Temp/Humidity control systems?
In message 20161027134312.GA18158@panix.com, Ron Bean writes:
And since this is time-nuts: Measuring humidity accurately is tricky.
According to people who have tested them, commercial electronic humidity
sensors, when tested in a lab, have never come anywhere close to the
accuracy claimed in the data sheet.
The main problem in measuring humidity is physical gradients: It is
incredibly hard to create a volume of homogenous humidity on a planet
which has gravity, and for that reason, a lot of labs are not anywhere
near as accurate as they think they are.
The exception is the "cold mirror" type of sensor, which measures the
dewpoint by cooling a mirror and bouncing a light off it to sense the
temperature where dew condenses on it. Those are expensive, and they
require maintenance to keep the mirror clean.
And they are comparatively slow, last I saw one it could only do
a measurement every second minute.
--
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.
Dew Point measurement technology is limited by surface energy problems,
in the near vicinity of the surface,Van der Waals or London forces create uncertainties
in the physical processes and dew point instruments always seem to have these uncertainties.
The physical equilibrium on the surface of a water film has proved reliable, hence the reliance
of reference instruments using psychrometery. Dew point technology is mainly used for measurement
at extremes of dewpoint well outside the conditions in ambient weather and in in-line situations
in industrial control where precision gives way to convenience.
cheers,
Neville Michie
On 28 Oct 2016, at 11:05 AM, Poul-Henning Kamp phk@phk.freebsd.dk wrote:
In message 20161027134312.GA18158@panix.com, Ron Bean writes:
And since this is time-nuts: Measuring humidity accurately is tricky.
According to people who have tested them, commercial electronic humidity
sensors, when tested in a lab, have never come anywhere close to the
accuracy claimed in the data sheet.
The main problem in measuring humidity is physical gradients: It is
incredibly hard to create a volume of homogenous humidity on a planet
which has gravity, and for that reason, a lot of labs are not anywhere
near as accurate as they think they are.
The exception is the "cold mirror" type of sensor, which measures the
dewpoint by cooling a mirror and bouncing a light off it to sense the
temperature where dew condenses on it. Those are expensive, and they
require maintenance to keep the mirror clean.
And they are comparatively slow, last I saw one it could only do
a measurement every second minute.
--
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.
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Years ago I had to deal with this and the instruments and sensors we
used matched well against dry and wet bulb measurements. I suspect
consumer level stuff varies considerably in reliability and accuracy.
The capacitive sensors are tricky to use because they require AC
excitation to prevent damage from electromigration. I see a lot of
integrated sensors are available now and I wonder how well they really
work
On Fri, 28 Oct 2016 09:39:04 +1100, you wrote:
You are correct to question commercial humidity sensors.
It seems to have come about because no-one can make a dollar by selling humidity.
Manufacturers do not tell the truth, they think ours is as good as theirs so we
should claim the same accuracy. People buy these sensors, believe them, and buy more.
It is not hard to measure humidity/temperature. (they should be measured together).
Cover the bulb of an ASTM32C thermometer with cotton gauze. Insert it through the wall
of a short length (18) of 4 metal tube, insert another ASTM32C thermometer through the side
of the tube 4 upstream. Put a computer fan on the outlet of the tube sucking air over
the thermometer bulbs at about 4m/s. Wet the thermometer bulb, but NEVER touch it with your fingers.
In about 3 minutes you can take two temperature readings. There are a number of tables and calculation methods,
some much worse that others that will convert these values to air temperature, Relative humidity,
Dew Point temperature etc. You get accuracy of 1% from temperatures measured to 0.1C.
If anyone is interested I have basic routines for XCEL spreadsheet use to do the hard work.
This is based on the WMO Reference Psychrometer developed by Russel Wylie of NML Australia.
On 28 Oct 2016, at 12:43 AM, Ron Bean time@rbean.users.panix.com wrote:
And since this is time-nuts: Measuring humidity accurately is tricky.
According to people who have tested them, commercial electronic humidity
sensors, when tested in a lab, have never come anywhere close to the
accuracy claimed in the data sheet. The best you can hope for is
consistent readings, not absolute accuracy.
...
Have you looked into wine cellar equipment?
