You weigh them when you start, and you weigh them as you go, and then you determine the percentage of weight lost. There is a certain percentage they should lose by lockdown. I don't do this (although I should) so I don't know the numbers offhand.This one does have a fan. I have a digital thermometer/humidity meter(?) at egg level. Maybe I should get a 2nd one just to be sure? I’m very interested in knowing how you can tell the humidity level by weight. I have an accurate food scale that I could use.