This optical refractometer will let you measure the moisture content of your honey. Simply calibrate it with the included oil, add a drop of honey to the test window, and look through the refractometer at a light source to measure the water content of you honey. Honey that is less than 17.5% water qualifies as No.1 grade, and is not at risk of fermentation. Honey with higher moisture levels could ferment. Don't take chances, be sure that your honey will keep by measuring the moisture level with this easy to use refractometer.