A New Way of Testing Fan Airflow

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May 2, 2007 by Devon Cooke and Mike Chin

What is that in the photo below? Three guesses — no cheating!

Need a hint? Remember the title of the article.


Is it modern sculpture? No? Then what...?

Neither the photo nor this article would exist if it weren't for the fact that, occasionally, we get things wrong. We make a recommendation, people take us at our word and whamour forum is full of people complaining that our recommendation didn't make their system any quieter. These situations don't arise often, but we take them seriously when they do.

Most recently, we identified the Noctua's NF-S12 series as the fan with the best airflow-to-noise ratio. Naturally, we were surprised when reports began to surface that, while the Noctua was indeed very quiet, some users were noticing their system temperature going up, not down. Since our recommendation assumes that higher airflow normally leads to lower temperatures, these reports cast our measurements into question.

Could a fan that we measured as having higher airflow provide poorer cooling in the real world?

Since our usual method of holding an anemometer directly in front of the fan to find the peak airflow didn't seem to be working, we needed to get creative. And that brings us to the odd-looking piece of modern art in the photo above. In proud SPCR tradition, we decided to build what we couldn't buy, and the strange device you see above was the result.

PROBLEM FANS

To the best of our knowledge, our technique of holding an anemometer directly in front of the fan to find the peak airflow works perfectly well for the vast majority of fans. The ones that cause problems are like the Noctua: They have some quirk that sets them apart from other fans. In the case of the Noctua, that quirk is thin, propeller-like fins with a sharper pitch than most fans, but we discovered several other quirks that seemed to affect measurements. Among the most serious examples:

  • Arctic Cooling's frameless, reverse-direction Arctic Fan 12L and its smaller cousins, including the Arctic Fan 3 we reviewed last November. These fans spin in the opposite direction of the impeller of our anenometer.
  • The Noctua NF-S12 series.with their very thin fins.
  • The 80mm Mechatronics fan with its thin, stubby fins.
  • SilenX' Ixtrema Pro series, with their wide, scooping fins and tiny diameter hub.

All of these are problem cases because our standard measurements show an RPM-to-Airflow ratio that is significantly different with other fans of similar size. Are these differences real or errors caused by our measurement technique? With the feedback on the Noctua fans, we weren't sure.

Fan manufacturers use complex multichamber testing tools to measure airflow. The cost of these tools runs into many thousands of dollars, possibly into the tens of thousands. The excerpt below from Laboratatory Methods of Testing Fans for Aerodynamic Performance Rating, document ANSI / AMCA STANDARD 210, shows just how complex such tools can be.

Such tools are far beyond SPCR's reach. We'd have to find our own way.



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