They sell equipment that's designed to control temperature and humidity for custom built wine cellars that can be anything from an retrofitted insulated closet to a small room. I think that they also sell prefab cellars to put into an existing space.
KE6CD
On Oct 26, 2016, at 8:59 AM, John Ackermann N8UR jra@febo.com wrote:
I may have the opportunity to build a small "clock room" and am considering whether I could make it an environmentally controlled space. I'd like to learn about the options for doing this.
The space would probably be 6x8 feet or so, in a basement with one outside wall.
Can anyone point me to purveyors of the hardware to do something like this? Because I'll have a limited time to build this, I'm looking for something that uses more-or-less off the shelf gear, and not a whole lot of custom engineering.
Thanks!
John
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There are people making a lot of money installing heating cooling systems
that take advantage of the Earth's thermal stability. What they do is dig
a hole the size of a swimming pool and lay out a zig-zag pattern of pipe on
the bottom of the hole then backfill the hole. Assuming the pipe is
buried deep enough which depends on where you live, the water comping out
of the pip is always about 58F. In the summer they use the 58F water to
cool the air, in the winter with snow outside 58F water seems almost warm
and ty use it for heating, pre-heating. If the pipe farm is big enough
you can keep a house comfortable with just a water recirculation pump.
My point here is that if you really want to keep something at constant
temperature what you want is huge amounts of dirt, rock or concrete, not so
much a heater/cooler
A basement is not quite deep enough nor far enough away from heated living
space to be stable. But I think rather then fighting it, measure the
day/night year round average temperate of the basement room and make THAT
your set point temperature. This minimizes the temperature difference
and minimizes the amount of work out heater/cooler will have to do. It
should save money but also reduce the amount of temperature change. So,
try using bricks or blocks all around and set the thermostat for about 58F
or whatever the local average is
For something as small as a 10Mhz oscillator I've wanted to try an
experiment where I drill hole drop in the electronic part then fill in the
hole.
On Thu, Oct 27, 2016 at 12:41 AM, Poul-Henning Kamp phk@phk.freebsd.dk
wrote:
In message 20161026210517.26c0fd397b1cae5ba9c12def@kinali.ch, Attila
Kinali w
rites:
Probably the easiest is to get some glass/mineral wool insulation and
put it over all the walls, including ceiling and floor. I do not recommend
any foam or styropor based insulation as almost all of them are
inflamable.
This should get you into the area of 10-100W/K thermal resistance for your
closet (assuming something like 4cm thick insulation gets about 40W/K).
Stop!
Over insulating is a 100% sure-fire way to get unstable temperature inside,
because it amplifies the consequences of any change in power dissipation.
It is a classic mistake to build a 100mm insulated enclosure inside an
office-like enviroment and end up having less stable temperature on
the inside than the outside.
Cinderblocks is a much better material for that scenario, because they
have both thermal mass and inertia (= heat capacity and heat impedance)
--
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.
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Chris Albertson
Redondo Beach, California
In message CABbxVHuZLKkWKyYP_v68a+Gk0RNKSW9RO-trv77-c7NdT3WzBA@mail.gmail.com, Chris Albert
son writes:
[...] the water comping out of the pipe is always about 58F.
Unless you live close to a volcano or an artesian spring, the
temperature will be exactly the yearly average air-temperature of
your climate.
Precisely how deep you need to dig for stability that varies with
your geology and precipitation, it may be as little as one meter,
it may be as deep as 10 meters.
My point here is that if you really want to keep something at constant
temperature what you want is huge amounts of dirt, rock or concrete, not so
much a heater/cooler
This is certainly true for any passive object, but as soon as we talk
about heat-emitting objects, this no longer holds, and you have to
take thermal conductivity, -capacity and -impendance into account.
If you happen to dig a deep trench anyway, by all means plonk a
PE40 tube down there to run liquid through, but unless you are
really dedicated, don't start digging just for constant
temperature.
--
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.
On 28 October 2016 at 01:05, Poul-Henning Kamp phk@phk.freebsd.dk wrote:
The exception is the "cold mirror" type of sensor, which measures the
dewpoint by cooling a mirror and bouncing a light off it to sense the
temperature where dew condenses on it. Those are expensive, and they
require maintenance to keep the mirror clean.
And they are comparatively slow, last I saw one it could only do
a measurement every second minute.
