The Difference Between Pressure and Flow

Of all the concepts that keep parts changers from becoming hydraulic troubleshooters, understanding the difference between pressure and flow is the most crucial.  A surprising number of technicians use the terms interchangeably.  An understanding of this one concept   Most people think that the pump is the source of the pressure.  If the pressure is low, their first inclination is to change the pump.  Unfortunately, this rarely works because the pump does not pump pressure.  The pump delivers a rate of flow.  That flow meets with (and hopefully overcomes) a resistance in the system and the result of this is pressure.  Therefore, what is read on the gauge is NOT the amount of pressure the pump is putting out – it is the amount of resistance that is currently being overcome in the system.  How many times have you seen a pump changed for no other reason than because the pressure was low?  The fact is that the system pressure, in and of itself is no indication whatsoever of the condition of the pump.  We need some other reason to replace the pump.

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