Test system and test conditions
The core of the measurement setup is again a powerful pump capable of delivering 200 l/h (and more), a large expansion tank in the compressor cooler and an external one with a combined volume of almost 10 liters (here I reduced the amount of water a bit again), as well as the compressor cooler itself, whose hysteresis was adjusted to keep the temperature in the flow between 19.5 and 20.5 °C up to 1000 watts of supplied waste heat. Smaller deviations are not feasible, however, the temperature in the flow can be measured and thus a possible delta can be used to correct the collected measured values via a uniform time stamp. Clean distilled water is used, nothing more. Any suspended particles would only falsify the measurement.
To avoid fluctuations of the volume flow (“flow rate”) in the area of the pump, I let it run unregulated at maximum power and instead regulate the volume flow down using bypass A. This also lowers the pressure a little bit to the usual level in a normal PC. For this purpose, a quite well controllable ball valve is used, which already helps to reduce the volume flow in the large circuit through the “short circuit” via bypass A quite roughly. As a result, the volume flows are divided between the large and small circuits, so that the pump itself no longer receives any significant counterpressure, which in turn could then depress the speed.
In order to be able to carry out an initial reference measurement here, the reference measuring device is first activated via the 2-way switch. With the quite precise pressure regulating valve in the return of the large circuit, which relies on a large reduction by a screw thread, I can now adjust its volume flow relatively precisely and above all also constantly and without large fluctuations. Even with larger volume flows around 3 l/m (i.e. 180 l/h), the measured fluctuation is only 0.01 l/m at the most.
For those wondering why I didn’t position the valve in the flow, the answer is relatively simple. For the volume flow as such, the position in the circuit is rather unimportant, however, turbulence occurs at the valve, which one would like to avoid as far as possible before an inlet and we have a pressure reduction again. In addition, the build-up of air bubbles in front of the components to be tested can be almost completely eliminated. By the way, the whole structure lies flat on the floor and, except for the components, the tubing is also largely symmetrical in design.
To determine the volume flow, I therefore rely on a firmly defined measurement set-up that is not “picked apart” in the meantime. At the heart of the system is a special, high-precision ultrasonic flowmeter in the form of the Keyence FD-Q10C. This clamp-on flowmeter operates completely contact-free and also does not affect the flow in the circuit. The accuracy exceeds that of the mechanical solutions by far, so that a real reference value for the instantaneous volume flow is obtained here after calibration.
The whole thing is powered by a special 4-pin M12 cable from an external power supply. In order to do exact justice to the tube section used under the clamp, I had the assembled components calibrated once again in a friendly laboratory in series connection to a calibrated device (M 1). The instantaneous flow rate is generally measured in litres per minute (l/m), not per hour. In addition to limit value determination, the device also offers differential measurements and a freely definable zero point.
Equipped in this way, it is now possible to evaluate flow sensors in the measuring circuit and, for multiple value determination, always have the option of switching between this test object and the reference sensor in real time in order to detect a possible drift in the volumetric flow and, if necessary, readjust it slightly. The measurement results on the next page show how the whole thing works in practice and what insights can be gained from it.
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