BANANA PURPLE PUNCH AUTO

Calibrate them. Best to do it at the beginning of a grow and even mid way to ensure its operating correctly. typically the solutions are 4.01 + 7.01 + 10.01, in growing we really only need a two point calibration from 4.01 to 7.01, food industry uses the 10.1 and other solutions. Some units are automatic other you need to turn a screw to move the meter numbers up or down. Good Luck!