[time-nuts] TECs in cooling below ambient

[Dana Whitlow]

The optical sensing of despoint by dew-on-the-mirror was used in a device
designed
at the Whirlpool Research labs in St. Joseph, MI.  It apparently never made
it into
production, but a number of units were built and sold or given away (I'm
not sure).
I know about this because I interned at the lab during college summer
breaks, and
on one occasion I was asked to replace the Peltier (TEC) module in one of
these
units.  These units had a thermocouple buried under the cold mirror's
surface, and
use of the device required that the user provide the instrument to read the
thermo-
couple's temperature.

I've long wondered how this system dealt with the fact that the desired
operating
point of the loop is on a sharp corner of the light versus temperature
curve where
dew is just beginning to form, and there is no "negative dew" on the warm
side of
that point.  I hadn't thought of this complication when I was working with
this thing,
however, or I would have asked.  Oh, well, an opportunity missed.  Sigh!

Dana


On Wed, Dec 23, 2020 at 6:53 PM Bruce Griffiths <bruce.griffiths at xtra.co.nz>
wrote:

> You could always use a TEC as the heart of a dewpoint sensor by optically
> sensing the temperature required for dew to form on a cooled mirror.
> In practice the temperature at which the dew vanishes is typically used.
> A collimated light beam together with a photodiode is typically used to
> sense the presence of dew droplets on the mirror.
>
> Bruce
> > On 24 December 2020 at 08:57 ed breya <eb at telight.com> wrote:
> >
> >
> > This recent TEC talk reminded of some of my long term planned projects,
> > and related issues. I have at least four "someday" projects involving
> > TECs, to regulate device temperatures near or below "normal" room
> > temperature, including a high precision DC voltage standard, a sub-fA
> > electrometer circuit, a constant temperature block for nonlinear analog
> > computing elements, and a small general purpose heat/cool box for device
> > and circuit testing. Each has its own particular system, application,
> > and environment issues, but common to all is the lower limit of running
> > temperature, based on the local climate conditions and dew point. I plan
> > to estimate the lowest possible operating temperatures for expected
> > conditions, that avoids condensation, and not having to resort to
> > special packaging.
> >
> > An essential thing for this is a dew point calculator. I found lots
> > online, but this is my favorite so far. It's slider-based, so you don't
> > even have to enter numbers.
> >
> > http://www.dpcalc.org/
> >
> > The harder part is finding the normal range of local climate conditions.
> >
> > Ed
> >
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