What the....? (analyzer question)

[Doc Harry]

I'm getting some funny results with my brand-new O2 analyzer (won't tell you the brand name). Maybe someone could give a clue about what's happening.

First, I calibrate on air to 20.9%. Then I crack the valve and analyze my compressed air tank. It reads 21.6%. Hmmmm...... Bad detector?

So then I took the thing to work. Calibrated on room air to 20.9%, plugged in pure oxygen and it reads....100.0%.

Then I calibrated to 100.0% on oxygen and disconnected the oxygen. The analyzer read..... 20.9% on room air.

The I re-checked my compressed air tank and it reads..... 21.6%.

Okay, is this humidity issue? A compressor issue? An analyzer issue????

WTFO???

 

[Somethin-Fishey]

Maybe your tank has 21.6 in it??

 

[Web Monkey]

Your tank probably contains 21.6%.

Terry

I'm getting some funny results with my brand-new O2 analyzer (won't tell you the brand name). Maybe someone could give a clue about what's happening.

First, I calibrate on air to 20.9%. Then I crack the valve and analyze my compressed air tank. It reads 21.6%. Hmmmm...... Bad detector?

So then I took the thing to work. Calibrated on room air to 20.9%, plugged in pure oxygen and it reads....100.0%.

Then I calibrated to 100.0% on oxygen and disconnected the oxygen. The analyzer read..... 20.9% on room air.

The I re-checked my compressed air tank and it reads..... 21.6%.

Okay, is this humidity issue? A compressor issue? An analyzer issue????

WTFO???

 

[Charlie99]

Humidity will affect the reading. One of the analyzers manufacturers had a calibration table that showed how much, but I can't seem to find it. The sort of error you are seeing is around the amounts I recall for high humidity.

There is also another potential error caused by a slight increase in pressure at the sensor when you are using the tank. You can easily eliminate that as a possibility by purging the dry air of the tank into a big plastic baggy, and then using the sensor to measure it.

Just remember that what an O2 analyzer is really measuring is the partial pressure of O2, not the percentage or fraction of O2.

 

[Diver0001]

Humidity will affect the reading. One of the analyzers manufacturers had a calibration table that showed how much, but I can't seem to find it. The sort of error you are seeing is around the amounts I recall for high humidity.

There is also another potential error caused by a slight increase in pressure at the sensor when you are using the tank. You can easily eliminate that as a possibility by purging the dry air of the tank into a big plastic baggy, and then using the sensor to measure it.

Just remember that what an O2 analyzer is really measuring is the partial pressure of O2, not the percentage or fraction of O2.

You mean to say that high humidity would cause higher readings or high humidity would cause lower readings? The air in his tank should be as dry as a rattlesnake's backside.

R..

 

[Charlie99]

You mean to say that high humidity would cause higher readings or high humidity would cause lower readings? The air in his tank should be as dry as a rattlesnake's backside.

R..

Right. The high humidity of the ambient air has a lower FO2/ppO2 than standard dry air. If you adjust the analyzer to read 20.9% in this low FO2 air, then when you test the dry air of the tank, it will read high.

 

[Charlie99]

I don't have the table handy, but some back of the envelope calculations:

ppH2O at 25C is 25mm Hg, at 37C is 47mm Hg. 1 ata = 760mm Hg

So at 100% RH, 25C H2O vapor is 25/760= about 3.3% of the total pressure, which results in ppO2 in the humid air being 3.3% lower. 3.3% lower ppO2 will show up as 3.3% * 21 = 0.7% error in the "% O2" scale of the meter.

That is pretty close to the 3.3% gain error of 21.6% vs. 20.9%. (0.7% difference/21%= 0.033 = 3.3%)

 

[TX101]

I wouldn't worry about it too much as most sensors are +/- 1% - which shouldn't make much difference to your dive planning.

 

[roakey]

This is why you should always calibrate against a cylinder of air, not ambient air.

Roak

 

[boomx5]

I would also check the tank contents with another analyzer. I'm not sure of how (or if) your LDS blends gas, but if your LDS blends gas on the same panel they give air fills, then it's always a good practice to check your tank contents even if there is supposed to be air in it.