[time-nuts] TECs in cooling below ambient

[Bob kb8tq]

Hi

One of the (many) parts of the EG&G empire made a dew on the mirror 
setup and (apparently) sold quite a few of them. It was one of the few gizmos
that could reliably tell you about very low temperature (like -50C) dew points. 

Bob

> On Dec 23, 2020, at 8:37 PM, Dana Whitlow <k8yumdoober at gmail.com> wrote:
> 
> 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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