Fan Test System, SPCR 2010

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THE TEST PROCEDURE

The gist of the fan test system is fairly straightforward:

  • A CPU simulator, in the form of copper block that has the same dimensions as an Intel i7-1366, has a small heater coil embedded within, capable of handling 150W.
  • A large heatsink with somewhat high airflow impedance is mounted to cool the CPU simulator.
  • The heater coil is powered by a regulated lab power supply to 137W. (Typically, 64.6VDC x 2.1A). This is the maximum power that the heater coil can pull from the lab power supply, although there is a bit of headroom in the coil as wenn as the PSU, which is rated for maximum 3A at 64V (192W).
  • The fan to be tested is mounted on the heatsink and driven by a regulated 0~12 VDC power supply at standard voltages, speeds, and SPLs.
  • The SPL is recorded (in dBA@1m) in the anechoic chamber at every voltage level.
  • The temperature of the CPU block and that of the air 6" in front of the fan is monitored closely using T-type thermocouple wire sensors and a dual-input digital thermometer.
  • A precision anemometer is used to record air velocity (Feet Per Minute) at every speed and SPL.
  • The most important parameter is Temperature Rise vs SPL.

We refer so often to temperature rise at SPCR that we sometimes forget that not everyone lives and breathes it. Basically, it refers to the difference between ambient temperature and the temperature of an object under themal load. Better cooling results in lower temperature rise; worse cooling results in higher temperature rise. In this case, the ambient is the temperature of the air 6" in front of the fan, and the thermal load temperature is that of the CPU die simulator.

In the past, we've used 12, 9, 7 and 5 volt settings for the fan drive voltage as test points. It made sense for a long time, as these voltages are fairly easy to obtain in any PC (except for 9V). Today, there are many more ways to adjust fan speed. Most motherboards are equipped with speed controllers for their fan headers, and monitor fan speeds for any standard 3-pin fans or 4-pin PWM fans. In most cases, the RPM can be displayed right on the desktop using any number of fan / thermal utilities.. So now, we're using specific RPM for the primary test points. Since we have little reason to change our long-standing reference of the Nexus 120 fan, its RPM at 12, 9, 7 and 5 volts will be used for standard test points. Above the 1080 RPM maximum speed of the Nexus 120, we will choose test points based on the performance of other, faster fans.

A PEEK AT EARLY RESULTS

The assessment of a fan is always comparative; performance can only be judged in comparison with other fans. There are a small number of fans that we've been using routinely in our lab due mostly to their low overall noise and benign sonic qualities. Here is a summary of the test data for a few fans, obtained from our new test system.

Astute readers will note that the temperature rise figures obtained here are lower than with the i7 CPU on our current heatsink test platform, despite the similar 137W load. This is to be expected as the entire copper top of the die simulator is radiating heat more evenly than a real i7; hence it runs a bit cooler. This is not particularly important in our fan testing procedure, however. The instant, easy, repeatability of the power settings is far more important.

Each fan was tested at maximum speed (12V), and then at voltages that provided 1080, 860, and 720 RPM. These are the speeds that the Nexus 120 achieves at 12, 9, and 7V. The high speed Scythe Ultra Kzae fan could only be tested at one speed to match the Nexus 120, 1060 RPM at 4.6V. It would not start reliably much below that voltage.

120mm Fans on Thermalright U120E + 137W Thermal Load
RPM
VDC
FPM*
SPL (dBA@1m)
°C RISE
Nexus 120 (reference fan)
1080
12
240
16
21.0
860
9
180
13
24.2
720
7
150
11
29.2
550
5
100
<10
38.2
Scythe Slipstream SY1225SL12M (medium speed)
1390
12
370
21
18.7
1080
7.8
285
16
21.6
860
5.4
270
13
25.2
720
4.3
150
11
29.0
Noctua NF-S12-1200 (original design)
1430
12
420
21
20.7
1080
8.4
325
16
23.9
860
6.3
285
13
28.0
720
5.1
220
11
32.3
Scythe Ultra Kaze DFS1238H-3000 (120x38mm)
2800
11.9
650
44
14.8
2280
9
530
38
15.5
1800
7
420
32
17.0
1230
5
270
25
21.0
1080
4.6
240
22
22.5
*FPM = Feet Per Minute. This is the actual value that an anemometer measures, the velocity of the airflow through its vane. The widely used CFM is Cubic Feet per Minute, obtained by multiplying FPM by the area of the inlet or exhaust. In previous reviews, we measured FPM directly at the fan, and multiplied that value by the area of the fan blades (area of diameter minus area of center hub). This was always a bit of of a scientific guess; no more guessing. The FPM is provided, and we don't believe it differs much from CFM for fans of the same diameter. In other words, our FPM measurements can be compared much like CFM, if you feel this is important. We caution you, however, that like CFM, FPM does not correlate that closely with temperature rise.

