Computer CPUs are typically "air-cooled", meaning that after the heat has conducted to the heatsink, it is extracted by blowing cool air across the fins. While it’s cheap, quiet and effectively does the job, it’s impossible to cool the CPU down below the ambient air temperature. In liquid nitrogen (LN2) cooling, LN2 (boils at nearly -200°C) is poured onto the a specialised heatsink to extract heat rather than blowing air.
I’ve had the opportunity to try this out first hand at the Gigabyte OC Workshop this Monday, with the TeamAU crew (dinos22, deanzo, and uncle fester). LN2 overclocking on the Z68X-UD4, Core i5 2500K, GTX470 and the GTX580 SOC.
How does it work?
The reason why LN2 cooling works so well is not just because we’re pouring really cold liquids onto the heat source; it’s also because a lot of the heat energy is absorbed by the process of evaporation. To make sure that the LN2 doesn’t just boil away and splatter away, there are custom designed CPU "pots" that are essentially a heatsink base with a cup to hold the LN2 while it evaporates.
I’ve been told that the colder the CPU gets, the faster it can run. (That, and to save the CPU from blowing up from pumping an insane core voltage into it.)
Why is sub-zero overclocking more difficult?
The biggest difference between the conventional air cooling and even cooling below room temperature is that condensation builds up as moist air settles on the cool surfaces on the equipment. As we all know, water and electronics don’t mix. That’s why we see so many fans, hairdryers and mounds of paper towels in LN2 setups.
Given that the CPU can only be overclocked so much below a certain temperature threshold, on the one hand we need to keep the CPU below a certain temperature, while on the other hand we can’t drop the temperature too much otherwise we’d get what’s called a "cold bug" and the computer would freeze – as in, lock up and stop working.
Usually when the computer locks up, it’s easy enough to hit the reset button to reboot and try again, but with subzero cooling there’s also the "coldboot bug" where there computer will not start unless it’s warmed up above a certain temperature. I suspect the coldboot bug is due to arithmetic underflow on the temperature sensor that falsely triggers the thermal protection circuit. The quickest way to fix this is apparently putting a butane torch to the LN2 pot…
Because temperature and voltage regulation is so critical, external temperature probes are used to get to most accurate readings from as close to the chip as possible. We can’t rely on the built-in sensors because they’re not designed to operate at extreme subzero temperatures, and many motherboards are simply incapable of reporting temperatures at that range.
What are the risks?
As mentioned before, the biggest risk in going sub-zero is the build-up of condensation wherever that’s colder than ambient. There’s a good amount of preparation work to be done before anything is plugged in, including insulating exposed circuitry on the motherboard against moisture and the cold, as well as making sure that the cooling device and temperature probes is mounted correctly.
Even when all precautions are taken, things can still go wrong. We were shown a dead $600 GTX580 SOC with a resistor gone kaput.
The Gigabyte OC Workshop
The workshop was an incredibly fun experience, and it’s a rare opportunity to have the professional overclockers share their insights and experiences as well as an excellent tutorial on LN2 overclocking.
It’s a shame we didn’t beat any records on our first try, but I did walk away with a Gigabyte X58A-OC Motherboard having guessed the closest top 3DMark11 score. The runner up (Justin) walked away with a Gigabyte Z68X-UD4-B3 Motherboard.
A big thanks to PC PowerPlay and Gigabyte for hosting this awesome event. It’s incredibly difficult to even have a go at LN2 overclocking because it’s just a different beast. Reaching out and teaching us how it’s done really saves the stress of not knowing what’s right and would definitely save a bunch of dead hardware.
Let’s do this again!