# Power/pressure washers: GPM & PSI - are they accurate, what is more important

Ed Pawlowski wrote:

Pressure is in the hoses and lines up to the nozzle, there is no pressure after the nozzle, only velocity.
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So when the water hits the surface, there is no pressure against it? You can hold your hand against it and feel nothing? Velocity is the rate of change of position, pressure is what you feel.
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Ed Pawlowski wrote:

There is force from the mass of the water impacting the surface at a high velocity, but this is not the same as the fluid pressure inside the hose.
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wrote in message

True, but that force, or pressure is what counts, not the pressure inside the hose. As you move the nozzle closer and farther from the target, the force is going to change. Even though the figure is given in psi, the hole in the nozzle is very small too, not an inch. Just as light falls off as you move to increase the distance between the light source and target.
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Some Guy wrote:

They are independent.
PSI = Cleaning power as it dictates the water velocity and thus cleaning force.
GPM = Cleaning efficiency as it dictates the volume of high pressure water and therefore the area that the cleaning force can be applied to. The smaller the area, the longer it will take to clean a given surface area, just like a 22" lawn mower vs. a 60".
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Pete C. wrote:

Well, that ain't exactly the whole story either...
The nozzle tip geometry has a great deal to do w/ the actual application of the water to the surface and the effect thereon for whatever the actual PSI/GPM ratings are...
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dpb wrote:

Yes, but most any given pressure washer can accept different nozzles, the max PSI and GPM available to drive the nozzle don't change.
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Another way of looking at these two washers is to make a more common electrical comparison. This comparison difference will hold true in pressure washers also.
2gpm @2000 psi would be represented by 2amps @2000 volts
4gpm @4000 psi would be represented by 4 amps @4000 volts
Power = Voltage x Amperage So........ The 2gpm washer equals 4000 units of cleaning power The 4gpm washer equals 16000 units of cleaning power
* units of cleaning power undefined * units of cleaning power are for reference/comparison only
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