Protect You & Your Family's HealthIdeal for Household or Laboratory Use, Suitable for Testing Ph Balance of Drinking Water, Pool, Aquarium, Ro System, Spa, or Hydroponics; Appropriate for Wide Range of Applications. Instructions for use of PH calibration powder:Calibration powder: pH standard solid substance. It can be used after adding water. Suitable for the calibration of any type of ph meter. Each package can prepare 250ML standard solution.(Be sure to add water according to the required ratio, otherwise it will affect the accuracy of the calibration and increase the error) Step 1: Take 50ml of purified water or distilled water, and then import one pack of 6.86 calibration powder, stir evenly, put the PH pen probe in water for One Hours.Step 2: Then turn on the pen, press and hold the CAL button for five seconds, then let go, the value flashes, and the END calibration is completed.Except the ways above, you could long press the switch and calibration keys at the same time for five seconds, restore the factory settings, and then test.( The test difference might within 2% with reset control , and the powder calibration's test difference is within 0.1%-1% ) Please Note:Clean the electrode with distilled water every time before and after use.Don't touch the electrode by hard tools or fingers. Always replace the protective cap to keep electrode from drying out.Please do not immerse the meter into the solution over the "warning line", to avoid damage to the circuit board.When the meter cannot be turned on or the display fades, open the battery case and replace battery (pay attention to the polarity), then need to recalibrate the meter.