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Radiation Shield accuracy

Discussion specific to Davis weather stations
Pete B
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Weather Station: Davis VP2 Wireless
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Re: Radiation Shield accuracy

Postby Pete B » Tue Mar 30, 2010 9:30 am

steve wrote:Can I ask you chaps where you bought your remote anemometer transmitters? I'm thinking of getting one so I can relocate my anemometer, but they seem quite expensive. I was thinking of importing from the US, but I think the frequency would be wrong. Exchange rate isn't good, either!


Steve

Mine is the Davis anemometer transmitter kit, Model No 6332, see here. The ISS & anemometer are then completely independent of each other.

I got mine at the same time as all the other bits from ProData Weather Systems.

Details on this page down towards the bottom (along with the Daytime FARS that began this thread :) ), price list here under 'Wireless Accessories for VP2 Stations'.

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steve
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Re: Radiation Shield accuracy

Postby steve » Tue Mar 30, 2010 9:56 am

Thanks, Pete. £198 :( . The (incompatible) US version is about half that. I know that's not John's fault, but it's hard to fathom.
Steve
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EvilV
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Re: Radiation Shield accuracy

Postby EvilV » Tue Mar 30, 2010 12:00 pm

steve wrote:Thanks, Pete. £198 :( . The (incompatible) US version is about half that. I know that's not John's fault, but it's hard to fathom.


It is mainly because retailers here expect very high margins and habitually over-charge us all. Remember when European cars were sold with huge premiums in Britain? There were extra margins here of nearly 20% on some only a few years back. The same problem can be seen on almost any goods imported here from the USA. Some of it may be due to VAT (except from Europe where VAT is already applied) but obviously not all, because it is very common to see dollar for pound prices set. This was true even when the dollar was only £0.60. I have seen it apply to electronic goods, jeans, CDs, bike parts, and software to mention only a few.

RayProudfoot
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Re: Radiation Shield accuracy

Postby RayProudfoot » Tue Mar 30, 2010 4:29 pm

steve wrote:Thanks, Pete. £198 :( . The (incompatible) US version is about half that. I know that's not John's fault, but it's hard to fathom.

Steve,

I raised the price variation between the US and the UK and John Dann of Prodata provided this answer.

I also bought my kit from Prodata. He will price match if you can find a cheaper price in the UK but it's difficult for accessories.
Cheers,
Ray, Cheshire.

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Pete B
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Re: Radiation Shield accuracy

Postby Pete B » Tue Mar 30, 2010 10:03 pm

RayProudfoot wrote:Hi Pete,

Thanks for the pic of the anemometer. My first thought is why is the transmitter so high? Would it not be more accessible closer to the ground? There is enough cable. Fully understand your reluctance to place the ane so high it becomes difficult to access. I took the decision to have mine fitted by an aerial guy but it does mean I need to get him around when it malfunctions. Time will tell if I made the right decision.

Hi Ray

It's not easily visible on the photo but about 0.5m below the transmitter, there is a joining sleeve to join the upper 1.5m pole to the lower 3m one. Therefore, come maintenance time, I just lower the whole pole down by loosening but not removing the bracket clamps, then remove the top section with transmitter & anemometer then onto the bench for service. I did think about mounting the transmitter separately to the wall & may yet do, the main advantage would be lower wind loading on the pole. A 'virtual visit' to your location via streetview a couple of weeks ago and it looks like that's what you have done. It does get full light exposure all day at that height though.

RayProudfoot wrote:I've been thinking about the round aluminium pole fitting in a round spike. To remove the rain guage you have to twist it with quite a bit of pressure. There could be a danger that the pole rotates with it. That would not be good. That's one advantage of post mounting. Is it securely fitted inside the hollow spike? Have you had to remove the gauge yet for cleaning and if so, was it a problem.

We'd have to go a long time without rain for clay to dry out. I think you'll be okay.

I have clamped the outer ground spike tube tightly around the inner mounting pole using a Jubilee clip after 'slotting' about a 4cm length of the ground spike across it's diameter with a hacksaw so rotation when removing the rain gauge is not a problem.

steve wrote:Thanks, Pete. £198 . The (incompatible) US version is about half that. I know that's not John's fault, but it's hard to fathom.

