Aqua-Macro Buffers Solution pH 4.01 are used for factory calibration. Regular use of these solutions is recommended to ensure specified instrument accuracy. Frequency of conductivity recalibration depends upon use, but once every month should be sufficient for an instrument used daily. pH models, depending upon use, should be re-calibrated with pH 7.00 Buffer every 1-2 weeks, and checked with pH 4.01 and/or 10.01 Buffers at similar intervals. pH Sensor Storage Solution is recommended for keeping the pH sensor wet.