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

Hi I think you will find that things work out a whole lot better if you target something just above room temp. If your room runs 22 +/- 3 C , a set point of 27C likely results in better operation than 17C. Bob > On Dec 23, 2020, at 2:57 PM, ed breya <eb@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 > > _______________________________________________ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there.

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@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 > > _______________________________________________ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there.

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@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@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 > > > > _______________________________________________ > > time-nuts mailing list -- time-nuts@lists.febo.com > > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > > and follow the instructions there. > > _______________________________________________ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. >

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@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@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@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 >>> >>> _______________________________________________ >>> time-nuts mailing list -- time-nuts@lists.febo.com >>> To unsubscribe, go to >> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com >>> and follow the instructions there. >> >> _______________________________________________ >> time-nuts mailing list -- time-nuts@lists.febo.com >> To unsubscribe, go to >> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com >> and follow the instructions there. >> > _______________________________________________ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there.

The method still appears to be in use: http://www.michell.com/nl/documents/Optidew_Vision_97144_NL_Datasheet-V6.pdf Bruce > On 24 December 2020 at 14:37 Dana Whitlow <k8yumdoober@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@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@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 > > > > > > _______________________________________________ > > > time-nuts mailing list -- time-nuts@lists.febo.com > > > To unsubscribe, go to > > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > > > and follow the instructions there. > > > > _______________________________________________ > > time-nuts mailing list -- time-nuts@lists.febo.com > > To unsubscribe, go to > > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > > and follow the instructions there. > > > _______________________________________________ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there.

Many years ago, I got some junker DP sensors - the chilled mirror type. I never did find any specific info on the model, or get around to figuring out the guts to run them. I can't recall if the support electronics were missing, or maybe potted, or if I just didn't want to reverse engineer them. I think I junked the non-usable parts, but may have saved some of the heads. I vaguely recall powering the TEC on one, and saw the dew show up on the mirror - very cool, so to speak. I always assumed they cycled the temperature over a narrow range once dew appeared, and used lock-in techniques to get a reliable optical signal telling the dew versus NOT dew condition, to home in on the temperature. Interestingly, the reason these were probably junked was that the little DC fans in the heads had all crapped out due to worn out bearings. Fans are so critical to many things, but are often the weakest link. Ed

-------- Bob kb8tq writes: > I think you will find that things work out a whole lot better if you target something > just above room temp. If your room runs 22 +/- 3 C , a set point of 27C likely results > in better operation than 17C. This decision should be based on expected abnormal situations. If your ambient might reach 35-40°C when cooling fails, running lower than ambient will cope, running higher than ambient will not. If your ambient might drop to -20°C when the heating fails or the lid is opened, then running higher than ambient is the robust thing to do. And as others have said: If you run lower than ambient, you have to plan for condensation. Bidirectional TEC setups need special attention: You have to take into account the 3:1 efficiency difference between heating:cooling, and only seldom, and then gently, switch direction, in order to reduce thermal stress in the TEC element. A sound bidirectional design use an inner oven to keep the payload temperature constant, and use a more coarse outer TEC loop only to keep the oven's operating conditions inside a narrow window. That way the TEC can operate with a suitably slow time-constant. -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 phk@FreeBSD.ORG | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence.