Re: Re:Wierd UrQ cooling issues

[Robert Myers <rmyers@inetone.net>]

CAUTION!  Pedant alert!  Read further at your own risk.

At 11:13 AM 4/19/99 GMT, Phil Payne wrote:
>In message <002d01be8a49$29390140$9f791bd0@sony> "Mik Tip" writes:
>
>> Pressure is what keeps the coolant(water) from boiling too soon!
>> Water boils near 120 degrees at sea level.  The "antifreeze" in your
>> system helps to elevate this number, as does the pressure.

True, Mik.

>Over here, it boils at 100 degrees.  Over there, it boils at 212 degrees.

Yes, Phil, pure water boils at 100 degrees C *at one standard atmosphere*
of pressure.  Higher or lower pressures cause higher and lower boiling
points.  The _normal_ boiling point of water (normal = at 1 atm pressure)
is, by definition, 100C (or 212F if you prefer).  A higher pressure will
cause a higher boiling point for the water.  Additionally, "antifreeze" or,
for that matter, any other relatively non-volatile solute will increase the
boiling point of the solution as compared to pure water at the same pressure.

The boiling point of a substance is the temperature at which the vapor
pressure of the substance is equal to the applied pressure (whatever that
pressure might be).  It requires a higher (or lower) temperature to raise
(or lower) the vapor pressure higher (or lower) in order for the vapor
pressure to be equal to a higher (or lower) applied pressure.  Solutes
decrease the relative proportion of solvent molecules at the surface of the
solvent thus reducing the opportunity for solvent molecules to escape from
the solvent.  This has the effect of reducing the vapor pressure at any
given temperature.  It also requires that the solution be heated to a
higher temperature to produce a given vapor pressure thus raising the
boiling point.

I have no problem with the rest of your statement, though.

>Anyway - cooling systems are pressurised to a limit (on Audis) of about
>1.4 bar.  The cap contains a safety valve - in my own experience, this
>operates once and once only.  After an event, it has to be discarded.

>It's _supposed_ to survive and reset itself - they never do.


___
   Bob
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