My nat geo digital hygro/thermometer also stopped working yesterday. I took it apart and it was all rusted inside and the battery was corroded. Apparently, despite saying it measures humidity up to 90%, it's not made to stay in humid environments. I will buy another one, but will put it in to check things and then pull it back out, I left mine in 24/7. I guess I shouldn't have spent so much money yesterday setting up a betta!