Topic: FARS vs Passive Temperature Shield

[kcidwx]

For the first time I have both a FARS and passive radiation shield operating together. Now that November is over, I decided to take a look at the first full month of data. The FARS is a RM Young with a Nimbus temperature probe inside. The passive shield is a Davis with the Weather Monitor II temperature/humidity sensor inside. Before installation in October, both sensors were tested across the range at six different temperature points between -20°F to 100°F. Here's a photo of the sensor suite and a copy of the data collected for November.

[Aardvark]

The difference isn't that much.  I have the same thing as well set up and I am noticing a little bit between both.  I was told that the differences are noticeable during hot weather and humid climates.   

I am keeping data as well and probably within a year of amassing data, I will have a much better comparison that I have now

[kcidwx]

That is true and I expect to see the biggest difference in next summers data. I was surprised at November's data. I was expecting the passive shield to be 1.5°F to 2°F higher.

[Skywatch]

Here in Texas it gets pretty hot. I rarely ever see anything more than a 0.5 degree difference from neighboring stations. Some of them are Fan aspirated and most are passive but on the hottest days in summer or the coldest days in winter I rarely ever see more than a fraction of a degree off. Same for humidity.

As I mentioned in another thread The Weather Observer's Handbook the author did a similar test with the same radiation shields and for those who have the book provides a graph on page 106. Basically the same thing. No more than a fraction of a degree off. I'm not sure about the WMII shield but on my VP2 shield there is a ring of holes around the middle 3 plates. My theory is this allows for convection aspiration. In low wind conditions as the plates and surrounding air heat up the holes allow for excess heat to dissipate.

[johnd]

Rigorous comparative studies of the 7714 Davis screen are of course reported on in both:

Davis's own application note #24 (admittedly some years old now, but no less valid for that)

http://www.davisnet.com/product_documents/weather/app_notes/AN_24-temp-radiation-shield-comparison.pdf

and in the WMO report at:

http://www.wmo.int/pages/prog/www/IMOP/publications/IOM-106_Ghardaia/IOM-106_Report.pdf

[Aardvark]

It hasn't really got nasty as far as winter goes, but I am guessing  that the temperature differences won't be that significant.  Now summer will be a different animal. We get the hot muggy summers and of course those where the humidity goes way down and temperature goes up.

I guess what I would recommend if someone wants a VP2 with a Fars,  how accurate do you want +/- 2.3F  or so?

[LFWX]

How high, above ground level, are you taking measurments?

The photo seems to indicate something much higher than 5 feet, where the air is less likely to be "still".

[kcidwx]

The top of the pole is 6 feet AGL.

Interesting notes below from the WMO study. On some of my temperatures, the passive shield temperature was lower than the FARS. I didn't think that would ever happen. Seems the WMO noticed the same thing.

• Some small passive multi-plate screens exhibited warmer temperatures than the
reference (~0.5°C). Two had results close to the reference. One model, the DAVIS
07714, gave surprisingly good results, with colder measurements than the reference in
case of solar radiation. This result of the DAVIS is surprising because past
intercomparisons in other environments did not exhibit such good results in some other
tests, done by individual members.

• Other artificially ventilated screens gave disappointing results, with quite warm
temperatures in case of solar radiation. This may be due to their design and/or some
faults in the ventilation during the test (dust and sand reducing the ventilation
efficiency). For example, the ventilated DAVIS gave worst results (warmer
temperatures during day) than the passive DAVIS, which was not expected.

[LFWX]

Very true!

A new station, no more than a 1/4 mile from me, came online recently and his daytime temperatures were always higher (as much as 10°F) than mine on sunny days (and that was with a broken fan on my FARS).

The radiation shield on his station is quite small. http://www.wunderground.com/wximage/viewsingleimage.html?mode=singleimage&handle=DocWilkey&number=0#slideanchor

[kcidwx]

Here is December's data. I was asked about the accuracy of using two different brands of sensors for these comparisons. I guess I forgot to note that each sensor was tested at -20°F, 0°F, 20°F, 40°F, 60°F, 80°F and 100°F. The sensor deviations were noted and applied to the high and low temperatures on the table. That's the only way you can truly do this test accurately.

[ValentineWeather]

Thanks for the comparison, your not getting much difference during low sun angle November,December. I'm in Arizona and summer you really need the airflow. I've modified my Davis VP2 FARS with an AC fan Cooltron FA8025B11T7-51. Very easy to do, if my calculations are close I'm pulling approx 5 m/s across sensor vs the 1.1 m/s with the Davis solar powered fan. I've been using AC fan connected to GFI plug for several years and no problems. One of the fan tabs needs removed to fit, and I use rope putty to seal around fan, making sure all air is drawn through sensor tube. Anyone doing this make sure you pull the air across sensor and not blow down on it.   

[meteolarissa]

Ηello guys,

I tested for 13 months 6153 model Davis VP2 24hrs fan aspirated unit with the new Davis Vue passive shield. They both installed on my  terrace 8 metres from ground and 2,5 metres from terrace.

Maximum Temps: They had almost 0 dgrs Celsious difference at summer time and from 0,2 to 0,6 to the other autumn winter and spring time, with higher Temps in the passive Davis Vue shield. Its very clever that Davis put the sensor on upper place of plates under the big plastic base of weather station. So even in winter and less autumn and spring the sun angle is lateral and hit the plates for some hours ,its ''cost'' only some tens of degrees. I think that the VP2 passive shield with sensor between the plates get more differences overall in a year.

