Abstract: An improved water pressurizer comprises a pair of piston and plunger type intensifiers each driven by pressurized oil on one side of the piston and returned to the starting position by air pressure on the opposite side of the piston. Magnetic sensing switches detect proximity of the piston as it nears the end of a power or return stroke and send enabling signals to electropneumatic relays controlling air actuated ball valves on input and output oil lines. Because of the displacement volume and piston to plunger ratio, the intensifiers reciprocate at unusually low frequencies, reducing down time for maintenance or repair and thereby increasing reliability and capacity factor. Installed on a trailer for transportation, the apparatus can be attached to any available water supply for providing pressurized working fluid to a remote site.