Father Doug
Quite right. The only thing that I would add is that many organic
surfaces are even somewhat hygroscopic (including many epoxy resins), so
that you do not even need to drop below the dew point to promote a
surface film in liquid phase. 'Tis a question of the equilibrium forces
between the water molecules beetling around, where the bonding forces
may be sufficient to prevent the molecules from shooting out into orbit
in the vapour phase (pictorially speaking). This is why dendrites can
form at 85% RH. Of course, some ionic (and non-ionic) contaminants are
also hygroscopic, so this can exacerbate the situation.
Brother Ellis in Cyprus has expounded!
Brian
[log in to unmask] wrote:
>
> David Douthit exclaims:
>
> First; trying to grow dendrites in a 85/85 environment is difficult at
> best. Dendrites
> require liquid water. the free energy at 85 degrees C is to high. It will
> not allow a liquid
> water film to form. you must be below 40 degrees C to ensure the formation
> of liquid water.
>
> Doug Pauls, well into his second Mt. Dew of the day, responds:
> No it doesn't, no it doesn't, no it's not, yes it will, and no you don't,
> respectively.
>
> It is not hard to grow dendrites, I do it frequently. Anyone who does SIR
> testing will say the same. I've even grown dendrites on gold samples.
> Dendritic growth takes moisture, bias and an ionic contaminant. The
> greater the magnitude of these factors, the greater the propensity for
> dendritic growth. Yes, most dendritic growth requires liquid water, but
> you only need a few molecular thicknesses to do this, which is easily
> attained at 85/85. Yes, liquid water can form at 85/85. All you need is a
> 1-2 degree difference in temperature between the environment and your
> substrate and poof, water. When water forms, temperature wise, depends on
> the relative humidity and the temperature. SIR testers, if they have done
> it long enough, have had an occasion where power goes out on the chamber
> and the temperature drop while the humidity remains high. You rapidly
> reach the dew point well above 40C.
>
> I'm sure Brother Ellis in Cyprus can expound on the physics in greater
> depth.
>
> Doug Pauls
> Rockwell Collins
>
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