Measuring Method : glass electrode (pH) Measuring Range HM-20P : pH0.00~14.00 HM-21P pH0.00~14.00, ORP 0~±1999mV Repeatability HM-20P : pH: ±0.02pH, HM-21P : pH: ±0.02pH, ORP: ±2mV Temperature Measurement range : 0~99.9oC Repeatability : ±0.2oC Temperature Compensation HM-20P : pH: auto/manual HM-21P : pH: auto/manual, ORP: Nil Calibration : pH: up to 3 points. Data Storage : 300 data points Auto Hold Function : provided Interval Function : provided (5 secs~99mins, 59secs.) Clock Function : provided Power Source HM-20P : AA size dry battery x 2pcs. HM-21P : AA size dry battery x 2pcs (or optional AC adapter) RS-232C Output HM-20P :Not available HM-21P : Provided Analogue Output HM-20P :Not available HM-21P : Provided Water proof : IP67 Ambient Temperature : 0~40oC Dimensions : Approx. 187.5(w) x 37.5(h) x 75(w) mm Weight : Approx. 300~310g