In the first Passively-cooled CPU Thermals post I looked at how the completely fanless Streacom DB4 I built performed under various CPU loads. A question over on Silent PC Review asked how the thermal curve looked when the CPU was unloaded. Good question — let’s find out!
It seems logical that a passively-cooled computer that uses heat pipes and large slabs of aluminium will heat up and cool down differently than an air-cooled system with a (relatively) small fin stack.
When I performed the previous round of tests, I ended up with graphs like this:
Now, one thing that niggled away at the back of my mind was how the temperature seemed to spike at the start and then flatten off relatively quickly. Given that I was sampling the temperature sensors at 5s intervals, I wasn’t sure if the sudden change in angle was just due to a lack of resolution, or because of something else. Hmmm…
Anyway, the SPCR question gave me an excuse to run another test but instead of putting the CPU under load and heating the system up, this time I’d be unloading the CPU and cooling the system down. I wondered if/how that would be different.
Test 5 — CPU unloaded from 100% at an ambient temperature of 20°C
The Ryzen 5 1600 was first placed under a 100% load (all 12 threads pegged at 100%) for about an hour until an equilibrium temperature was reached (60⁰C). The load was then removed and the CPU temperature recorded:
My previous set of results used wider graphs, but when websites scale them
the text ends up a bit small and blurry, so this time I made the graph narrower.
Otherwise the testing and recording setup was identical.
The CPU cooled down from 60⁰C to 49⁰C almost instantly, and then gradually made its way down to 34⁰C over the course of an hour. It would have cooled down a couple of degrees more, but I wasn’t prepared to wait — that sudden change in angle was even more pronounced and needed investigating.
Obviously 1 hour graphs sampled every 5s were too coarse to shed a whole lot of light on what was happening in those first few seconds, so I had to run some more granular tests. I figured 1 minute sampled every 1s would do the trick.
Test 6 — CPU loaded to 100% for 60s at an ambient temperature of 20⁰C
Test 7 — CPU unloaded from 100% for 60s at an ambient temperature of 20⁰C
Nope, it wasn’t a figment of my imagination. The sudden change in angle is definitely there — both when heating up and cooling down.
Let’s put the results ‘side-by-side’:
What we’re seeing are two different response curves. When the CPU is initially (un)loaded the temperature changes at ±1⁰C every 1s for about 8–9s. It then flattens out dramatically and changes by ±1⁰C every 16–24s after that. Huge difference.
Changes in thermal conductivity can usually be explained by changes in the material or media being used. In this case we have a CPU, soldered to a copper IHS, plated in nickel, covered by a thin layer of TIM, covered by a copper shim, covered by TIM, clamped against an aluminium block with exposed copper heat pipes.
This image (from Streacom’s DB4 Manual) might make the stacking a bit clearer:
If we ignore the thin layers of TIM, it’s pretty-much metal all the way through to the heat pipes before we reach the first medium that could possibly have a 16–24x lower thermal conductivity — the water inside the heat pipes themselves.
I suspect that the thermal mass between the heat pipes and the CPU can buffer about 8–9s worth of heat output, and that — due to high thermal conductivity — this mass heats up and cools down rather quickly. The water in the heat pipes, however, needs to undergo a phase change (from liquid to vapour), travel to the end of the heat pipe, undergo another phase change (from vapour to liquid), and then flow back — a process that could easily be an order of magnitude slower at transferring heat.
At the end of the day it doesn’t matter exactly how thermally conductive your cooling system is — as long as it’s conductive enough to get rid of all of the heat your CPU is producing without impacting performance (i.e. thermally throttling), it’s fine.
The previous tests (and now weeks of daily hammering) show that the DB4+LH6 combination is easily capable of cooling a stock Ryzen 5 1600 — no matter how heavily you load it or how long your jobs run for. You can run a Ryzen 5 1600 at 100% load all day, every day, and it won’t even break a sweat.
AMmmD and Streacommm — I’m lovin’ it! 😉