe shop.. so you say verify every time I use it? Not weekly, or monthly..but everytime? That sucks a little, but I'd rather be safe then sorry.

 

[stuartv]

Thanks Stuart. That little knob is finicky. I had always thought a bad sensor would drift a little, I just didn't expect a reading of 8 when it was supposed to be 21.

I have plenty of O2 atthe shop.. so you say verify every time I use it? Not weekly, or monthly..but everytime? That sucks a little, but I'd rather be safe then sorry.

You observed it to go from working to "totally screwed" very quickly....

It's up to you how often you verify. Use some common sense.

For example: If I have a cylinder that I analyzed and marking "EAN30", but it has been sitting for a few weeks, I'll re-analyze it before I use it. In that case, I might just check the analyzer calibration in air and then check the cylinder. If it still comes out 30%, then I'm pretty confident I don't need to verify my analyzer against O2.

It's YOUR life you're gambling with, so you do what you are comfortable with. I was just trying to make sure you understand what the possibilities are.

 

[rob.mwpropane]

You observed it to go from working to "totally screwed" very quickly....

It's up to you how often you verify. Use some common sense.

For example: If I have a cylinder that I analyzed and marking "EAN30", but it has been sitting for a few weeks, I'll re-analyze it before I use it. In that case, I might just check the analyzer calibration in air and then check the cylinder. If it still comes out 30%, then I'm pretty confident I don't need to verify my analyzer against O2.

It's YOUR life you're gambling with, so you do what you are comfortable with. I was just trying to make sure you understand what the possibilities are.

Makes perfect sense, thanks.