Abstract: An ultrafiltration cell utilizing two membranes with a magnetic stirring bar system in between. This permits stacking of stirred cells and doubles the efficiency of each cell. Automatic stop to prevent drying is accomplished by employing a floating valve. When using compressed gas for pressure filtration, the floating valve sinks when empty and the valve mechanically stops gas pressure. When an electric motor pump is used, the floating valve keeps a switch on by pressing it upwards and shuts it off when it sinks. The switch could be magnetically or mechanically operated by the floating valve.