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Davis: Fan Aspirated versus Passive Shield

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ocala:
http://ams.confex.com/ams/pdfpapers/118190.pdf

weatheroz:

--- Quote from: "ocala" ---http://ams.confex.com/ams/pdfpapers/118190.pdf
--- End quote ---


The way I read that, I was getting the impression that the passive screen was  'better' than the fan aspirated one ?

I think I might have to read it again. :)

Mark / Ohio:

--- Quote from: "weatheroz" ---
--- Quote from: "ocala" ---http://ams.confex.com/ams/pdfpapers/118190.pdf
--- End quote ---


The way I read that, I was getting the impression that the passive screen was  'better' than the fan aspirated one ?

I think I might have to read it again. :)
--- End quote ---


Yeah that was my impression too at first.  After re-reading again, the NOAA sensor also used a passive shield.  So it is better in matching the NOAA readings.  But it could also be said the aspirated Davis was more sensitive than the passive shielded Davis or official NOAA unit.

weatheroz:

--- Quote from: "Mark / Ohio" ---

Yeah that was my impression too at first.  After re-reading again, the NOAA sensor also used a passive shield.  So it is better in matching the NOAA readings.  But it could also be said the aspirated Davis was more sensitive than the passive shielded Davis or official NOAA unit.
--- End quote ---


I also noted somewhere that they recommend the rain guage be separated from the screen, which I thought was a bit strange.

From what I can work out / guess, the rain gauge being black convects the air up past the screen, which to me is a good thing as it works like a tiny fan bring air past the temp sensor, particulary on a still day with no breeze. When there is a breeze, then it should not create any issues at all.

Also, the simply fact that the size of the rain guage also shades the screen, particulary during the middle of the day... remembering that the base of the rain guage is not absorbing heat like the rest of the guage as it is also in the shade.

Not having the rain guage above would in my mind, necessitate the addition of FARS, and would also skew this previous study made, with a more pronounce difference between the FARS and passive, as well is the base temp sensor.

Mark / Ohio:
It would have been nice if they would also have included the VP in it's factory original configuration in the study.

From the article..
"This study compared the MMTS to the Davis
Instruments passive shield and retrofitted daytime
fan because it was believed that NOAA would be
more likely to accept an installation where the
radiation shield is separated from the rain
collector. The most widely used weather station
that Davis Instruments manufactures has a rain
collector that is installed above the radiation
shield."

I never thought much about the daytime temps and the rain gauge's effects but I know mine would work better during the night without the rain gauge close to it.  In my case I use a homemade rain gauge heater with foil faced insulation around the inside of the cover. It would read a degree or so higher during a cold calm winter night with the heat on.  But I'm using a 130F clothes dryer bi-metal on the aluminum heatsink that I mounted the ceramic resistor to.  Davis I think uses a 110 thermostat.  This year I put a second thermostat to measure ambient air temp inside at the bottom of the gauge.  It cuts on at 35F and off at 45F.  So the 130F will now be just to keep the heatsink from getting too hot and melting the plastic.  The test will be if it melts the snow fast enough once the real cold weather gets here.  I've had it plugged in for about a month now and so far my night time temps seem to be better than before while running it.

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