Music Nomad MN305 HumiReader Hygrometer, Humidity, and Temperature Monitor Hygrometer comes with a long-lasting AG13 battery. AG13/LR44 batteries required (best with Polaroid AG13/LR44 batteries) Accurate sensor measures humidity and temperature every 20 seconds Innovative 3-in-1 design for sound hole, case, or room Hang from strings to monitor sound hole or use Case Holster to securely use in instrument case Reads Fahrenheit or Celsius and displays maximum and minimum levels