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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.
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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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