Watercooling Myths Exposed

Nice compilation of common misconceptions – Greenman100 (Tim Elmore)

Disclaimer:

The following is the work of a team of technically minded
individuals who worked together to make the following as factually correct as
possible. However, one should not believe everything they read on
the internet blindly; if you have evidence that contradicts
the information presented here, feel free to email comments/corrections.

Myths about Water and Flow

Myth: Water must slow down to fully absorb heat.

Reality: In a closed loop, a given water molecule actually spends
the same amount of time in the radiator, no matter how fast it is
moving, as long as the water is indeed moving.

If this is a
difficult concept to understand, think about a race car on a track.
If the track is one mile (5280 ft) long and the car is driving at 60
mph, the car will spend about one second in a 100 ft stretch. Think
of the 100 ft stretch as the radiator.

If the speed is
doubled, the car only spends ½ a second in the 100 ft section, but
it passes through that same section twice a minute, so it spends a
total of one second in the 100 ft section per minute.

Myth: The order of components has a significant impact on temps
(eg, the radiator must be before the CPU).

Reality: The order of components makes a difference of less than 0.5ºC
in most watercooling systems. The physics:

pump—->radiator—->CPU—->pump
pump—->CPU—->radiator—->pump

There is only one difference, and that is the position of the pump
in the loop, be it before or after the CPU.

Assuming the pump dumps about 50 watts of heat into the water and the flow
rate is 1 gallon/minute (gpm – very reasonable assumptions):

Water has a thermal capacity of 4186J/Kg-C at 22ºC and a density
of about 1g/mL

With a flow rate of 1 gpm, that’s ~3.75 liters/minute (lpm).

3.75 lpm / 60 seconds= 0.0625 liters or kilograms through the
waterblocks per second.

4186 * 0.0625 = 261.625 W/C

So that’s 1ºC warmer for every 261 watts; but only 50 watts of heat are present, so:

50 / 261.625 = 0.19ºC

Ergo there is a 0.19ºC difference in water temperature between the
inlet and outlet of the pump. This does not mean the water is only
0.19ºC warmer than air – that is an entirely different calculation.

And that’s with 50 watts. If you’re running a smaller pump, such as
the D4, you’re looking at about 15 watts.

So, do what allows for the simplest tubing runs – tubing
length/kinking will have a greater impact on temps.
{mospagebreak}

Myth: Pump power consumption has a significant impact on temps.

Reality: It is difficult to know exactly how much heat a pump
dumps into water, but a good rule of thumb is the following:

Most inline pumps dissipate 70-90% of their heat into the water, while a
submerged pump dissipates 100% of its heat into the water.

A dual 120 mm
radiator is good for about 0.03 C/W with a decent
pair of 120 mm fans. That is, the water temps will rise 1ºC for
every 33 watts in the water. So, if your pump dumps 33 watts into
the water, water temps will rise 1ºC.

Therefore, the difference
between a Mag3 at 40 watts and an Iwaki WMD-30 at 90 watts is fairly
insignificant; 32 watts into the water versus 72 watts, so about 1.1ºC. Note that
the performance of a waterblock will improve with diminishing
returns as the pressure increases.

Myth: A pump’s flow rate is the only consideration to make when
choosing a pump.

Reality: A pump’s maximum head pressure is just as, if not more,
important. Many waterblocks are relatively restrictive, and most
aquarium pumps are not made for high pressure drops. In order
to estimate one’s flow rate, calculate all pressure drops, then
overlay the result on top of the pump’s P/Q curve. In other words, it’s not
easy, but consider head pressure, too.

Myth: A T-line must be at the top of the system or water will leak
out when you take the top off.

Reality: If the water were to leak out, air would have to replace
the water. Since the rest of the system is sealed, air
can’t get in to replace the water. Thus, no water will leak out of
an opened T-line unless there is a leak in the top of your loop.
{mospagebreak}

Myths About Materials/Coolants

Myth: Aluminum absorbs/dissipates heat faster than copper.

Reality: All thermal properties of copper are better than
aluminum. Aluminum’s advantage is that it is lighter and easier to machine. So,
if one were given a pound of copper and a pound of aluminum, you
might make a better performing heatsink with aluminum, as it might have more surface area to dissipate heat than copper for a given weight. Considering footprint limitations for air cooled CPU heatsinks, however, copper is definitely favored.

Myth: Gold is the best thermal conductor.

