Quote: (Originally Posted by
Janos)
Question: Have you never found it to be more than 25%? Eg, if I have a ppO2 reading of 1.2, can I be sure that worse case is all three sensors have failed high and it should be 1.5? (1.2 * 125%)
Janos
In fact, the output voltage can go up indefinitely if the load fails. That is another reason we check the load inside the cell is there.
If the temperature compensation circuit fails such that there is no load, the output voltage from the cell will rise and rise, because the charge generated by the sensor will have nowhere to go. That is, until it causes the input protection circuitry on the ADC to fail due to the overvoltage.
Thanks for asking about that point. I should have been clearer in my original email.
On the maximum that it can be if the load is there: it does depend on temperature. Assuming calibration is at 20C, and the maximum temperature 50C (the cells are not specified to work above 50C by manufacturers), then the worst you should see is 25%, based on our empirical results.
I should add, we did quite a lot of testing on big sheets of the membrane used in O2 cells. It is Zitex A-105 or G-104 (numbers from memory, so hope I have not swapped them). There are a few other membranes also used which we tested, such from those from GE, but all are fairly similar. The response time of the sensor depends on the thickness of the membrane. These membranes are not impervious to water: one can force water through them with quite low pressure. The KOH electrolyte inside the sensor is a very strong alkaline, and if diluted, the output from the cell will fall in value. The arrangements to keep water off the face are important factors in deciding how much water vapour gets through. It is very hard to breathe through these membrane sheets: they resist gas, but resist water about 50 times more. You can look up the water and air resistance figures on the web by Googling "Zitex A-105".
Alex