 Pneumatics is a branch of engineering that makes use of gas or pressurized air. Pneumatic systems used in industry are commonly powered by compressed air or compressed inert gases. The centrally located and electrically powered compressor powers cylinders, air motors, and other pneumatic devices. A pneumatic system controlled through manual or automatic solenoid valves is selected when it provides a lower cost, more flexible, or safer alternative to electric motors and actuators. Pneumatics also has applications in dentistry, construction, mining, and other arias. Pneumatic systems in fixed installations, such as factories, use compressed air because a sustainable supply can be made by compressing atmospheric air. The air usually has moisture removed, and a small quantity of oil is added at the compressor to prevent corrosion and the briquette mechanical components. Factory-plumge pneumatic power users need not worry about poisonous leakage, as the gas is usually just air. Smaller stand-alone systems can use other compressed gases that present an asphyxiation hazard, such as nitrogen often referred to as the WFN oxygen-free nitrogen when supplied in cylinders. Any compressed gas other than air is an asphyxiation hazard including nitrogen, which makes up 78% of air. Compressed oxygen approximate. 21% of air would not asphyxiate, but is not used in pneumatically powered devices because it is a fire hazard, more expensive, and offers no performance advantage over air. Portable pneumatic tools and small vehicles, such as robot wars machines and other hobbyist applications are often powered by compressed carbon dioxide, because containers designed to hold it such as soda stream canisters and fire extinguishers are readily available, and the phase change between liquid and gas makes it possible. To obtain a larger volume of compressed gas from a lighter container than compressed air requires, carbon dioxide is an asphyxiant and can be a freezing hazard if vented improperly.