Pressure washing or power washing is the use of high-pressure water spray to remove loose paint, mould, grime, dust, mud, and dirt from surfaces and objects such as buildings, vehicles and concrete surfaces. The volume of a mechanical pressure washer is measured in gallons or litres per minute, which is usually fixed and cannot be changed. The pump’s pressure, which is measured in pounds per square inch, pascals, or bar, is fixed but can be adjusted by altering the unloader valve. There are machines that can produce pressures ranging from 750 to 30,000 psi (5 to 200 MPa) or higher. In many situations, the phrases pressure washing and power washing are used interchangeably, and whether they are genuinely different techniques is a point of contention.