I am receiving strings from Arduino and extract them to numbers in App inventor. That part works fine, the app displays the numbers correctly. Now I want a warning if a value goes under, let's say 200. This block does not work, if I understand correctly numbers in text is treated as numbers in App inventor, or am I wrong?
To help, we would need to know exactly what FL_Set contains. It can't be anything but numbers.
You could use the is a number block to check, if your value really is numeric
http://ai2.appinventor.mit.edu/reference/blocks/math.html#isnumber
Taifun
Thanks guys, I misunderstood it. That explains. It is a string sent from Arduino via serial, so it is a string but not a number, How do I convert a string to a number in app inventor?
One way, multiply the string by 1 
A number in app invetor can be represented as a string, but this string cannot contain any hidden characters. So you have to make sure that the string does not contain anything but numbers. There can be no spaces, no line breaks, etc....it is best to display in a label what the received string contains for debugging purposes.
Thank you for your suggestion, but it made no difference. I decided to do it differently, from Arduino but it would still be interesting to know if it´s possible