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Radiation Shield accuracy

Discussion specific to Davis weather stations
THX
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Radiation Shield accuracy

Postby THX » Thu Jan 14, 2010 2:57 am

Can someone comment on the accuracy of a Vantage station with a standard radiation shield versus a Vantage station with a 24-hour fan-aspirated radiation shield?

Is the difference really that dramatic?

If so, under what circumstances?

Regards,

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beteljuice
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Re: Radiation Shield accuracy

Postby beteljuice » Thu Jan 14, 2010 3:09 am

If I remember correctly from one of their datasheets, it suggested that in strong Sun, calm conditions, the normal shield could be +2C compared to their aspirated model.
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TNETWeather
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Re: Radiation Shield accuracy

Postby TNETWeather » Thu Jan 14, 2010 9:42 am

There are a lot of areas where a 24hr FARS is not really needed.

Here in AZ it makes a difference because of all the direct sun and heat late into the nights during the summer. I've had one for both my Vantage Pro1 and Vantage Pro2. Both have been full instrument Plus units.

IF you think you are going to need one, you are best buying it with the station. I don't think you can add it later (24 hour FARS that is).
Kevin
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George Richardson
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Re: Radiation Shield accuracy

Postby George Richardson » Thu Jan 14, 2010 3:07 pm


matt4595
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Re: Radiation Shield accuracy

Postby matt4595 » Thu Jan 14, 2010 7:59 pm

You might want to check out AJ Tuck Co., they specialize in electroforming and radiation/ ir shields. http://AJTuckCo.com

THX
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Re: Radiation Shield accuracy

Postby THX » Fri Jan 15, 2010 5:30 am

That PDF document is an amazing read.

It sure does say that Davis instruments are accurate especially when it comes to temperature readings.

It makes one wonder if the added expense of the fan-aspirated shield is worth it?

adam5
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Re: Radiation Shield accuracy

Postby adam5 » Wed Jan 20, 2010 2:29 pm

It makes one wonder if the added expense of the fan-aspirated shield is worth it?

I suppose it all comes down to your level of weather "geek-dom." Myself...I'm a Meteorologist by profession and somewhat obsessed with data accuracy, to the extent possible with the equipment I'm using. If there is any way to improve temps even by a couple degrees (i.e. the fan aspirated shield), I'm in. I can say it does make a difference, not only on the overall accuracy, but also with sensor response time to rapidly warming/cooling situations.

I've even gone as far as to hotwire the fan unit to house power (via a power adapter of course) so that I have no dependency on the degree of solar charging and am guaranteed 24/7/365 fan aspiration. Like I say, it depends on your level of weather "geek-dom."

I'd say for the majortiy of home users who just want a solid system and good data, the FARS probably isn't necessary as long as you make good effort to site the sensor properly (well exposed grassy area 2 meters above ground and well away from buidings, concrete, etc). That is of course unless you live in Arizona.

-Brian

RayProudfoot
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Re: Radiation Shield accuracy

Postby RayProudfoot » Wed Jan 20, 2010 7:40 pm

Hi Brian,

I wonder if I could ask your opinion on how I have my station setup? I appreciate it's not easy without being here but I'll try to describe it as best as I can.

My wireless VP2 is attached to a wooden post approx 5ft from an east-facing 5ft high fence and the same distance from a 4ft high fence facing SW. The east-facing fence comprises 6ft lengths supported by concrete posts and is typical of those in many UK gardens. The SW facing fence is the same construction but a foot lower. The rain guage is higher than either fence and the temperature sensor is close to 1.25 mtrs above ground level. The ground immediately below the station is soil but the lawn is only a couple of feet away. There is no concrete paving within 20 feet.

My garden is surrounded on three sides by semi-detached houses which might explain why my temperature readings in hot summer days are sometimes 2-3C higher than stations at Manchester Airport (3 miles west) and Woodford (3 miles east). Both those are located in open grassed areas. At other times of the year the temps are similar to the other stations and other PWSs although my recent overnight low of -11.1C was somewhat higher than Woodford's -17.8C.

I have considered buying a daytime aspirated fan but I'm concerned that it won't make any difference because the enclosed garden area is quite sheltered and exposed to radiated heat from the nearby houses so the fan will only circulate that air and not make any difference.

Appreciate your thoughts. :) Here's my station. http://www.wunderground.com/weatherstat ... =ISTOCKPO4
Cheers,
Ray, Cheshire.

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adam5
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Re: Radiation Shield accuracy

Postby adam5 » Thu Jan 21, 2010 1:48 am

Hi Ray,

The ideal goal is to mount a temp/humidity sensor in the open and well exposed, sensor shielded from direct sunshine, 2 to 3 meters above the ground, in a grassy area and far enough away from structures (buildings, pavement, etc) to be impacted by an artificial heat source. Now of course, this ideal siting location simply is not possible for many home weather station owners due to space limitation, particularly when you also need to find a location with a good deal of sunshine if you own a solar powered station. After all most folks don't live at a wide open airport. All you can do is try to strike the best balance possible.

In your case, and without seeing the site, try to get the sensor as far away from those houses and into the grass if possible, and perhaps just a little higher (at least 1.5 meters). Also, is the wooden post that the sensor is mounted on painted white? If not, a brown or dark post will likely add another source of heat. Try painting it a nice bright white. Those measures alone might take half to 1C off you midday temps.

