Palm O2 analyzer reading higher than fill station/other analyzers?

[Rearviewmirror]

Hi there.

I recently did my first nitrox trip and dove EANx for both days (five tanks total). For all of the dives, my Palm O2 analyzer read at around 33.1% O2, while the tanks had been labelled as 31.8% by the fill station, and were testing pretty close to 32% using the dive shop's own analyzer. I calibrated it each time and was seeing 20.9/21% on room air.

I was using the low pressure inflator hose attachment, and giving things about 45-60 seconds to stabilize, with the tank cracked open enough to get consistent flow.

Given this is my first time analyzing gas "in the field" post nitrox cert I was kind of surprised, and honestly not sure how to interpret those results. I set my computer to the higher reading of 33%, but I'm honestly kind of surprised as to why I'd be seeing quite a bit higher than both the fill station and the other analyzer. I bought the Palm from DGX and it had been tested etc.

Is this a common occurrence? Or do you think something is up with my analyzer? Any other thoughts?

Cheers!

 

[jborg]

Were you using the same inflator hose attachment for both analyzers? The analyzer is measuring ppO2, so it's sensitive to the actual pressure at the sensor...

I also think this sounds like a fairly small variance but I'm not experienced enough with different analysers to be sure. I know I often get a difference of about a percent between measuring with the club's analyzer at fill time and checking later at home. The sensors are also humidity and (somewhat) temperature sensitive. (They're supposed to be temperature corrected, but that might not be 100%.)

 

[Johnoly]

The sensors are also humidity and (somewhat) temperature sensitive.

Perfectly said......
A common mistake is the sensor is still warm from sitting in a drybag in the sun or hot car and the reading is shifting slightly. Best if everything (room temp, tank temp/sensor temp/calibration temp) is somewhat equalized and has been stable for atleast an hour or more.

 

[inquis]

Did you calibrate with the LPI inflator as well? You want the same flow rate for both cal and test for best results.

 

[Rearviewmirror]

Good questions! I'll answer below:

- No, the other analyzer was the plastic tube type where they put it directly up against the tank valve.
- The analyzer was sitting in a locker in the dive shop so was probably pretty close to room temp for the later tank checks (it was consistent across 5 tanks)
- I didn't calibrate off of the LP hose - I just calibrated it without flow as I would have had to ask to use a separate air tank/connected my BCD to use the LPI hose. Do other folks typically calibrate off a reference tank?

 

[parzdiver]

From my understanding, calibration should be done from a known tank, not relying on ambient air.

 

[inquis]

Ideally calibrate off a known tank because of humidity and flow rate differences. However, that may be tough on a typical charter for the reason you mentioned. (You may get lucky if someone else is not diving nitrox to ask if you can cal off their inflator.)

 

[Sevenrider860]

The other thing to consider about the Palm O2 Analyzer that is not part of the sales pitch.

The manufacturer states:

The devices are designed to meet the requirements for both ambient and elevated oxygen measurements but should NEVER be calibrated with air or 21% oxygen with the intent of taking oxygen measurements at oxygen levels above 30% oxygen.

The manufacturer does not certify this device to measure a 32% Nitrox fill if you are going to calibrate with air.

The manufacturer is also very specific about how to calibrate the device. "Calibrate with a known source of dry air, 21% or 100% oxygen". Waving it around in the air and pressing "CAL" is not a correct calibration.

With that said, I have the same analyzer. I replace the O2 sensor every 24 months, and I verify it with air and 100% O2 a few times each year to verify. When on a recreational dive trip I do calibrate it in ambient air like you did and I would have no concerns if I am reading 33.1% on my sensor when the tank is labeled 31.8%. I would set my computer to 33% and enjoy the dive. A 1.3% difference falls within the < 2% device accuracy that the manufacturer claims.

 

[Jcp2]

Analox has a table to adjust for humidity if using atmospheric air as the partial pressure of water can lower the calibration point by 1-2%. Maybe that’s the source of the difference.

 

[stepfen]

When on a recreational dive trip I do calibrate it in ambient air like you did and I would have no concerns if I am reading 33.1% on my sensor when the tank is labeled 31.8%. I would set my computer to 33% and enjoy the dive.

Same here, but in that case I would set my computer to 31% (lowest of the readings rounded down to the nearest integer). I choose to be on the safe(r) side of things.
In practice 1-2% doesn't make much of a difference since in any way we should avoid going near NDL/MOD especially for repeated dives.
YMMV