When to discard analyzer O2 sensor?

[Pao]

I can't seem to find any info on the subject for analyzers, most are recommendation for rebreathers.
I am doing a project on an Arduino based O2 analyzer. I would like to know if there is a cutoff voltage value (in air) for the sensor to be considered "bad" and lose its linearity significantly. I would like to incorporate a warning into the code when the sensor might be failing or has failed. Thanks in advance.

 

[Bobby]

With a single data point, air, you can't determine the sensors linear deviation. The best way to know is to use air and pure O2 to determine linear deviation and/or limiting. I've seen sensors with a very low mv reading that are near perfectly linear and I've seen sensors very close to their specified mv in air that have as much as 15% linear deviation.

 

[Pao]

With a single data point, air, you can't determine the sensors linear deviation. The best way to know is to use air and pure O2 to determine linear deviation and/or limiting. I've seen sensors with a very low mv reading that are near perfectly linear and I've seen sensors very close to their specified mv in air that have as much as 15% linear deviation.

I think I can incorporate that into the code. So what is "acceptable" loss of linearity?

 

[Chris H]

Narked@90 has a spreadsheet that is used with their cell checkers. It may shed more light in the linearity question, but it also requires many data points. I have a copy of the spreadsheet, but I don't know that it is mine to distribute. I don't see it on the Narked@90 website right now.

 

[Pao]

Narked@90 has a spreadsheet that is used with their cell checkers. It may shed more light in the linearity question, but it also requires many data points. I have a copy of the spreadsheet, but I don't know that it is mine to distribute. I don't see it on the Narked@90 website right now.

Like how many? I Will 20.9, 32, 50 & 100 suffice? What is their percentage deviation threshold?

 

[michael-fisch]

Like how many? I Will 20.9, 32, 50 & 100 suffice? What is their percentage deviation threshold?

Kind of difficult with those values, Air is usually around 20.9% but in a location with lots of trees and forest can easily be a half percent higher. As far as 100% goes, good luck, medical O2 is specified to 2.5 meaning that it has to be at least 99.5% O2, welding grade O2 has to meet the same purity standards as medical grade. So finding 100% O2 is going to be determined by how lucky you are unless you are willing to pay for reference grades of O2 with impuritys specified as under 0.1ppm

Calibrate with O2 to 100%, check it with air, then calibrate with air and check it with O2 - this will give you a good idea if your sensor should be used for measuring O2 percentages. If you have a pressure pot with a calibrated digital pressure gauge you can slowly fill it with air and measure every bar untill your sensor shows 100% O2. This will give you a good idea of the sensor's linearity in the 20,9% to 100% range.
rebreather divers using pressure pots fill them with O2 and measure linearity from 1.0 PPo2 to 2.0 PPO2 which is not a range that will ever show up while mixing gas.

Michael

 

[tmassey]

Isn’t this a question the sensor makers would have a much better handle on? We only know it from the user side, and for a fairly odd use to begin with. The sensor makers are going to have a much better handle on this.

Have you looked up the specifications for a particular sensor, say an R-17? This might give you a lot more information. I know when I was looking for some specific information regarding voltages and capacity, Teledyne had a decent amount of information about their sensors. As did Maxtec, though I wasn’t using their sensor.

 

[fullytek]

A sensor is considered bad when it's no longer able to achieve max MV in 100 percent o2.

A bad sensor may calibrate in air fine or may measure 32%or 40 but you will find a threshold where the sensor more or less stalls. Let's say that's 44 mv, that might represent 75 percent o2 then you move to pure o2 but the sensor still will not exceed 44 mv and the display will show 75%

The values I provided are purely for descussion you will need to consult with the sensor manufacturer for proper working range as diffrent models will have diffrent output

 

[michael-fisch]

Many rebreather divers, after replacing their RB sensors, then use the sensors for another year or so in their analyser(s).
Reason is a RB sensor is useless if it doesn't work well at 1.6Bar O2, but in most cases it isn't yet current limited at lower PO2s.
Asking a RB diver nicely for 1 or 2 of the old sensors that he would otherwise throw away, is a good idea and as long as you calibrate and check often (every couple of weeks) it's a way of getting free O2 sensors that will usually work fine for another year.

Michael

 

[Bobby]

There are two common failures for O2 sensors. Linear deviation and limiting. For your application limiting is a concern if it reaches the point that it will affect the percentage of O2 content to be analysed. With linear deviation the amount of deviation will affect any O2 percentage being measured. For your application you need to test for both limiting and linear deviation and decide how much deviation is acceptable. My personal comfort level with deviation will depend on the the application (dive profile). Keep in mind that we already allow for a variance of 0.1 on either side of output for measuring inaccuracy, any linear deviation from there will increase the variance. I calculate in mv, when calculating in PO2 there is less accuracy.