[time-nuts] Re: Long term stable environmental sensors

[Lux, Jim]

On 11/29/22 6:34 AM, Bob kb8tq via time-nuts wrote:
> Hi
>
> If you have a giant pile of bonus points on your credit card, you can go investigate
> a “chilled mirror hygrometer”. There are a variety of brands. They all attempt to get
> around the drift issue by taking it all back to dew point and thus a temperature reading.
> They are very expensive and I’m sure there are subtle issues with them ( maybe
> keeping the mirror clean ….who knows ).

They can be made to work quite well with contaminated atmospheres.  I 
was looking into building one for a meat smoker (where you need to 
measure humidity at temperatures above 100C - the solid state sensors 
don't work up there). The basic sensor is really simple - it's basically 
an optocoupler behind a glass plate. (historically, it was a light 
source and a photodetector on a prism, but I suspect that's because 
"back when" the source and detector were "vacuum tubes" and you needed 
something physically large.

There were two designs I saw: both "servo" around the dew point, but 
some try to track the "beginnings of condensation", others ramp up and 
down around the dewpoint and look for the change in reflectivity as it 
crosses the dew point. I think, today, you could cobble something 
together with an arduino, etc.   Of course, if your time has any value, 
then you might as well buy the $1000 sensor.

Dry/wet bulb also works, but requires maintenance.


>
> For pressure, the local airport might be an adequate substitute. A lot depends on just
> how much your building self pressurizes and how far that airport is from you. Building
> issues can be addressed to some degree with a differential (indoor / outdoor) measurement.
> Those are pretty easy to calibrate with a tube of water.
>
> Bob
>
Either a manometer or a magnehelic gauge.  And if you need it in digital 
form, a camera looking at the water manometer.  Of course, you WILL need 
to correct for changes in the acceleration due to gravity for the 
manometer. You could use silicone oil, so it doesn't evaporate.

https://dwyer-inst.com/series-magnehelic-2000.html

Referencing to published outdoor sensor of some sort.. I don't know how 
accurate the standard weather service system is? I would think that it's 
"good enough that planes are the right height" but I'll bet that's no 
better than 0.01" of mercury (0.03" = 1 mbar/hPa).  Note that reported 
baro pressure is "reduced to sea level" using the ICAO standard 
atmosphere, which is almost certainly NOT what your local atmosphere is 
like.  (you would need what's called "station pressure")

Fun fact - at some airports, the baro reading they report (particularly 
on Unicom) comes from them having a calibrated altimeter, and then they 
set it so it reads the known field (or altimeter) altitude. Then they 
use the setting in the window as what they report.  After all, the goal 
is to have pilots all have a setting that makes the altitude read the 
same as the field (plus height of sensor).  FWIW, I don't recall ever 
having my altimeter recalibrated in the several years I owned a plane.  
Granted, you'd set to the pressure setting you're given, then check that 
it reads reasonably close to the field altitude.  But that's more a 
check on "is it broken?".