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Pressure Washer Terminology

PSI (pounds per inch)

This indicates the amount of pressure the unit will create if concentrated on one square inch of surface area. The greater the PSI, the better the cleaning efficiency.

GPM (gallons per minute)

This tells you how much water (gallons) will flow through your pressure washer (flow rate). A higher flow rate will cut down on your cleaning time.

PSI versus GPM

PSI and GPM are both equally important when choosing a pressure washer. The PSI indicates the amount of pressure and GPM indicates the flow. You must have the correct combination of PSI and GPM to have the most ideal pressure washing system for your needs.

CU (cleaning unit)

This is calculated by multiplying PSI by GPM. The number is useful for comparing models. The higher the number of cleaning units, the more cleaning power your pressure washer has.


Pressure washer pumps are driven by the motor or engine and push the water through your pressure washer. The two types of pumps are direct drive and belt drive.


The actual pressure is created by the nozzle. Different nozzles sizes (orifices) are available to create the spray angle or pressure needed to fit your cleaning need. The wider the spray, the less cleaning power. The narrower the spray, the more cleaning power.

Unloaders and Relief Valves

Pressure washers are equipped with safety features designed to reduce pressure buildup. Unloaders and thermal relief valves prevent heat and water pressure from reaching dangerous levels.

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Chappell Supply & Equipment
6509 W. Reno Ave
Oklahoma City, OK 73127
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PO Box #270960
Oklahoma City, OK 73137

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