Quote: (Originally Posted by
Gibbon)
I noted that Alex's ORR project has gone with cells that do not contain this circuit?????
If I may explain the reason why, as it may help on this thread.
As part of a safety case, we did a 5 year study of O2 sensors. We identified 14 failure modes. The test plan is attached so you see the rigour applied when it came to testing the sensors themselves. We liaised closely with several O2 sensor manufacturers to ensure we understood the sensor and its failures very well.
The only failure modes that give rise to a higher voltage than the correct voltage, in a sensor of appropriate design, is if the temperature compensation circuit fails. By eliminating that failure point, and chosing the right sensor construction, we know that when an O2 sensor fails, the output is lower than it should be. This means we can take the highest reading O2 value as being the correct one.
Failures in temperature compensation circuits can be sudden, or from the outset. None of the Teledyne sensors we tested had temperature compensation working to spec. The error of a completely failed temperature compensation circuit can be as high as 25%.
All galvanic O2 sensor produce a charge, which turns into a voltage when it is loaded with a resistor, usually about 270 Ohms (so do not waste time debating whether a 10K load or 100K load is best). The load should be as small as possible, from the point of view of the sensor manufacturer.
The smaller the load, the better, but the smaller the output voltage.
In the DL Open Revolution submission, we use cells that have 100 Ohm (1% tolerance) load internally, produce 4.5mV in air, and have a SMB male connector on them. We calibrate in air. This is an AII sensor: their quality is head and shoulders above all others we tested.
We check if the source is 100 Ohms by putting a small current via a 100K resistor onto it, from a waveform generated by a DAC. We also check the voltage level. We chose 100 Ohms and the low voltage to prevent someone plugging in compensated sensor from an Evolution by mistake, which has the same connector.
The unit has lots of very accurate temperature sensors, so the temperature compensation is performed digitally.
Cheers,
Alex