Reality: Diamond (6-50w/cm-k, dependent on purity) is actually
best, but too cost prohibitive. Silver (4.173w/cm-k ) is second,
and copper (3.937w/cm-k) is a very close third.

Gold’s thermal
conductivity is 2.913w/cm-k. It’s primary use in electronics is for its electrical
conductivity and resistance to corrosion. The reason for this is
that gold is largely chemically inert and thus will withstand
many harsh environments. Graphite has many different thermal
conductivity values, ranging from .6-10 w/cm-k, dependent on the
formulation.

Sources: ai.mit.edu
pyrographite.com

Myth: Mercury would make a good coolant.

Reality: Mercury is way too environmentally unfriendly to be used
in a non-industrial cooling system. In fact, engineers studied
using mercury to cool a power plant. If mercury is too unsafe for
workers working around a controlled nuclear reaction, it then
has no business in the home PC.

Besides, if you
ever did have a leak, liquid mercury is an excellent electrical
conductor and would leave you with a dead motherboard rather
quickly. Mercury vapor is poisonous.

Myth: Antifreeze improves the thermal properties of water.

Reality: Antifreeze actually worsens the thermal properties of
water and is 18 times thicker at room temperature,
resulting in more back pressure and slightly lower flow.

However,
in a system with dissimilar metals (eg, aluminum and copper), antifreeze or
some other anti-corrosive is required to keep metals from reacting with each other to prevent corrosion. Antifreeze is used in a car to
raise the boiling point of water and lower the freezing
point. Unless you are boiling your CPU (not recommended) or
running below ambient (see condensation myth), then you are not in
need of those two characteristics.

Myth: Bleach is a good coolant additive.

Reality: Bleach is actually very corrosive, as indicated by both
its datasheets and ionic makeup. The chemical formula for bleach
is NaOCl, Which is ionic and in water becomes Na+ and OCl-. Ions
in solution will lead to galvanic corrosion, as previously noted,
as well as bleach’s natural tendency to corrode.

Source: Fact-index.com
{mospagebreak}

Myths About Temperatures and Measurement

Myth: On-board temperature sensors are accurate.

Reality: You shouldn’t even ask what someone else’s temps are.
This is one of the biggest problems I see in forums. YOUR temps
are not even accurate relative to your own system, much less
someone else’s.

Let me elaborate: Just because your board reads
41ºC, it does not mean that if the temps drop 5ºC in real life the board
will read 36ºC. Motherboard temperature sensors are not “absolutely accurate”, nor are
they “relatively” accurate.

This is even worse with in-socket
monitoring while watercooling. It is quite possible for the area
around the socket to heat up to 50-60ºC and influence the
temperature measurements of the in-socket thermistor.

Myth: Digidoc5/ThermalTake/Vantec temperature monitors are
accurate.

Reality: While generally not as easily influenced by other
factors, generally these sensors are off by more than +/- 3ºC. That
means if your sensor is reading 3ºC high and your friend’s is
reading 3ºC low, you could be off by 6ºC on your comparison.

More
accurate sensors are available – the question is whether that is
important to you. The point is, do not believe on-board/digidoc
temps. You shouldn’t even tell other people what they are, they
are so inaccurate. Tell them your overclock – there is very little
inaccuracy in MHz measurements.

Myth: Condensation will form when watercooling.

Reality: The water in a watercooling cannot get any lower than the
temperature of the air around the watercooling system flowing
through the radiator. In order to make the water colder than the
surrounding air, energy must be applied either in the form of a
compressor system or a thermoelectric device, both of which will
cool the water, sometimes below ambient. The temperature point at
which water condenses is a function of both temperature and
humidity.

Myths About Waterblocks

Myth: If a block is shiny, it will conduct heat better.

Reality: In reality, the flatness of the block is what counts.
Flatness ensures maximal contact with the CPU and minimal TIM
necessary to bridge the gap between the waterblock and CPU. Shiny just looks nice. It is
also of note that hand lapping a flat machined base will only make
the base worse, if the base was fairly flat from the factory.

Myth: You can tell how a waterblock will perform just by looking
at it.

Reality: The only way to know for sure is to run a highly
calibrated test with a well thought out methodology.

Thanks to:

  • Cathar for the racetrack analogy, and several corrections and
    comments;

  • Crimedog for doing a search on OCers for me when I was unable to;
  • nikhsub1, Etacovda, Razor6, AngryAlpaca, and DrMemory for
    corrections and comments;

  • All of those not named previously who have given me an education
    in the field of watercooling in general.

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