The 15 ppm sensor of OWS (Oily Water Separator) is calibrated usually once a year. In this a Calibration liquid is supplied to 15 ppm sensor. If the display shows the ppm to be exactly same as the ppm of sample, sensor does not require any calibration / adjustment / replacement and OWS sensor is said to be performing well. Supply clean and distilled water into the sensor. The monitor should display 0.