Archive: SPCR's Unique Heatsink Testing Methodology

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Technical Complexities, or What Does All This Tell Us Anyway?

The test procedure outlined above explains what we do, but it doesn't explain why we do it that way, or what the end results tell us. It turns out that thermal testing is quite complex, and, although our test procedure is simple and repeatable, it glosses over a few issues that need to be addressed to understand what's going on here.

Accuracy of CPU Thermal Sensors

First of all, there is virtually no way of knowing whether the thermal sensor in our test CPU is accurate, and no reliable way of calibrating it in the likely event that it is wrong. There, we said it: Our testing produces inaccurate results. There is plenty of technical documentation out there that explain how accurately testing CPU temperature is practically impossible. One of our favorites is a piece from Arctic Silver, entitled Why Thermal Measurements are Not Valid.

So, why do we bother testing at all? Fortunately, accuracy, in absolute terms, is not what really matters in heatsink testing. What we want is not a tool that tells us that our test chip is exactly 42°C, but a tool that detects fluctuations in temperature and produces consistent results under similar thermal conditions. And it turns out that the thermal sensor on the CPU works just fine for these purposes.

Consider your bathroom scale. Chances are, it has a small notice on the back that says not legal for trade. That's because the accuracy of most bathroom scales is not considered good enough (or, it's not certified to be good enough) to yield the same result as government-approved, trade-legal scales. However, that doesn't mean it can't tell you when you gain or lose weight. That's because, as long as you always weigh yourself on the same scale, it will always produce a higher result when you gain weight, and a lower result when you lose weight. It can also tell you whether you weigh more or less than your wife, your best friend, or your dog. It can even tell you how much the difference is, though perhaps not with quite as much precision as a better scale.

Heatsink testing doesn't require exact numbers. What matters is how a heatsink performs in comparison to other heatsinks, not what CPU temperature it achieves on our test bed. And, as long as all heatsinks are tested using the same test bed, it is possible to make valid comparisons between them without ever knowing exactly how hot the CPU was — just like it's possible to use the bathroom scale to gauge changes in your weight without knowing whether it is giving you exactly the right number. In fact, even if they were accurate, the actual thermal results would be useless on their own. All they tell us is how hot our specific test bed was during the test, but unless your system runs exactly the same parts, in exactly the same thermal conditions (i.e. on an open test bench at ~21°C), and you can guarantee that your thermal measurements are 100% accurate, these numbers won't tell you how the heatsink will perform in your system.

Thermal Rise

How do we go about converting the inaccurate thermal measurements into valid comparisons between heatsinks? We do two things: All tests are done on the same test bench, and comparisons are based on thermal rise to avoid errors based on different ambient temperatures. On its own, this is enough to evaluate any heatsinks that we test. Heatsinks with the lowest thermal rise are the best performers.

Thermal Resistance

However, we attempt to go one step further, in order to make the result useful to you, our readers. Thermal rise tells us how a heatsink performs versus other heatsinks that we've tested, but not how it compares to your heatsink. Thermal resistance, on the other hand, factors our test bed out of the equation. In theory, the thermal resistance for a given HSF running at a specific fan speed should never change. If you can determine the thermal resistance of your heatsink, you should be able to tell which heatsinks will be better performers based on our testing.

Of course, the reality is a bit more complicated than that, mostly because it's difficult for a casual (or even a not-so-casual) user to calculate thermal resistance. Essentially, it involves duplicating our test procedure — including measuring the amount of power consumed by the CPU and hoping that minor differences in VRM efficiency are not enough to compromise the results. On top of that, variables such as system airflow (which is not taken into account by our test bench) and the aforementioned accuracy of the onboard thermal sensor can also affect results.

Despite the difficulties in making good use of them, we shall continue publishing the thermal resistance results as we have since we began testing heatsinks. If nothing else, thermal resistance is still the most "correct" way of expressing how well a heatsink cools.

VRM Efficiency

One final source of variance is worth mentioning: VRM efficiency. Our measurements of CPU power — and the thermal resistance results that are derived from it — include power losses in the VRMs. As a general rule, VRM efficiency does not change significantly between tests — though VRM efficiency can vary quite a bit from board to board.

However, there is one specific instance when VRM efficiency can affect our results. Like any other electronic component, the VRM efficiency begins to drop once it is above a certain temperature. If the VRMs are not cooled adequately, the power losses in the VRMs increase and the total amount of heat that must be dissipated by the heatsink goes up. Because the major source of heat near the VRMs is the CPU, the VRMs often overheat when the CPU is undercooled, but they can also overheat in a system with poor system airflow, as outlined in the yellow box below.

Obviously, this increase in power draw makes our thermal resistance results invalid, since they are calculated on the assumption that the CPU and VRMs draw 78W. For this reason, AC power is monitored during testing, and if it increases above normal (120W under load), the change is noted and CPU power consumption is re-measured for the relevant data points. This increase in power consumption is unhealthy, and it's unlikely that a heatsink that demonstrates this kind of variance will be highly praised by SPCR.

VRM COOLING

Motherboard makers generally assume a certain level of "spillover" airflow from the heatsink fan across the voltage regulator module (VRM) components that are placed around the CPU socket. These components include capacitors, power transistors and inductors (coils). When the CPU fan speed is reduced to minimal levels in order to achieve low noise, cooling for the CPU may be perfectly adequate with a good heatsink, but the VRM components may be prone to overheating, which can impair electrical efficiency and reduce component life.

Tall tower (or high rise) heatsinks with fans that blow air parallel to the motherboard rather than down at it are more likely to cause VRM component cooling problems — even when the fan is not run at minimal speed, because the airflow is sometimes blocked by the fins from reaching the surface of the motherboard. When the fans on such heatsinks are slowed to minimum speed, VRM cooling can suffer quite a bit.

Users should be aware of this potential issue and ensure some additional airflow from at least one case exhaust fan in most systems, especially in systems with hot (100W+) processors. The quality, efficiency and intrinsic cooling of VRMs varies substantially from motherboard to motherboard, however.



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