Finally my big surprise was at summer time. My place is in a middle of a biggest plain of Greece, Larissa city. Its one of the hottest place in Greece with calm winds and foehn winds sometimes that push the temp over 42s Celsius. In this ''no windy'' place the diferrences at high temps was zero !!! June July and August.

Minimum Temps: Stable -0,2 to -0,3 Celsius degrees differences all the months with Davis Vue passive cooler. Its normal cuz the cool air and some humidity stuck on sensor at cold calm nights.

Relative Humidity was the same with 1-2% differencies some days.

Precipitation was almost the same at stable rain  and light rain. At thunderstorms and high rain rated the Vue count more 5-8% than the Davis VP2 rain gauge.

Wind was slightly lower at Vue( mean daily and gusts). Generally till 40km/hr differencies was very small but over 45km/hr was significant.

All my test on my Blog but unfortunately is in Greek language.
http://www.e-meteolarissa.blogspot.gr/2012/05/vp2fan-aspirated-vs-vuepassive.html

Vassilis

Larisa, Greece

[ValentineWeather]

Davis Instruments also did their own test. http://www.davisnet.com/news/ars_vs_c.pdf

[VaJim]

For the first time I have both a FARS and passive radiation shield operating together. Now that November is over, I decided to take a look at the first full month of data. The FARS is a RM Young with a Nimbus temperature probe inside. The passive shield is a Davis with the Weather Monitor II temperature/humidity sensor inside. Before installation in October, both sensors were tested across the range at six different temperature points between -20°F to 100°F. Here's a photo of the sensor suite and a copy of the data collected for November.

Very nice looking station!

Jim

[kcidwx]

January

[kcidwx]

I decided to skip a few months as it was pretty much the same results. Now that solar radiation intensity is increasing I'll start posting data again. I'm also dropping the low temperature data as I don't feel it's relevant. My focus is the maximum error for the month using the fan aspirated shield as the standard.

[Aardvark]

I did that as well with the same results.  Humidity differences were a tad better.  I am wondering if there is another way to compare, that is to take some calculated value such as heat index and see if it shows a difference  or for the matter wind chill and see if the differences are significant enough in calculations.

For a 0.3 or so temp difference, it makes no sense in having a FARS, but if the difference is in a calculation  it might prove significant.

[kcidwx]

I did that as well with the same results.  Humidity differences were a tad better.  I am wondering if there is another way to compare, that is to take some calculated value such as heat index and see if it shows a difference  or for the matter wind chill and see if the differences are significant enough in calculations.

For a 0.3 or so temp difference, it makes no sense in having a FARS, but if the difference is in a calculation  it might prove significant.

What results did you get when comparing humidity? I'm curious now. I would think the only difference you might see is the humidity sensor in the passive shield would respond slower to changes on calm to very light wind days.

[ValentineWeather]

I did that as well with the same results.  Humidity differences were a tad better.  I am wondering if there is another way to compare, that is to take some calculated value such as heat index and see if it shows a difference  or for the matter wind chill and see if the differences are significant enough in calculations.

For a 0.3 or so temp difference, it makes no sense in having a FARS, but if the difference is in a calculation  it might prove significant.

There are several ongoing studies I've followed, and it does show a much greater than .3 difference at times as much as 1.7C difference with exactly calibrated instruments.
That's the key both comparisons need calibrated as true as possible not just somewhere in the ballpark. If the non aspirated is already reading slightly low the difference is not going to be accurate.
I'm not saying this is happening but in my comparisons years ago I saw a big difference between the aspirated and non, but you need to also realize this was in Arizona where very high temperatures and no grass ground cover so radiation heating was coming from everywhere. Because of my own experiments I don't even trust the low air flow Davis fan powered by solar so every weather station I've done includes a 3-4 times higher air flow 110v ac unit. 

On going study with just a solar powered fan in Utah: http://weather.usu.edu/htm/file40499/24-hour-thermistors

[Aardvark]

I did that as well with the same results.  Humidity differences were a tad better.  I am wondering if there is another way to compare, that is to take some calculated value such as heat index and see if it shows a difference  or for the matter wind chill and see if the differences are significant enough in calculations.

For a 0.3 or so temp difference, it makes no sense in having a FARS, but if the difference is in a calculation  it might prove significant.

What results did you get when comparing humidity? I'm curious now. I would think the only difference you might see is the humidity sensor in the passive shield would respond slower to changes on calm to very light wind days.

about 3% difference.

[baranij]

Hello, please also have a look at this Davis FARS vs MeteoShield Pro comparison. If anyone wants to test the new helical MeteoShield Pro, let us know. We will offer a number of users free shields for just shipping costs if they want to do and publish comparisons.
MeteoShield Pro vs. Davis FARS:

• https://www.wxforum.net/index.php?topic=34936.msg398911#msg398911
• http://www.kwos.org/marinadimontemarciano/indexDesktop.php Sensirion sht30 in a BGT solar radiation shield compared to sht30 in a Meteoshield pro, next days the sht30 will be replaced by a state of the art sht35 sensor
• https://www.allmeteo.com USA distributor eshop run by COMPTUS who is the Made-in-USA manufacturer of the MeteoShield Pro