Pump Curves or P/Q (P is pressure, Q is flow) curves are derived from two measurements, flow and pressure, and the P/Q curve is just a visual representation of the relationship between flow and pressure. For the curve, maximum flow rate (GPM or LPM) is the X axis and Pressure (PSI, mH2O, ftH20, mBar) is the Y axis.
Almost every pump specification I have ever seen comes with a defined maximum head (or lift) and maximum flow rate. Problem with these curves is they are based on perfect world conditions, where no restriction or resistance are factored into the data and resulting P/Q curve. The key for testing is setting up a test fixture to collect data for all capable ranges of the pump and top in order to develop a real world P/Q which uses the common components in a PC Liquid Cooling System, and that is exactly my goal for each Pump and Top test performed in the lab, we aim for real world scenarios and the resulting data.
One other item to note on P/Q curves is the Trendlines used on the scatter plots. DDC’s trendline has a best fit polynomial using a 3rd Order, where as D5’s have a best fit with 4th or 5th Order. Now the problem I am running into with Excel is a defect that has been around since Excel 2000 and polynomial trendlines and forecasting/prediction. Sometimes no matter what you do, the trendline will not work on a best fit with prediction, the code inside Excel is not consistent. As a result, some of the trendlines you see on the individual test reports will not represent the proper trendline. Additionally, I did not forecast or predict ahead on the compiled P/Q curves for all results, some did not have the Excel bug and some did.
In short, dynamic head pressure is the pressure measurement at the outlet minus the pressure at the inlet or the differential. Dynamic head pressure is a better measurement for the actual pressure the blocks will see in the loop. Many other pump tests have only measured the outlet pressure, which does not take into account the inlet pressure and does not represent a true pressure measurement for your cooling loop. Dynamic head pressure is quite easy to add to the Pump test fixture and requires a T fitting at the pump inlet and a T fitting at the pump outlet. The pump inlet T is hooked up to the negative side of the manometer and the Outlet T to the positive side of the manometer.
I took Martin’s lead on this measurement and included these in the individual test results that you will find at the bottom of each speed setting graph. For each flow setting the voltage was regulated to 12.00 volts as set on the DC Power Supply and verified on the multimeter hooked up for all tests. In addition, the current (amps) draw was recorded for each flow setting, luckily the DC power supply I have has a display for amp draw, initially I had a multimeter hooked up but as tests went on the two displayed identical readings. As with Martin’s tests, the voltage regulation and amp draw data recording is really used for the efficiency charts, but power consumption is displayed on the P/Q curve charts for your reference as well.
No shocker here, another test result that was started by Martin and I only feel that it is right to continue providing the information. The efficiency charts are simply a graphical depiction of the water horsepower to break horsepower of the pump at the given flow rate, showing which flow rate the pump (and pump top) will perform with the best efficiency.
The pump testing loop is a completely separate loop on the test bench. The pump test loop is comprised of a custom built half gallon PVC reservoir, and gate valve for controlling flow. Bitspower 1/2″ High-Flow fittings were used for consistency across all D5 pump top tests. I use an assortment of measurement tools for pump testing, here is the list.
For each configuration tested, the loop was bled and ran for 20 minutes prior to measurements taken. After 20 minutes, power to the pump was shutoff, and the Manometer was reset to zero PSI. This step ensures a proper measurement of the total dynamic pressure for the pump is recorded. Voltage is adjusted to 12.00 volts in order to achieve a constant voltage across all tests, voltage was verified prior to the 2 minute runtime and verified before measurements were recorded. Also to eliminate and pressure variation, each flow setting was , given a 2 minute runtime before measurements were recorded. For each flow setting the PSI, Voltage and Amperage are recorded.