Hens go broody when you don’t want them to… and won’t go broody when you do. 
I've had better success with testing/calibrating with a medical thermometer:
https://www.backyardchickens.com/ar...incubator-thermometers-and-hygrometers.73634/
https://www.backyardchickens.com/ar...incubator-thermometers-and-hygrometers.73634/
