Thanks to the team from MeteoWare, their MeteoWare Plus service is working without producing interruptions of the Wunderground Current Conditions anymore. With my existing free WU Key I can use my own Netatmo PWS as input source for the OpenSprinkler watering adjustments. This allows for perfect local temperature, humidity and precipitation measurements.
I hope that WUnderground is not cancelling the API service for existing free API key owners in the near future?
What I can say is, that the Zimmerman method with its adjustments for OpenSprinkler is quite efficient in regards to calculating the watering times needed for a sprinkler system. It incorporates historical weather data from the day before and actual precipitation data for today. This results in predictable watering times for the ongoing day. If an acceptable access to local weather data can be provided, there is also no really need for the installation of an additional rain sensor. The water level is automatically set to 0% if it is raining.
I like to mention that any alternative to WUnderground should take care for using the Zimmerman method with OpenSprinkler in the way it is implemented right now – preferably with the same functionality.
Thanks and regards