Author Topic: Temperature Readings w/out FARS  (Read 2905 times)

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Offline LIWeather

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Temperature Readings w/out FARS
« on: March 25, 2009, 01:22:43 PM »
I am sure this question will yield a variety of answers and I'm not looking for a specific number, but curious to know how much of a change in temperature has been seen when using the Standard shield vs a FARS or Daytime only FARS. Again, I'm sure different results take place in different locations but just wondering if there was an average departure that anyone noted.

Offline offroadjosh

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Re: Temperature Readings w/out FARS
« Reply #1 on: March 25, 2009, 01:52:57 PM »
i know there is a link somewhere for this..



josh
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Offline LIWeather

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Re: Temperature Readings w/out FARS
« Reply #2 on: March 25, 2009, 03:21:46 PM »
Within the forums? I did a search and found nothing. (could very well have searched with the wrong terms)

Offline offroadjosh

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Re: Temperature Readings w/out FARS
« Reply #3 on: March 25, 2009, 03:41:03 PM »
yea i tried looking for it. i know i saw it on here, it was a link to the davis site.....
can someone chime in???

Josh
Josh Patten

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Offline mackbig

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Re: Temperature Readings w/out FARS
« Reply #4 on: March 25, 2009, 03:42:10 PM »
Other than some smothing of graphs, I doubt there are many hard findings.  Too many variables to consider.  Time of year, intensity of sun, average daytime high.... natural shade at location where shield is located.

Andrew

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Offline johnd

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Re: Temperature Readings w/out FARS
« Reply #5 on: March 25, 2009, 04:03:17 PM »
What you're thinking of is probably:

http://www.davisnet.com/support/weather/support_docs.asp?dtype=3

See App Note 24 'Comparison of 5 radiation shields'

With irradiance of ~1000W/sqm and calm conditions the passive shield probably over-reads by 1.5C or so relative to a FARS shield (which itself overestimates true air temp by maybe 0.5C). But don't forget that many official temperature values are still recorded in passive shields, which have much the same error as the Davis shield (which actually does a pretty good job as a passive shield). There are many pros and cons to FARS vs passive - too many to summarise quickly here. FARS will be more accurate under calm sunny conditions but also has certain disadvantages, especially in higher latitudes where there's less sun to power the fan.
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Offline JOE

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Re: Temperature Readings w/out FARS
« Reply #6 on: March 25, 2009, 05:13:28 PM »
The fan won't make it any cooler than the air around it.

I think the idea of the FARS is to remove any heat build-up within the shield from outside sources.

You can remove heat, but you can't cool it.  Significately, at least.
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Offline DanS

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Re: Temperature Readings w/out FARS
« Reply #7 on: March 25, 2009, 05:28:35 PM »
Although I plan on installing a FARS in the next month or so, one concern that I have is the fan causing abnormally low temp readings during the first hours of daylight. For example, since I live in the South where we have relatively high humidity levels and consequently, high dew points, my concern is that on balmy mornings when the dew collects on everything, including the temp sensor, that when the fan kicks in in the moring that it causes the dew to evaporate, thus artificially driving down the temp-sensor readings. Overall though, I think the benefits of a FARS outweight the possible negatives, so I plan to install one soon.

Bruce
Bruce, also if your FARS is a daytime only setup you can have your solar panel pointed more S/W away from the morning sun slightly. This way all morning dew will be dried out and evaporated long before temps rise up enough to need the fan on.

Offline johnd

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Re: Temperature Readings w/out FARS
« Reply #8 on: March 25, 2009, 05:29:08 PM »
The fan won't make it any cooler than the air around it.

If the air movement caused by the fan were to be actively evaporating any dew condensed on the sensor then that would make the sensor read lower than the surrounding air, potentially by quite a noticeable amount. This would obviously only be a transient effect, ie until any residual dew had evaporated, but it would potentially be real for as long as the dew lasted.

But as I alluded to above there are several second order effects to consider in FARS vs passive. It does also make a difference where in the world you're located. In Northern Europe (lower max solar irradiance, usually more breezy conditions, daylight hours often too short in winter to recharge solar power adequately), I'm not sure that the cons don't outweigh the pros for a FARS, or at least for the 24 hour FARS - the DFARS is more debatable perhaps. But if you're in eg the Southern US then the issues might well weigh differently and very possibly in favour of the FARS.
Prodata Weather Systems
Prodata's FAQ/support site for Davis stations
Includes many details on 6313 Weatherlink console.
UK Davis Premier Dealer - All Davis stations, accessories and spares
Cambridge UK

Sorry, but I don't usually have time to help with individual issues by email unless you are a Prodata customer. Please post your issue in the relevant forum section here & I will comment there if I have anything useful to add.

 

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