Orchid XP v7 nous apporta ses lumieres en ce 2007/11/10 17:17:
> Alain wrote:
>> Orchid XP v7 nous apporta ses lumieres en ce 2007/11/10 04:19:
>>> Nicolas Alvarez wrote:
>>>
>>>> I read on a website about an interesting CPU watercooling technique: 
>>>> submerging the motherboard. They still had trouble because of ions 
>>>> on the water. It's hard to get *totally* pure water...
>>>
>>> We have a machine at work that produces "20 mega-Ohm water".
>>>
>>> Of course, water reacts with *air* to absorb various ions. (For 
>>> example, CO2 disolves in water to yield carbonic acid.)
>>>
>>> But seriously - computers work at 12V. The water's resistance would 
>>> have to plummet to yield any meaningful current.
>>>
>>> I would imagine a far bigger problem is water's viscosity; I'd think 
>>> fans wouldn't like that...
>> Main power is 5V for all the electronics. The 12V is for the fans and 
>> drives motors.
>>
>> A good deal of the currents are counted in miliamps or less. In 
>> particular, the data transmition use very low curents at 5V. A leak of 
>> less than a 1 miliamp can easily cause malfunctions. And then, those 
>> 12V going to various motors can push or pull some curents into the 
>> data and address paths...
> 
> My point was more that at a mere 12V, the resistance has to drop *very* 
> low indeed for any noticable current to happen. (Ohm's law, and all 
> that...)
Not that much. When the distance is short and the surface relatively large, even 
10K ohm can let pass to much durrent.
On a typical computer mother board, there are 1000's of places where the 
distances are less than 0.5mm, with neigoing surfaces in the 2~3 mm square 
range. Have some 200 MHz signal passing there and you can easily get currents in 
the 0.1 miliamp range, and a parasitic current of about 0.01 miliamp is 
sometimes enough to cause malfunctions.
So, no, you don't have to have a big drop in resistivity at all.
The 20 Mohm water quickly drop to the 100Kohm range. And that's for a 1cm cube 
of water. A 1 cm travel trough a 1 cm square section. In a case where it's 0.5mm 
with a section of 2 mm square, you divide the resistance by 4! Add some 
contaminant buildup to the mixture, whose concentration is the greatest near 
those conductors, and you easily get a HUGE resistivity drop.

-- 
Alain
-------------------------------------------------
You know you've been raytracing too long when you stop using a protractor to 
measure angles because you can do it just by looking.
Taps a.k.a. Tapio Vocadlo

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