A pressure washer or power washer is a high-pressure mechanical sprayer used to remove loose paint, mold, grime, dust, mud, and dirt from surfaces and objects such as buildings, vehicles and concrete surfaces.
The volume of a pressure washer is expressed in gallons or liters per minute, often designed into the pump and not variable. The pressure, expressed in pounds per square inch, pascals, or bar (deprecated but in common usage), is designed into the pump but can be varied by adjusting the unloader valve. Machines that produce pressures from 750 to 30,000 psi (5 to 200 MPa) or more are available.
Washers are dangerous tools and should be operated with due regard to safety instructions. The water pressure near the nozzle is powerful enough to strip flesh from bone. Particles in the water supply are ejected from the nozzle at great velocities. The cleaning process can propel objects dislodged from the surface being cleaned, also at great velocities. Pressure washers have a tendency to break up tarmac if aimed directly at it, due to high-pressure water entering cracks and voids in the surface.
At extreme high pressure, water is used in many industrial cleaning applications requiring the removal of surface coatings and for dust-free cutting of some metals and concrete. For exterior applications, gas powered washers provide greater mobility than electric ones, as they do not require use in proximity to an electrical outlet. For indoor applications, electric washers produce no exhaust and are much quieter than gas or propane washers.