For anyone interested in the relationship between RPM, air speed, and cooling, the above table is fascinating. Here are some observations.

1) Nexus 120 remains an excellent choice for a quiet reference fan. It is quieter than the other fans at the 1080rpm max speed, and its cooling is a half degree better than the Slipstream, 3°C better than the Noctua and 2.5°C better than the Ultra Kaze. At 860rpm and 13 dBA@1m, its advantage increases over the Slipstream and Noctua, to 1°C and 4°C. Its overall noise signature is more pleasant than the Noctua, and a bit of a tosseup against the Slipstream, though if pressed, I'd choose the Nexus.

2) Each fan has a somewhat different relationship between RPM, FPM and cooling. The Nexus measures the lowest FPM at any given RPM, while the Noctua measures highest. It's obvious that lower RPM leads to lower FPM. So reduce the speed of a fan, and the cooling suffers. But there is no logical correlation between FPM and temperature rise from one fan to another. For example, 180 FPM on the Nexus gave 24.2°C, a degree better than 270 FPM on the Slipstream. On the other hand, an even higher 285 FPM on the Noctua gave the much worse result of 28°C.

3) Much higher fan speed does not mean proportionately better cooling. The Sycthe Ultra Kaze is a 38mm thick fan rated at 3000 RPM. It didn't quite reach rated speed in our test rig, but note the differences in cooling at the top three speeds — they are separated by little more than two degrees, while the noise spread is a massive 12 dBA! At 7V, 1800 RPM and 420 FPM, we saw 17°C temperature rise with 32 dBA@1m SPL; increase the speed a thousand RPM and the noise by 12 dBA (to a whopping 44 dBA) and the cooling improved by only 2.2°C. This suggests that for the thermal load of our test rig, the relationship between 120mm fan speed and cooling becomes exponential around the 2300 RPM mark. Increasing fan speed beyong that point provides only marginal increases; there's only 1.3°C improvement going from 2280 to 2800 RPM.

Interestingly, when the powerful 38mm Scythe is set to the 1080rpm reference speed, the temperature goes 1.5°C higher than the Nexus, while the noise level is 6 dBA higher — with a terrible, tonal quality in stark contrast to the smoothness of the Nexus. Certainly, there seems to be no pressure advantage in the deeper 38mm blades compared to the standard 25mm ones at this speed. Comparing the best cooling results of the Nexus and the Ultra Kaze, you have to ask if anyone would be willing to accept a 28 dBA@1m noise penalty for 6°C cooling improvement.

4) With a lower impedance (less tightly spaced fins) heatsink of similar quality and size as the U120E, less airflow will be needed for the same results, In other words, the lead of the Nexus 120 at low speeds will naturally increase. Tighter fins spacing in the same size heatsink will mean a higher number of fins and greater fins surface area, so a converse result with this hypothetical heatsink is that higher airflow will probably provide better cooling. Most readers should have little interest in a heatsink with tighter fin spacing than the U120E, as it will require a faster, noisier fan to reach the same cooling, even though its ultimate cooling capability could be higher (due to the combination of higher fin surface area and higher airflow).

Suitability of the Thermalright Ultra 120 Extreme as a General Purpose Thermal Load for Fan Testing

The Ultra 120 Extreme was not just randomly chosen. It was one of four heatsinks considered. We think it is a good balance between high and low impedance.

