OK – I tried running the unit on batteries and that problem went away. I will need to filter my DC supply better! I am surprised that the internal voltage regulator in the unit does not remove this AC ripple.

Kind of a different question: why does this unit show ozone levels in the warning zone when it is in some really fresh air – near a forest and away from major highways, etc.?

Ken

The unit is fed from a 24 volt supply and POE injector/splitter.
Actual voltage arriving at uRad is about 20 volts when using the 24 volt supply.

The center portion of this graph is with the 18-volt battery supply (3×6.3).
The right hand side is back on the 24 volt supply with some added filtering – I can do more.
I can also try lowering the voltage, as I found everything else on the system works at 18 volts.
You can see the readings average about the same values with either supply.

I an quite sure the O3 sensor is responding to other compounds in the air. I found it is quite sensitive to alcohol and the forest here emits natural compounds that are chemically similar to alcohol. The second photo shows the month of September. Readings are rarely below 100 ppb and tend to show a peak right after it rains, when soil bacteria naturally emit a compound that the O3 sensor seems to detect quite well.

Ken

I have been watching the O3 with the new power supply and it’s quite the same. It is just that – during the daytime – there is a lot of activity that covers up the tendency to “stick” around 120 ppb. Since this sensor is designed to measure PPM, it is probably unrealistic to ask it for readings in PPB range. The resolution is 10 ppb and I think the zero point could easily drift by 100 PPB.

I would be nice if there was a way to set the zero calibration in software. In my case, zero gas is giving a reading of around 100 PPB (and has always been so).

If replacing the sensor with another one from China would be certain to fix the problem, I would be willing to give that a try.

Ken