This digital pen style pH meter is an ideal instrument for any aquarium, the fishing industry, swimming pools, school laboratories, food & beverage etc. Please be sure to calibrate with buffer solution before use. Calibration: -- Immerse the tester up to the maximum immersion level in the PH 4 or 6.86 buffer solution. (Package Includes 2 pack of PH substance which makes 250 ml buffer solutions, PH 4 or PH 6.86).-- Allow the reading to stablilize and using the small screwdriver adjust the calibration trimmer to reading 6.9