Finally I have the solar sensor working properly. 
After being calibrated, I have compared the readings with another weather station and the readings are quite the same. 
As I have said:
I am doing a Pre-University research project about Sun that will punctuate me for the access to the university; and one part of this project will be record data of the solar radiation in my city. But it do not have to be very accurate.
So, is there some way to save the data recorded with WD?
I have seen that in “Advanced Graph” you can save the graphs of the last hours.
But I wonder if there is some other way apart from this to do that.
In summary: What are the possible methods to save the data acquired by the sensor? :?: