After three weeks of humidity measurement, Azure data shows the following behavior:

I expected to see straight lines for sensors blue and green as they are in test tubes.
The measurement is not random though because:
1 - The fully immerged sensor (blue) gives higher values than the half immerged (green).
2 - Green and blue follow exactly the same pattern
3 - Red sensor which is in a pot, shows similarities in its evolution.

I copied local temperatures history from the same period from the Web and compared the two graphs on a day per day basis:


It is obvious that my "sensor in a test tube" is a pretty good thermometer!! 
In order to know the humidity of my pots, I need to correlate sensors information with local temperature and add some filtering mechanisms.
I added a DHT11 Temperature and Humidity sensor to the Azure Watering system (another option would be to get it from the cloud...)

Now my Azure watering system has been running for a few weeks and I can use collected data to undestand how it behaves.
The data on Azure lead to the following graphs:

The big watering bars correspond to manual watering, from this graph I understand several key points:
1 - Humidity level should be maintained between 50 and 300
2 - Cooler temperatures in November have minimized evaporation ...or plants consumption
3 - The humidity variation during a day may be important compared to the interval given before
4 - All humidity sensors seem to follow an identical pattern, but my pots are not identical!!

To know more about humidity Sensors, i will modify the configuration, two sensors will be installed in test tubes, one completely immerged and the other one half immerged .

With this 100% stable configuration and Azure data recording, I expect a clarification on humidity measurement.