How accurate is an analyser at high % of oxygen?

[broncobowsher]

I wonder what difference purging a new deco cylinder with oxygen first makes

Lets do the math together. This will be the one time metric makes it easier. Take a cylinder, any size, won't matter. Fill it to 200 bar of pressure, that is about 3000 PSI. So starting with a completely empty cylinder, it will still have a volume of 1 filled to 1 bar. Fill 199 bar of O2. That is ½% of the complete fill. So ignoring the oxygen in the air and just say 100% nitrogen (makes math easy), you should still read 99.5% O2. Purging the fill lines will matter more, but how much will depend on how long the lines are, how well they are purged. But the new cylinder itself, irrelevant.

 

[BRT]

I wonder what difference purging a new deco cylinder with oxygen first makes

Less than 1%

 

[Jack Hammer]

I've had air calibrated sensors read 105% when checking O2. Understanding calibration is important.

 

[jgttrey]

For calibration, there are a couple of issues going on.

One, as mentioned, is linearity. The cell merely generates a voltage based on a galvanic reaction when exposed to O2. More O2 = more voltage. In a healthy cell, the response is mostly linear, but not perfectly. The analyzer is assuming linearity. So, even if you calibrate perfectly on air, you may be off a little at 100% O2 or vice versa. Frankly, with good cells, this has not been much of an issue for me. When I calibrate on 100% O2, I'm very, close when I check against a known 21% gas.

The other issue is just to understand the nature of what the analyzer is doing. The analyzer is just doing math. When you calibrate, the sensor locks in the relationship between the mV it is reading from the cell and whatever O2 percentage you've told it you are using for calibration: e.g., the analyzer might calibrate by "locking in" the relationship that "12.5 mV = 21% O2". After that, the analyzer just does multiplication based on that ratio to convert mV to an O2% when you test gases: if 100% O2 generates 60 mV on calibration, then when you test a gas and the cell puts out 30 mV, the analyzer will report that as EAN50.

Thus the desirability of calibrating on 100% O2 when you have that ability. Any error in an air calibration -- failure to adjust for humidity, altitude, gas flow, etc. or just the fact that cells have some inherent error range -- is going to be multiplied by almost 5 when you analyze 100% O2. If you're off by 1% on air, you'll be off by about 5% on 100% O2.

Conversely, if you calibrate on 100% O2, any errors will be smaller when you analyzing gases with less O2 (about 0.2% on air, leaving aside the inherent small +/- error in cells). For recreational stuff, it's not that significant, but this is why for high O2 mixes, you are better off calibrating off 100% O2. It doesn't solve for linearity, but it is much better than calibrating on air and then trying to measure really rich mixes with accuracy.

Better analyzers allow two or three point calibrations and then the analyzer can correct for cell linearity, which is obviously even better.

87% is grossly off. Either the cell in the analyzer is current limited, the calibration was horrible, or you've got something other than 100% O2. No way I'd dive that cylinder until it is resolved.

 

[Lorenzoid]

. . .
Better analyzers allow two or three point calibrations and then the analyzer can correct for cell linearity, which is obviously even better. . . .

Which manufacturers offer analyzers with 2-point calibration? I'm sure they're pricey, but maybe worth a look.

 

[michael-fisch]

Which manufacturers offer analyzers with 2-point calibration? I'm sure they're pricey, but maybe worth a look.

Divesoft on their He/O2 Analyser.
Expensive but worth every penny!

Michael

 

[Lorenzoid]

Divesoft on their He/O2 Analyser.
Expensive but worth every penny!

Michael

I suspected trimix analyzers might have that feature, but what about O2 analyzers? Does it not exist because single-point calibration is "good enough"?

 

[rjack321]

Which manufacturers offer analyzers with 2-point calibration? I'm sure they're pricey, but maybe worth a look.

For diving, none
The main reason is because getting calibration gas is impractical. 100% is do-able. But getting 20.9% is actually harder than it might seem because 1) you have to either dry it to 0% humidity or 2) measure and account for humidity
On top of that, have to account for barometric pressure and flowrate.

Net it's not worth it for diving where plus minus 1% is all that is required.

 

[michael-fisch]

I suspected trimix analyzers might have that feature, but what about O2 analyzers? Does it not exist because single-point calibration is "good enough"?

I think you are right since O2 cells output a voltage that should be absolutely linear to the O2 percentage, if the cell is any good, made by a reputable manufacture, is reasonably new and hasn't been dropped. Then the analyser doesn't need expensive electronics and will work fine with a battery, 3.5 digit display, DVM chip, a potentiometer and the O2 cell. Total cost of parts without the case < $90.
Trimix analysers work using either delta-temp, or as in the case of the Divesoft using delta-speed of sound. Both methods require dedicated processors/programming code. With a few extra lines of code, they can implement 2 and 3 point calibration of the O2 sensor.

Michael

 

[jgttrey]

For diving, none
The main reason is because getting calibration gas is impractical. 100% is do-able. But getting 20.9% is actually harder than it might seem because 1) you have to either dry it to 0% humidity or 2) measure and account for humidity
On top of that, have to account for barometric pressure and flowrate.

Net it's not worth it for diving where plus minus 1% is all that is required.

The Divesoft analyzers offer 1, 2 or 3 point calibration. Three point uses air, 100% O2 and a third gas with 0% O2. The analyzer also measures barometric pressure and temperature and uses a flow limiter. It's pretty slick.

I agree that for most diving it is not required but extremely deep dives need a lot of accuracy (not that I'm doing those, just sayin').

I don't think it is particularly challenging to get the gasses to do a two or three point calibration. I don't think it is any more critical to get a perfect 20.9 when you are doing it as part of a three point calibration than if you were just calibrating off air as your only reference, probably less so.

All that said, I calibrate on 100% O2 when it is convenient and air when it is not. Occasionally, I'll do a two point calibration, but I just haven't found linearity to be a big deal. If I calibrate on 100% O2 and then watch it fall back down to 20.5 or 21.4 or something when it exposed to air, I'm plenty good with that for the kind of diving I'm doing. I also keep track of the raw mV readings so I can see if something is out of whack. If my analyzer measured 10 mV on air, I'd expect to see the mV on 100% at something very close to 50. If it showed 35 mV on 100%, that is a big red flag that it is current limited or just about there.

I know you already know this well, but paying attention to the cell mV also a good thing to do vis a vis cell replacement for a CCR. When the cell voltage drops 10% from what it was when you first put it in, it's probably time to think about replacing, even if it hasn't been a year. At least that's the expert recommendation from the guy that started Vandagraph. This is a pretty good reference: https://www.amazon.com/Oxygen-Measu...nt+for+divers&qid=1570041424&s=gateway&sr=8-1