Am 2022-11-29 10:01, schrieb Attila Kinali via time-nuts:

I have bought a sensor from Honeywell some years ago with the intent
to build an altimeter. Never tried it since since I have -7 diopt
now and I'm probably no longer air worthy as a glider pilot.

If I still can find it, you can have it. Either per mail or across
the table in the Zwickel in Dudweiler, some Friday evening.

Cheers, Gerhard

PT100 or PT1K are fine and stable for temperature.

We have no technology which can make long term stable precision
measurements of humidity and pressure:  You will need periodic
calibration no matter what kit you get.

"Long" calibration periods are two years, and if you can avoid
condensation, you can get down to 0.5%RH drift over that period.

The big player in this space is Vaisala.com and they are not cheap.

Calibrating RH and pressure sensors are essentially just highschool
physics experiments, and if that's your hobby: Go for it.

You dont need to do it /all/ from scratch, see for instance:

https://store.vaisala.com/en/products/HMK15

--
Poul-Henning Kamp      | UNIX since Zilog Zeus 3.20
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Never attribute to malice what can adequately be explained by incompetence.

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 ).

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

On 11/29/22 4:45 AM, Poul-Henning Kamp via time-nuts wrote:

One can measure humidity a variety of ways - one way is a dewpoint
sensor - a peltier device on a mirror and you sense the temperature at
which the mirror stops being specular. If the air is run through a
filter, then you don't need to clean it so often.

One can also measure humidity by measuring the loss or propagation speed
at microwave frequencies.

Measuring pressure or density is a bit trickier. I would think that the
standard piezoresistive aneroid sensors (sealed vacuum behind a strain
gage, typically micromachined silicon, but could also be on silica) are
long term stable. These sensors often have a significant tempco.

On 11/29/22 6:34 AM, Bob kb8tq via time-nuts wrote:

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.

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?".

Many 🌙 moons ago I scored a bunch of DPM 7885 pressure sensors off eBay.

The herd of 10ish units were all inside 70cm (pressure converted to altitude by standard atmosphere model).

The local guy calibrating private aircraft altimeters was not interested. The sensors were likely superior to his (transfer) standard.

/Björn

Sent from my Phone

…

On 11/29/22 9:43 AM, Björn wrote:

A longer time ago (1980s) I was building SAW based pressure sensors as
altimeters - they wanted 1 foot accuracy from SL to 40,000 ft.  We had a
fancy calibration setup that used some form of sensor that used a force
balance of some sort (there was this motor that moved a bead chain on
some sort of mechanical device.. I can't remember the details.)

I learned then "do not let Helium near a sensitive absolute pressure sensor"

It turns out this is a problem on ISS - the atmosphere on ISS has a
remarkably high helium concentration (a few percent IIRC) because it's
used for packaging, as a propellant, etc.