Fan Test System, SPCR 2010

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The role of 12V DC axial fans in PCs dates back over thirty years. They're the quick and easy cooling tool of choice for engineers, and they're often the bane of noise-haters. SPCR has been studying, testing, analyzing, discussing and thinking about DC axial fans since our inception. Fans seem unbelievably cheap to produce, they are made in enormous quantities, and they're ubiquitous around computers. They are usually too noisy.

Our first assessments of fans was based almost entirely on listening, and this method was good enough to identify several fans that were markedly better than others available at the time. It took some years for us to develop a fan test methodology that was better than careful listening. We combined subjective listening, airflow measurements and SPL measurements at a wide variety of speeds to establish a more complete profile of each fan. The airflow measurement methodology evolved over several sets of test runs, with the last one being A New Way to Test Fan Airflow... but we were never fully satisfied with out results.

Fan manufacturers use complex multi-chamber tools to measure airflow. The cost of these tools runs into many thousands of dollars, possibly tens of thousands. The excerpt below from Laboratory Methods of Testing Fans for Aerodynamic Performance Rating, document ANSI / AMCA STANDARD 210, shows one of the simplest fan airflow measurement devices illustrated in that document. It's no wonder that our self-designed and built airflow measurement systems didn't quite hold up.

Manufacturers' most commonly cited airflow measurement is free-air — that is, the airflow in cubic volume per minute (Cubic Feet per Minute, for example) when the fan sees no resistance or impedance to its movement other than still air, at sea level pressure in moderate temperature and humidity. CFM always varies with impedance, and "serious" fan manufacturers measure this under varying impedance, often at different RPM with each fan. For fan makers, CFM is one of the main selling or defining parameters for a fan. Thermal engineers have access to details on typical and safe maximum temperature, impedance to airflow, the heat transfer parameters of heatsinks, and many other complex factors other factors. From such data, thermal engineers can use heat transfer and fan equations to calculate airflow requirements, which are then used to specify appropriate fans.

That is for thermal engineers. For typical PC enthusiasts, CFM is a purely abstract concept. This is not to say that there's no relationship between airflow and cooling, but that there is no recognizably linear relationship, and it differs for each thermal system. We simply do not know clearly enough the relationship between the CFM rating of a fan and the cooling it effects in our thermal system.

This is very different from many other technical issues around computers that DIY PC enthusiasts deal with. With power, for example, a watt is a watt, whether it's in AC or DC, and a watt drawn by the PC means there's a watt of heat being dissipated by the PC. If you need to know how much power the components are using, as opposed to the whole PC, then you apply the AC/DC conversion rate that applies. For example, a system that draws 200W from the wall and uses a PSU know to be 80% efficient at that power level is delivering 160W DC to the components within. This methodology of estimating DC power use remains the same for any type of PC. There is no similarly neat way to calculate how airflow and cooling work together.

A simple example: If a fan spinning atop a CPU heatsink effects a temperature rise of 20°C in the CPU under 100% load, all we can predict is that if the fan is slowed down, the CPU will run hotter, and if the fan is sped up, the CPU will run probably run cooler. There is no way for us to predict with any scientific certainty just how much the airflow must change to effect, say, a 5°C change in the temperature rise. Furthermore, the amount of airflow change needed will not be the same going up in temperature compared to going down. We also cannot change airflow directly; it changes in response to the RPM, which we can change, but there's also no precise correlation between RPM and CFM.

Finally, CFM has no direct bearing on cooling, which is measured not by airflow but by a drop in temperature, usually in a device in the PC. Without the benefit of a thermal engineer's knowledge and detailed parameters about the components and conditions, CFM might as well be APH (angels per pin head). The relationship between CFM and cooling is at least as complex as that between SPL and perceived noise. The CFM value has no real meaning beyond itself. In contrast, with a bit of experience, 30 dBA/1m does have some meaning. Still, DIY computer tech geeks want to compare fans by their CFM rating, and in the SPCR (and other PC tech web) forums, some have gone so far as to specify what CFM rating they believe is needed for their application. This is a reliance on CFM numbers that has obfuscated the role of airflow in cooling. It's not really a surprise; CFM is one of the very few performance specs that fan manufacturers make available.

Over the years, we have observed one clear phenomenon about fans and cooling: The relationship between airflow and temperature invariably becomes exponential at some point. Increase airflow from nothing to something, and the drop in temperature can be dramatic. Keep increasing airflow, and the cooling improvement becomes less and less significant, until at some point, the temperature hardly drops at all. The trick, for the PC builder who seeks both good cooling and low noise, is to find the point where any decrease in airflow (or fan speed) effects a significant increase in temperature, while only a very large airflow increase effects a significant temperature drop. In other words, once you have enough airflow, additional airflow has very little cooling effect, so all you're doing is increasing noise. "Enough airflow" is not a constant, of course, it varies for each system of components.

When our previous fan testing strategies were reviewed against this brackdrop, we realized that our attempt to accurately measure CFM was a kind of search for the holy grail. It simply was not going to be achieved successfully, not without a dramatic, multi-fold increase in funding and expertise. As is so common, this realization opened up a new opportunity, a new way of looking at the performance of fans in the context of silent PC cooling. Many months of planning, thinking and experimentation later, we have a new fan testing system, one that's a dramatic departure from all of our previous systems.

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