System Pressure Drop and Flowrates

Test Results for various waterblocks and tubing sizes – Joe

SUMMARY: Minimizing system pressure drop and maximizing flowrates requires large IDs throughout the system.

I ran a series of tests to determine impacts of waterblocks and tubing diameters to determine impacts on system pressure and flowrates and thought the results were interesting.

For this test series, I used the following waterblocks:

The Intake Diameters listed are NOT tubing sizes – these are measurements I took of the narrowest opening of the intake nipples; for example, while Comserver’s slip fittings are for 3/8″ tubing, it narrows down to 3/16″ at its base. This is an important distinction as it directly impacts pressure.

To measure pressure drop, I used an Omega PX2300 Low Differential Pressure Transducer (0-5 psid). This unit measures the pressure drop between two points – in this case, between the intake and outake of the waterblocks tested. I measured flowrates with the King Flowmeter. I used the Eheim 1250 and set the flow rate at 1 gpm for each waterblock, a flowrate typical of watercooling systems in use.

¹Pounds/square inch differential.
²This value was with the Little Giant (#3E-12NYS) 500 gph waterpump; the Eheim could not reach 1 gpm due to the high pressure drop.

Predictably, narrow openings and complex interiors take their toll – waterblocks such as these require a hefty pump to maximize performance. However, designs that feature large openings, even with pin fin designs, tend to be more flow-friendly overall than others. These results closely track waterblock flowrates HERE.

As a further test, I changed the nipples on the AquaJoe and ran tests with 3/8″ ID openings and again with 1/4″ ID openings:

¹Pounds/square inch differential.

“WO” stands for “Wide Open” – these are readings with no flow restriction in the system; by using a control valve, I restricted flow to 1 gpm and took readings at that flowrate. The Deltas give the impact of the waterblock on system flowrates and pressure drops.

Interesting to see that the differences, for this particular waterblock, are not as large as one might expect – most likely, the pin fin design extracts more of an effect than intake diameters.

As a further test, I ran the test rig first without anything in the tubing loop, then introduced two step-down adapters to take tubing size from 3/8″ to 1/4″ to 1/8″, with the following results:

¹Pounds/square inch differential.
²The Eheim 1250 could not deliver 1 gpm in this test.

While the impact from 3/8″ to 1/4″ is “tolerable”, introducing anything in a system using 3/8″ ID tubing using a 1/8″ ID intake will exact a considerable toll in flowrates due to the high pressure drop. Users who add GPU or Northbridge waterblocks with such openings may take a performance hit overall.

Minimizing waterflow resistance in watercooling systems is an important factor affecting performance, all other things being equal. Many newer designs sporting 3/8″ openings are clearly less resistive than others, although interior designs (pins, spirals, etc) have a significant impact on system resistance as well.

Matching waterpumps to systems requires some thought as to probable system resistance, considering components used. For example, any system with small diameter GPU or Northbridge waterblocks will exact a toll on flow rates, as does using a spiral or pin-fin CPU waterblock with very narrow intake diameters.

However, users are cautioned that while maximizing flow rates and minimizing pressure is one key determinant of system performance, waterblock design plays a critical role. Waterblocks that maximize cooling area (pin fin) or accelerate flow over the CPU core (WhiteWater) may take a heftier pump for maximum performance but pay back in terms of superior performance compared to less complex designs.

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