Les, with the dozens, perhaps hundreds that I've used with table grapes, watermellon, etc.. if you clean the two plates well with distilled water, then place a drop or two of distilled water onto the receiving plate you will, if you look towards light, see the "O" callibration. Simply move the marker to this. Perhaps you'll see the difference as blue and white, or perhaps another color. Blue representing sugar is what I've always seen. Contact me at [log in to unmask] anytime. Best Tim