1.5-volt Pen Batteries come four per pack and power most pH, EC and TDS pens. This refractometer measures the sugar content of plant material directly derived from photosynthesis and reads in Brix. The user squeezes plant juices onto the refractometer's reading lens and then the user looks through the meter. A scale is observed known as the Brix scale. The higher the Brix reading on the scale the more sugar content in the plant juices. This allows the grower to record data on the plant and confirm that more or less photosynthesis has occurred since the last reading. Things that can change Brix reading are: the spraying of minerals/vitamins/enzymes or hormones onto the leaf, the adding of specialized nutrients to the growing medium, the reduction or addition of humidity or the simple addition or reduction of more light to the plant leaves. It is truly great tool to own.