5 1/2" Diameter Round Pine Sauna Thermometer. A thermometer is a necessary accessory in the Sauna as it is important to know the correct temperature. After heating your Sauna room for at least thirty minutes, an average temperature would be about 180°F. Hang your thermometer 6" below the ceiling directly above your Sauna heater. Another useful item is the hygrometer. It will indicate the Sauna humidity, which usually ranges from 10 to 35%, depending upon the amount of water used on the stones.