Oxygen sensors — formulas to check linearity with an ambient air and an oxygen reading

[inquis]

3H on the ChOptima is on the monitor circuit and I suspect there is a different resistance than on the controller circuit.

Doesn't matter since that resistance, whatever it is, is the same for both ambient and O2.

Ambient pressure is from my Garmin running watch so it is quite accurate.

We're those example measurements -- including the ambient pressure -- at sea level or near Denver?

 

[J-Vo]

Maybe I missed it in some other comments, but you should be deviding your air measure by FO2 (20.9%) not PPO2 to calc your expected linearity to 100%. Altitude is not relevant.

 

[LFMarm]

Doesn't matter since that resistance, whatever it is, is the same for both ambient and O2

Indeed. I use the same scaling factor regardless of the pO2.

We're those example measurements -- including the ambient pressure -- at sea level or near Denver?

Sea level

 

[LFMarm]

Maybe I missed it in some other comments, but you should be deviding your air measure by FO2 (20.9%) not PPO2 to calc your expected linearity to 100%. Altitude is not relevant.

Why isn’t altitude relevant? Readings from the sensor depend on pO2 and pO2 of air depends on ambient pressure. Or I am missing something?

 

[inquis]

I use the same scaling factor regardless of the pO2.

But not the same factor (or calculation therefore) for all cells, which is what I find a bit odd. Again, it doesn't impact the linearity assessment, it's only that a cut& paste of the Constant formula for cell 1, 2, or 3C into the cell for 3H does not give the numbers you show.

 

[inquis]

Altitude is not relevant.

For any particular day, that's true. @LFMarm, if you change the pressure, you'll see the %dev doesn't change. It becomes relevant only if trying to compare across days.

 

[J-Vo]

Why isn’t altitude relevant? Readings from the sensor depend on pO2 and pO2 of air depends on ambient pressure. Or I am missing something?

The scaling factor from air to oxygen is based on relative FO2, so for linearity checks you always use 20.9%. As @inquis says you do have to adjust for relative ambient pressure when comparing historical measurements.

 

[LFMarm]

For any particular day, that's true. @LFMarm, if you change the pressure, you'll see the %dev doesn't change. It becomes relevant only if trying to compare across days.

Got your point. I track daily readings over each trip to detect weird behaviors of sensors

 

[justinthedeeps]

Damn just noticed a dozen more columns I need to put in my divelog spreadsheet!

 

[justinthedeeps]

Hi all,

I am trying to do a better job at monitoring my O2 sensors and have been measuring every time I calibrate both the ambient air reading and the pure oxygen reading at ambient pressure. I am then calculating the mV constant from both readings and checking the percent difference as a quick measure of linearity in the field.

View attachment 852851​

Here are the formulas I am using:

• Constant based on ambient [mV] = Ambient [mV] / Ambient pO2 [bar]; e.g., 58.2 = 11.9/0.204
• Constant based on O2 [mV] = O2 [mV] / Ambient pressure [bar]; e.g., 55.6 = 55.4/0.997

I am surprised that even for new sensors I get about 4% difference (while I understand from the manufacutere specs it should be <2%). I suspect I am doing something wrong with the math and hence looking for your kind feedback on my calculation.

Attaching here the table in case anybody wants to play with the numbers:

Ambient [mV]	O2 [mV]	Ambient	Constant based on ambient [mV]	Constant based on O2 [mV]	Δ [mV]
1	2	3C	3H	1	2	3C	3H	Pressure [bar]	Amb. fO2 air [%]	pO2 [bar]	1	2	3C	3H	1	2	3C	3H	1	2	3C	3H
11.9	10.7	12.6	11.8	55.4	50.1	58.7	53.4	0.997	20.50%	0.204	58.2	52.4	61.7	56.4	55.6	50.3	58.9	52.3	4.8%	4.2%	4.7%	7.8%
12.0	10.8	12.6	11.9	57.0	50.0	59.0	56.0	1.000	20.51%	0.205	58.5	52.7	61.4	56.7	57.0	50.0	59.0	54.7	2.7%	5.3%	4.1%	3.6%
11.9	10.8	12.6	11.9	55.6	50.5	59.0	53.8	0.999	20.55%	0.205	58.0	52.6	61.4	56.6	55.7	50.6	59.1	52.6	4.2%	4.1%	3.9%	7.6%
12.0	10.8	12.6	11.9	55.3	50.4	58.8	53.9	0.998	20.52%	0.205	58.6	52.7	61.5	56.7	55.4	50.5	58.9	52.7	5.7%	4.4%	4.4%	7.6%

This is cool QC, but also a pretty clear demonstration of what normal cells should look like? I thought they all (or most) curved off just like like this, rather than being completely linear?

Since we are diving in target ranges between 0.7 to 1.6 ppO2, and mostly in the 1.0 to 1.3 range, what might matter more than linearity to down to FO2 0.21 is what the mVs and curve look like in more relevant ranges.

Having valid, expected, *responsive* mVs at 0.21 & 1.00 (within spec) and approximate linearity still matters, but if we do get acceptable mV's at FO2 ~1.00, we are not going to be critically wrong at 0.7 or 1.3--even if the curve from FO2 0.21 is noticeably non-linear. More maths would probably show that even with a substantial curve-off from 0.21, the error from 1.0 to 1.3 is going to be acceptably small.

I wouldn't begin to worry unless the mV's were out of spec, or a sensor was old, inconsistent or slow. ?