Hi Zoltan,
I also used concrete blocks in the start for testing, but they can contain water and you place them in an environment where you are not in control of the humidity, temperature, wind etc. it is difficult to say something about the actual weight.
So to conclude that the weight cell is bad is probably not fair. But the graph for the aluminum sheets looks ok - taking into account dew that will collect on any surface during the night and evaporating during the morning/day.
Try to add a beehive with bees, you will see that you can get a lot of information from the weight readings - you will not need accurate readings - something like ±100/200 grams is fine.
For your info: I have supported this kickstarter project (hope I get a product) - https://www.kickstarter.com/projects/hiive/hiive-link?ref=discovery&term=hiive%20link&total_hits=1&category_id=337
Also to compare how well the homemade system compares to an commercial product.
br,
Jesper