PSI vs. GPM, settled
Most people buy too much of one and not enough of the other. Here is how to tell which.
What each one does
PSI is pressure. It breaks the bond holding the contaminant to the surface.
GPM is flow. It carries the loosened contaminant away and off the work.
You need both, and they do not substitute for each other. A high-pressure, low-flow machine breaks everything loose and then leaves it sitting there while you make another pass.
The tell
If you are cleaning but it is slow — the surface comes clean where you point, and the job takes all day — you are short on flow.
If you are passing over an area repeatedly and it is not releasing at all, you are short on pressure, or you are short on the right chemical, or you need heat.
Most contractors we talk to are short on flow and are convinced they need more pressure.
Cleaning units, and their limits
PSI × GPM gives you a cleaning-units number, and it is a reasonable rough comparison between machines. It is not the whole story, because 4,000 PSI at 4 GPM and 2,000 PSI at 8 GPM produce the same number and behave nothing alike.
Use it to compare machines in the same general class. Do not use it to justify a machine with a badly lopsided balance.
What flow costs you elsewhere
More flow means more water. Check your supply before you buy — a great many well systems and older spigots will not deliver 8 GPM, and a starved pump cavitates itself to death. This is worth measuring rather than assuming.