Fans are used directly on heatsinks, and as exhaust or intake case fans. On a heatsink, a fan might face a higher impedance than as a case fan. In either case, the thermal loads which a fan is asked to cool are generally equipped with heatsinks (CPU, VGA, NB, VRM), and there is always some impedance. The other heatsinks considered... and rejected:

  • Prolimatech Megahalems - It was our first choice, as its mounting systems was the best, its fin spacing seemed about right, and its fan clips are very easy to use. After many days of fan testing and examining the results, we came to the conclusion that that the Megahalems is simply too good with low airflow; it shows too little difference in temperature between low and very low speeds. It also doesn't show much cooling improvement beyond about 1600~1800 RPM, which is a bit low considering the range of fans we have to test. The ideal setup is one that shows the greatest range of temperature differences for different fan speed/airflow settings.
  • Scythe Mugen 2 - Fin spacing and impedance appropriate but mounting incompatible w/ thermal test platform.
  • Noctua NH-D14 - Impedance about right, but load on both sides of fan (in middle position) causes unnaturally high turbulence noise with higher speed fans.

In any case, the test results do not turn out that differently. The Thermalright Ultra 120 Extreme has higher impedance due to its tighter fin spacing (1.5mm vs 2mm in the Megahalems), and the Megahalems also has the gap in the biddle between its two banks of fins for even lower impedance. On the U120E, the Nexus 120 shows a wider range of temperature drops at different speeds, with some 17°C between 1080 and 550 RPM, compared to 14°C on the Megahalems. The Megahalems also cools a bit better, especially at the lower speeds. But just as on the Ultra 120 Extreme, the 38mm high speed Ultra Kaze fared 1.5°C worse at the same 1080 RPM as the Nexus. The data basically shows the slower Nexus faring less well at slower speeds on the Thermalright than on the Megahalems, due to the former's higher impedance. The U120E shows greater differences in temperatures over a wider range of fan speeds than the Megahalems, so it is better choice for general fan performance testing.

Noteworthy points on the New Fan Test System

1) Using the die simulator and DC power supply is an incredible luxury compared to the CPU, heatsink, or fan testing we're used to doing all these years. You turn on the power supply, dial up the needed voltage and current, turn on the fan, and within 5 minutes you have a perfectly stable temperature reading. There are no mysterious software or CPU-based fluctuations in either power or temperature; they stay stable within a watt or a degree. Getting the fans to stabilize at the exact RPM we seeks is tricky, but we usually get it within 10 RPM. Keep in mind that with higher ambient temperature, the fans seem to run just a touch faster at the same voltage.

2) Testing fans smaller or larger than 120 mm diameter will require choosing other reference heatsinks, We already have a good candidate for 140 mm fans, the Prolimatech Armageddon. We'll choose an appropriate one for 80/92 mm fans when the need comes. Ditto smaller fans.

SUMMARY

We are reversing several years of fan testing by putting airflow measurements back to the very bottom of relevant parameters that we seek to measure. Those who have been reading SPCR articles and forums from the start know that this was our original position, and we were swayed to look more closely at airflow by popular opinion. Instead, we're focused now on the end result of airflow: Cooling, by comparing the temperature drop of each fan at specific speeds and voltages on a well-known and extensively examined heatsink with a static, consistent 137W heat source on a block of copper that emulates an Intel i7-1366 processor.

Some will argue that while this approach tells about the performance of a fan when mounted on a heatsinks, it tells much less about when the fan is used for case intake or exhaust. Our counter is that even in the role of a case fan, the main question is how much cooling effect the fan has on hot devices that generally have heatsinks mounted on them. Unless the temperature difference between the air outside the case and inside is huge, the relationship between airflow and temperature drop follows the same type of curve shown on the first page of this article; beyond a certain point, additional airflow becomes essentially useless. Precise setting of airflow for case cooling is also less critical than with fans for use on heatsinks, simply because most hot components in computers do have heatsinks and fans on them.

The best way to determine how much case airflow is needed for optimum cooling is to experiment with the system you wish to cool, under realistic conditions. There's usually a subjective/objective balancing by each DIY PC enthusiast between spot-cooling fans (such as the fans on CPU heatsink or GPU heatsink, or in a PSU) and case fans. What works best for one user and system doesn't necessarily work for another. Our reductionist point of view is that for a silent PC, what is easily perceived but very difficult to change is the fundamental sound character of a fan, so it's always best to choose a fan that sounds best and work with it (or them) to achieve the cooling you need. Look for fan reviews using our new fan testing system in the near future.

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Articles of related interest:
A New Way of Testing Fan Airflow
SPCR's Fan Testing Methodology
Anatomy of the Silent Fan
Recommended Fans

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