As far as the daytime fan is concerned...assuming that your sensor gets a good deal of sunshine I would guess that adding the fan would cut another 1C off your midday temps. Why? Even with the shielding, full sunshine can literally cause some heat build-up inside the passive radiation shield, especially on calm wind days, that a fan will continuously remove. Is that worth the cost? It literally depends on how much of a weather geek you are. For me it would be worth it.

Just to add one more thought to the conversation. I wouldn't worry too much about readings from neaby locations. It is not uncommon for temps on a nice clear/calm night to vary 5C or more over short distances. So those differences you noted are probably real. As long as you have done everything you can to site your sensors as good as possible, you're probably in good shape. After all, you bought a home weather station to monitor conditions at your location.

Hope that helped.

-Brian

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Re: Radiation Shield accuracy

Postby RayProudfoot » Thu Jan 21, 2010 12:44 pm

Hi Brian,

Thanks for your helpful reply. To place the sensor in the most open part of my garden would mean placing it in the middle of the lawn (and closer to the house) so that's not terribly practical. It is as far from my and other houses as possible - the gardens are all quite small.

What I am relieved about is that you didn't mention the wooden fences as possible heat sources. That was one thing I was concerned about but perhaps 5-6ft is sufficiently far away not to influence the temperature too much.

I'm surprised you suggested mounting the ISS higher as that would then place the temperature gauge above the recommended 4.5ft. I didn't mention that the anemometer is located on a 6ft pole lashed to my chimney stack so the ISS doesn't need to be higher for wind measurement.

The post the ISS is attached to has been treated to prevent rot and is dark brown. It's been driven into a MetPost for stability. I appreciate painting it white might help but it might also cause it to rot prematurely. It has to remain unpainted for the warranty to be valid.

But if by adding a daytime FARS it would improve accuracy then that is well worth considering. I appreciate every garden has a micro-climate. That has become very apparent since last year when I can now compare temps in my back garden to official sites. I don't take other PWS stations too seriously as I don't know how well the equipment is positioned.

On those very cold mornings 2 weeks ago the temp was -11C in my garden but on my way to work (close to Woodford) the temperature on the car thermometer dropped to -14C in the very exposed office car park providing that urban areas do remain considerably warmer than surrounding countryside.

Thanks very much.
Cheers,
Ray, Cheshire.

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George Richardson
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Re: Radiation Shield accuracy

Postby George Richardson » Thu Jan 21, 2010 5:41 pm

Hey Ray,
I just took a look at five or six of your Wunderground history pages and I don't think your wind was ever calm. Why pay for an artificial wind when nature does it for you?
JMO
George

RayProudfoot
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Re: Radiation Shield accuracy

Postby RayProudfoot » Thu Jan 21, 2010 5:58 pm

George Richardson wrote:Hey Ray,
I just took a look at five or six of your Wunderground history pages and I don't think your wind was ever calm. Why pay for an artificial wind when nature does it for you?
JMO
George


Hi George,

The anemometer is 30 feet up lashed to the chimney. Down in my very sheltered back garden there is far less wind. Hence the consideration for fan-assistance. Here's a picture of my station. The house is right behind me. The camera is pointing NW.
ViewStation.jpg
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Cheers,
Ray, Cheshire.

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adam5
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Re: Radiation Shield accuracy

Postby adam5 » Fri Jan 22, 2010 2:29 pm

Hey Ray,

Just took a look at the pictures of your temp/sensor placement. I guess I didn't realize just how close it is to that dark fencing. I would speculate that a day of good sunshine on that fence might just add a few degrees to your temps. If it were me, I would seriously consider getting your sensor farther out into the yard...if that is even an option for you. Some folks simply do not like the idea of sinking a post in a more open part of their yard.

As far as the daytime fan is concerned, given your current setup...a tough call. On one hand a fan will certainly keep the sensor aspirated and remove any build up of "heat" inside the shield, particularly on calm wind days. On the other hand a fan will pull in more air from around the sensor which could potentially be warmer due to that fence :?

In the end all you can do is strike the best balance between your space limitations, sensor exposure, and minimizing sources of artificial heating. Not always an easy task as you can see.

-Brian

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beteljuice
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Re: Radiation Shield accuracy

Postby beteljuice » Fri Jan 22, 2010 2:33 pm

Sun on a wet / frosty fence will also increase the (local) humidity ;)
Last edited by beteljuice on Fri Jan 22, 2010 3:08 pm, edited 1 time in total.
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George Richardson
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Re: Radiation Shield accuracy

Postby George Richardson » Fri Jan 22, 2010 2:37 pm

Ray,
A hobby isn't any good unless you can throw money at it so maybe you should get the fan. Actually, you will never know if it actually helps unless you buy a complete 6153 and mount it side by side and compare the readings. Whatever you decide to do it looks as if your grounds are in good enough condition to stand a couple of months inattention. Would you be willing to come over and spruce mine up?! :roll:

The more I think about it, I think a fan would pick up stagnant air from the fenced in corner and adversely affect your readings, both cold and hot.

JMO

George


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