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.?
Yes, bad power supplies are responsible for most sensor failures. The sensors are indeed of good quality, the wall warts supplied are super cheap.
Do you have access to an oscilloscope to check your power supply?
What levels of ozone does your unit show when you used batteries?
A dose of 30 to 50 ppb is already considered to be harmful to humans. WHO 2021 global air quality guidelines
Ozone is nothing to do with pollution like nitrogen or carbon oxides.
Concentration can be high on warm and sunny days.
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.