One of the first things I struggled with when I started reading water cooling reviews were the pressure/flow plots.  Essentially there are two types, one for blocks/radiators/parts and one for pumps.  The axis are the same on both.  The y axis normally measures pressure change across the part under test (measured in PSI or somewhat more confusingly in distance).  The x axis is normally flow (measured in gallons per minute (GPM) or litres per minute, or sometimes litres per hour).

Here is a typical “restriction” plot:

In a restriction plot the pressure drop across a part normally increases as the flow rate increases.  It normally increases in a non linear fashion.  An analogy to help with this could be thinking about pedaling your bike.  Pedalling a bike at 10mph takes only a little amount of enegy.  Pedalling a bike at 20mph takes more than twice as much energy, because the air resistance increases as a square relative to speed.  The same is true with water as with air.  They are both “fluids” and the faster the water flows the more resistive is the cooling block or radiator to that water.

Pump charts look similar but yet different and can become confusing because of the other information on the plot.  Instead of being pressure drop we’re focusing on pressure “gain”.  Pump “gain” typically looks like this:

The maximum pressure put out by the pump is usually highest at zero flow.  This you could think of again like your bike.  Maximum torque occurs when you’re about to start moving.  However there is a limit to how fast the pump can spin and how much water it can move.  At some flow rate the pump can’t go any faster and the pressure it outputs drops to zero.  This is not shown in the above plot because we haven’t measured at high enough flow rates.

One question I’ve been asked before is “what’s better then – pressure or high flow?”  Well it’s not a choice of one or the other because with a pump they are related.  However some pumps may have a higher “head” or static flow pressure rating.  A high static flow pressure rating helps when you have to move water vertically a long distance.  Therefore a geothermal cooling setup needs a beastly pump.  It also helps with bleeding a loop, as the pump needs to have good pressure to start pushing the water around the loop and the air (which easily compresses) out of the system.  However once bled and sealed a PC cooling system is much like an elevator.  The pump doesn’t have to be powerful enough to push the water to the top of the system anymore because in doing so there is relatively incompressible water being pushed back down.  With an elevator the motor doesn’t lift the entire weight of the elevator but instead it works to raise the difference between the elevator body and the counterweight.  So for most users what matters is the pressure at the flow rate that your loop will run at with that pressure.  If you were excel right now you’d complain that I just gave you a circular reference because you don’t know what the loop pressure drop is because you don’t know what the loop flow rate will be.  However this is just a simulataneous equation which is most easily solved graphically. By plotting both pressure drop of the loop and pressure gain of the pump on the same plot we can quickly and easily see where the two equations will balance.  That is the point at which the two lines intercept will be the flow rate you’ll see.  For example:

I.E.  The pressure gain of the pump equals the pressure drop of the loop.  This is equilibrium.  If the two didn’t balance, the pressure would be unbalanced and so the flow would be changing velocity and moving towards equilibrium.

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