Abstract: A flux density control system controls the density of flux or other liquid by measuring the density and adding a thinner solvent or other liquid with a lower density than the flux. The apparatus includes a tank having an overflow adapted to maintain a set level of liquid in the tank, a liquid container positioned to receive liquid from the overflow in the tank, and a liquid pump adapted to pump liquid from the liquid container to the tank. A thinner container is provided with a thinner pump adapted to pump thinner from the thinner container to the tank. A density probe is positioned in the tank with a controller to turn off the liquid pump when the density of liquid in the tank rises above a predetermined set point, and turn on the thinner pump. The controller also turns off the thinner pump and turns on the liquid pump when the density of liquid in the tank drops below the predetermined set point.