Re: [ATM] ATM Digest, Vol 28, Issue 20

["Arjan te Marvelde" <arjan.te.marvelde@hetnet.nl>]

Mark, Gil, e.a.

Before considering the response of the control electronics, it might be
worthwhile to decide what temperature difference to measure.
For minimum mirror deformation you would like to keep the gradient in the
glass down to zero. The convection however remains even when these gradients
are under an acceptable level.
Hence the ultimate temperature difference to measure would be between the
mirror surface and the ambient air. You might get away with measuring the
glass center instead of surface, but the error you may get is probably above
the acceptable limit of, say, 1 degree.

Would it not be better to just leave the fan on? Or is the circuit really
meant for knowing when the initial cooldown is close enough?

Arjan


> > Really, one might want a V shaped response curve.  One leg for cooling
> > temperature vs. time and the other leg for warming, just in case.  That
> > is, unless the mirror is equipped with the dew heater.  OK, the V shaped
> > response is probably gilding the lily.  It might be useful for a few,
> > but probably not for most.
> >
> Having written that, I am having second thoughts about the hedging
> sentences.  When air temperature is not dropping rapidly, the mirror
> might radiation cool below air temperature.  That is, after all, one of
> the situations that causes dew condensation.  Maybe the V shaped curve
> isn't so loony.  Use the fan for heating as well as cooling.  In some
> climates, it might be enough to keep dew at bay.  Plus, a cold mirror is
> going to form just as nasty a boundary layer as a warm one.
>
> Mark Holm
> mdholm@telerama.com
>
>
>
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> End of ATM Digest, Vol 28, Issue 20
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