Nitrate calibration solution or calibration standard set is needed to calibrate meters for extra precision. This Nitrate solution set is ideal for calibrating LAQUAtwin meters in two points (150 and 2000 ppm) before measurement. This set was designed for intensive testing and will last for 300 calibrations (for each concentration) of LAQUAtwin meters. We recommend calibrating your LAQUAtwin meters daily (if you measure daily) or after every 10 measurements within a single day if you are testing more intensively.