I was just reading the Keysight forum on VNAs. Someone was having issues
measuring a bit of waveguide 80 m long. Dr. Joel Dunsmore pointed out that
humidity has an effect in waveguide.
I'm guessing this is due to the change in permittivity in the air, which
would be most noticeable near the cutoff frequency of the waveguide. That
should change the magnitude of the reflection, not just the phase, so it
might be a relatively cheap way to measure humidity.
Just thinking aloud. I'm probably talking rubbish.
Dave
On Sun, 30 Oct 2016 20:38:49 +0000
"Dr. David Kirkby (Kirkby Microwave Ltd)" drkirkby@kirkbymicrowave.co.uk wrote:
I was just reading the Keysight forum on VNAs. Someone was having issues
measuring a bit of waveguide 80 m long. Dr. Joel Dunsmore pointed out that
humidity has an effect in waveguide.
I'm guessing this is due to the change in permittivity in the air, which
would be most noticeable near the cutoff frequency of the waveguide. That
should change the magnitude of the reflection, not just the phase, so it
might be a relatively cheap way to measure humidity.
Just thinking aloud. I'm probably talking rubbish.
This has been used for quite some time. Though cheap is relative :-)
A non-representative selection of papers that I have lying around here:
"A Reference Standard for Measuring Humidity of Air Using a Re-entrant Radio
Frequency Resonator", by Huang, Ripple, Moldover, Scace, 2006
http://www.nist.gov/customcf/get_pdf.cfm?pub_id=902995
"Automatic digital microwave hygrometer", by Hasegawa, Stokesberry, 1975
http://dx.doi.org/10.1063/1.1134331
"ELECTRICAL PROPERTIES OF ATMOSPHERIC MOIST AIR: A SYSTEMATIC,
EXPERIMENTAL STUDY", by Carlon, 1988
http://www.dtic.mil/dtic/tr/fulltext/u2/a199599.pdf
"The Refractive Indices and Dielectric Constants of Air and its Principal
Constituents at 24,000Mc/s", by Essen, Froome, 1951
http://dx.doi.org/10.1088/0370-1301/64/10/303
Don't ask why i have these papers....
Attila Kinali
--
Malek's Law:
Any simple idea will be worded in the most complicated way.
On 10/27/2016 6:43 AM, Ron Bean wrote:
BTW some of us are more sensitive to humidity than others. I can't tell
you the RH of a room, but I can tell you when it's too dry for comfort.
I want it as close to 50% as I can get it without growing mold on the
walls. Some "experts" claim that 30% is good enough for anyone, but
they're wrong.
Clearly, you don't live in Arizona :-)
Looking at it as a problem in thermodynamics, which has equations for
the flow and storage of heat, it might have a simple solution.
If you can have your equipment closet hotter than the basement will ever
be, we can use basement air for cooling the closet.
The basement air can be held to about +/- 5 degrees F with conventional
heating and cooling. You should dehumidify to 50% or less if that is a
problem. 30% is a reasonable minimum. You should have a fan or two to
stir the basement air.
Install the biggest standard air filter you can find in an inside wall
of the closet, which will filter incoming air. Install one or more
exhaust fans on the wall opposite the filter. At least one of the fans
must be variable speed. Or you could use an array of small fans with
individual switches to get controlled air flow - better yet, make two of
the fans in the array variable speed. Shouldn't be a problem if you use
common 12 VDC fans.
Now for the control system. You need sensors for closet temp and
basement temp and humidity, and also one for the power flow into the
closet. Electrical power leaving the closet on 50 ohm cables is assumed
to be negligible. You need a D/A converter to control the speed of the
fans.
You know the temperature of the closet air (maybe want an average) and
you know the temp and RH of air available to cool the closet. The RH
will affect the heat capacity of the air. Now you calculate the amount
of basement air needed to balance the heat flows at the desired
temperature, and adjust the fans to provide it.
Think of the fun you'll have determining the heat flow model constants
for the system. In particular, there's no air flow sensor because they
are expensive. You'll need to determine the relation between fan speed
and air flow.
I'd do this myself to determine the attainable precision, but I live in
an old folks apartment now. Let us know how it turns out.
Bill Hawkins
I know you wanted a COTS solution, but I think this is what you need for
1 degree control. You can't do it with on/off control.