Various types of pumps can be used for supercritical fluid applications. For medium to large volume processes, a pneumatic booster pump is most often used. A diaphragm pushes against a piston to compress the liquid carbon dioxide to a set pressure point. The air that drives the pump increases the liquid carbon dioxide pressure (boosts) in a ratio of about 100 to 1. So, for every 1 psi of air delivered to the pump, the carbon dioxide pressure is boosted by 100 psi (i.e. air at 50 psi will deliver about 5,000 psi of carbon dioxide). The CO2 pressure is controlled by an air regulator which in turn controls the pump operation. Once the desired pressure is selected, the pump pressurizes the overall system to this set point. When the restrictor valve is opened, the pump will continue to actuate to maintain the desired set point.