Meily(TM) Air Ion Counter Tester Meter test negative Ions with peak maximum hold LCD Display: Max reading of 1999. Display "-" means the ions are negative ions. Test range is between 1x10000 to 1999x10000 ions/cc Attention: don't touch the rear square metal of the tester when testing This Air Ion Tester is designed for test high level concentration of negative air ion of air ion generator. With it you can estimate whether the air ion generator or air purifier is working in good state. The reading of the tester is correlative to the air ion concentration, but not exactly the number of it. In factor, this tester can only provide you a rough measurement to the air ion source. Please don't take it as an precision instrument