Propylene Glycol.
It's cheaper than Ethylene Glycol (Car anti-freeze), is non-toxic with
rust and algae inhibitors. All anti-freeze lowers the freezing point of
the water it is mixed with. It also raises the boiling point of the same
mix. A 50/50 mix of "RV Anti-Freeze" may be what your looking for.
On 3/6/2017 8:04 AM, Manfred Mornhinweg wrote:
Okay, folks, can we then perhaps reach some consensus, or
recommendation about what exact liquid to use in watercooled electronics?
It has to be water-based, because there is simply no other liquid that
matches water in terms of specific heat, availability and cost.
We don't want buildup of anything in out heat exchangers. So we must
make sure that the liquid is pretty much free from calcium, magnesium,
carbonate, silicate, sulfate and other such ions that tend to
precipitate on hot metal surfaces and form scales. This rules out tap
water (unless it's very soft).
We don't want algae growing either. This pretty much requires some
sort of alguicide added to the liquid. Otherwise we WILL get them, in
the long run. What to use? Chlorine ions? Hypochlorite? Some alcohol?
We don't want corrosion. But a tiny amount of corrosion should be no
problem. If we load the amplifier with distilled water, and that water
rips some copper from our tubing and heat exchangers, until reaching a
balance, is that so bad? I think we can live with it. If not, then we
would need some sort of corrosion inhibitor if we start from distilled
water.
Conductivity may or may not be an issue. With a typical LDMOSFET amp,
that has the source (and cooling block) at ground potential, and every
other metal part of the cooling system at ground potential too,
conductivity is a non-issue, as long as there are no metals with very
different electrochemical potential there (because then one of them
would corrode the other, by electrochemical action). It should be easy
enough to make the whole system just from copper and plastic, so that
there would be only one metal, and no electrolytic corrosion. Then we
simply don't need to worry about conductivity.
But if I ever build the amp I planned years ago, that uses cheap
switching MOSFETs, I would have two cooling blocks at drain potential.
It would be easy to keep the entire water circuit at the supply
voltage, so there would be no DC between any parts of the water
system, but there would be RF voltage between the two cooling blocks.
Typical water solutions have a resistivity that's plenty high enough
to cause only negligible loss, but would the tiny RF current between
the two cooling blocks cause harmful electrochemical action? If yes,
then an insulating liquid would be needed.
Cars typically use distilled water mixed with concentrated
"antifreeze", which is usually a mix of glycol (the antifreeze proper)
with corrosion inhibitors, and possibly alguicides, etc. We don't need
antifreeze action on our amps, used at room temperature. And in
countries like mine, where in most places the weather never gets below
freezing, car parts stores sell a refrigerant for cars that doesn't
contain antifreeze, but just corrosion inhibitors and a lubricant for
the water pump. Would perhaps this stuff, mixed with distilled water,
be a good candidate? In cars it's typically used at a concentration of
just 1 to 3%.
A note on rainwater: Even in areas free from industrial pollution (if
such a place exists...), rainwater is still moderately acidic, because
the rain drops absorb carbon dioxide from the air, forming carbonic
acid (the same stuff that in much higher concentration makes soft
drinks fizzy). I live in a pretty clean, non-industrial forest area,
and some months ago I measured the acidity of freshly collected
rainwater. Its pH turned out to be 5.8.
And another note, on tap water: It doesn't seem to be terribly
corrosive, at least not to copper, brass and solder. Many houses here
have copper water pipes and brass fittings, soldered with either
tin-lead solder, or the more modern ones with pure tin, or with alloys
having mostly tin and a small amount of some other metal, and these
installations run reliably for many years, with hot and cold tap
water. Only in areas that have extremely hard water, scaling can
become a problem after several decades, mainly when scales shed off
and accumulate at the lowest spot of the piping, causing a clog there.
But corrosion doesn't seem to happen.
Instead when zinc-plated steel hot water tanks are connected to copper
piping, this forms a shorted galvanic battery! Given slightly acidic
tap water, it eats the zinc in a short time, and then starts eating
the steel below, and we get rusty hot water! Instead with slightly
alkaline tap water there is no big problem.
Anyway I can't avoid the feeling that it would be better to develop
good high efficiency linear amps, so that cooling requirements become
very low, and nobody would have any need to consider watercooling.
Manfred
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