The two metal plates on your Kelway may look clean, but in order to obtain the most accurate results, they must be chemically cleaned using Kelway Conditioning Film. Before using the meter, place the Kelway Conditioning Film around plates, squeeze lightly, and rotate a few times. Then wipe plates with a clean rag or paper towel. Be sure not to touch plates with your fingers or conditioning must be repeated. One sheet is good for apx. 20 uses. Sold by the package of 10.