Digital ORP Meter Pen Type Redox 999mV Digital Water Treatment Tester Measuring Range : -999mV ~ +999mV Accuracy : 4mV ; Resolution : 1mV Operating Temp : 0~60C (32~140F) ; Storage Temp : -10~60C (14~140F) Operating Humidity : &lt;80%RH ; Storage Humidity : &lt;90% Battery: 4 x LR44 buttons batteries (included and installed in the main unit)