Calibrate your pen or meter once every 3rd time you test your water.New meters should be calibrated before you use it.They cali. it at the company,but how long has it been sitting in the warehouse and shipping can knock it off calibration.Vinegar will bring down the PH and Bakeing soda will bring up the PH.Dont touch the electrode the oils from your fingers degrade the electrode and will not give you the correct PH reading.Hope this helps.
PEACE