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EXPERIMENT #3 PRELIMINARY RESULTS
Results for selected fans retested in the airflow box with the hot wire anemometer are listed in the table below. The 80mm fans are on top, followed by the 92mm and 120mm fans.
A few generalizations can be made from the above data:
1) The airflow readings are significantly lower than with the direct-at-fan method we've been using. This suggests some impedance in the box, which is expected. We no longer have any expectation that our measured CFM will match the free-air CFM specified by the manufacturer. Yet, some of our measurements are identical to the manufacturer-specified CFM; we believe this is mostly happenstance.
2) There is a general correlation between RPM and measured airflow for fans of the same size. Good examples are the Nexus and Scythe 80mm fans, and the Noctua and Scythe 120mm fans; each pair, which spins the the same speed, have the same or very close measured CFM. Another example is between the 1000 RPM 120mm fans and the 2000 RPM Antec Tricool 120: The latter has roughly double the CFM of the former, which is expected from the fan laws (assuming the same fan blade geometry).
3) The bigger fans with higher specified airflow do measure proportionately higher than smaller fans with lower specified airflow. In other words, there is no serious compression of measured airflow for larger, high speed fans, unlike the results with Experiment #2. For example, the Antec Tricool 120, rated at about double the CFM of the Fander 92, does give us roughly double the airflow measurement.
One fact not mentioned in the table is that there was virtually no difference in the RPM measured in free air compared to in the test box. The maximum variance we saw was a drop of about 40 RPM for a medium speed 120mm fan. This indicates that although the box does pose some impedance to airflow, it is very low, and the impedance does not load the fan enough to force any significant drop in fan speed. Low noise fans seem to be most susceptible to RPM drops. This may be a coincidence, but it's probably of interest to most SPCR readers.
The anomalous fans which were partly responsible for this revisit of our airflow measurement technique no longer give us anomalous results. The specific fans in the above table are the Arctic Cooling 12L, which is virtually frameless and also spins in the direction opposite to most fans, and the Noctua, which has very thin blades and gave high airflow readings in our original tests.
The Arctic Cooling 12L gave us very low airflow readings previously. Now the measured airflow is much more reasonable, although still lower than specified, and lower than the same speed but conventional design Nexus 120. This may be a real weakness of the Arctic Cooling open frame design; we suspect that some of the kinetic energy of the fan blades is dispersed and lost on the intake side before it has a chance to be directed into the output stream.
Our original measured airflow on the Noctua 120, on the other hand, was unusually high. At 63 CFM, it was 15 CFM higher than specified. The new testing brings this number down to earth; the measured 38 CFM is more in line with other fans of similar RPM and size. We must retract the comments we made about the Noctua's airflow in our original review. It does not push significantly more air for a given RPM than other fans. In this regard it is ordinary. But most importantly, it remains a very quiet fan, and comparison with both the Nexus and Scythe SFF21E 120mm fans, the Noctua still seems to have an edge in airflow at lower speed.
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