As others have said it's not the end retailers problem, it's the "Rip Off Britain" syndrome driven by the main single importer & possibly the overseas supplier as well, just as with everything else that comes in.

I did check the French price this morning via their single importer (the unit is the same overseas version) and things were not that much better (Euro219) at current £/Euro exchange rates (and you'd need to be there anyway or get someone you know who's going to bring it back).

RayProudfoot
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Re: Radiation Shield accuracy

Postby RayProudfoot » Wed Mar 31, 2010 4:48 pm

Pete B wrote:Hi Ray

It's not easily visible on the photo but about 0.5m below the transmitter, there is a joining sleeve to join the upper 1.5m pole to the lower 3m one. Therefore, come maintenance time, I just lower the whole pole down by loosening but not removing the bracket clamps, then remove the top section with transmitter & anemometer then onto the bench for service. I did think about mounting the transmitter separately to the wall & may yet do, the main advantage would be lower wind loading on the pole. A 'virtual visit' to your location via streetview a couple of weeks ago and it looks like that's what you have done. It does get full light exposure all day at that height though.


Hi Pete, Good plan! As you spotted my transmitter is on the house front facing SE and gets the sun until about 2pm. Access from the porch flat roof is easy too. I wish I could have easier access to the ane but it would have meant a long pole or poles and it was too much hassle. Touch wood it's been trouble-free in nearly a year.

I have clamped the outer ground spike tube tightly around the inner mounting pole using a Jubilee clip after 'slotting' about a 4cm length of the ground spike across it's diameter with a hacksaw so rotation when removing the rain gauge is not a problem.

Don't suppose you could post a picture please? Good to hear it's a tight fit.
Cheers,
Ray, Cheshire.

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Pete B
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Re: Radiation Shield accuracy

Postby Pete B » Tue Apr 06, 2010 11:42 pm

RayProudfoot wrote:Don't suppose you could post a picture please? Good to hear it's a tight fit.

Hi Ray

I finally managed to get some pictures, just after the first grass cut of the year today, the warmest day of the year so far @ 16.9C.

Mount 1.jpg

Mount 2.JPG

The 4 cm long diagonal slot cut in the ground spike together with the clamping "Jubilee" Clip can be seen. Also, the ultra high tech mounting (plastic clothes peg) for the grass minimum measuring temperature probe, feeding back to the leaf/soil transmitter is clearly visible. If I was doing it now, I'd have cut a second diagonal slot along the ground spike at 90deg to the first, giving 4 sections rather than just 2. All is well so far.
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RayProudfoot
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Re: Radiation Shield accuracy

Postby RayProudfoot » Wed Apr 07, 2010 4:22 pm

Thanks Pete. A picture makes everything clear. I like the technical solution to your grass temp sensor. :)

Since I last posted the owner of my 'reference' station 10 miles away has made contact with me and visited on Monday. He recommended I replace the wooden post with an aluminium one and also raise the ISS by about 12-18" to clear the top of the shorter fence. I'll be moving the station about 6 feet closer to the house and that should make a big difference to things. His ISS is attached to his chimney so he has no problem with air circulating around the sensors.

No such temps here today. The sun is now out and the high was 13.0C - identical to my reference station. :D There's a breeze which helps to keep the air moving. The airport recorded a high of 12C.
Cheers,
Ray, Cheshire.

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Pete B
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Re: Radiation Shield accuracy

Postby Pete B » Mon Apr 12, 2010 9:34 am

RayProudfoot wrote:Thanks Pete. A picture makes everything clear. I like the technical solution to your grass temp sensor. :)

Since I last posted the owner of my 'reference' station 10 miles away has made contact with me and visited on Monday. He recommended I replace the wooden post with an aluminium one and also raise the ISS by about 12-18" to clear the top of the shorter fence. I'll be moving the station about 6 feet closer to the house and that should make a big difference to things. His ISS is attached to his chimney so he has no problem with air circulating around the sensors.

It'll be interesting to see what result you get with that change to your setup, especially if you were to try the ISS mounted on the pole at both heights, i.e. the "std" 1.25m and the ~2m point to see how much, if any, difference there is there.

I've been away for a few days since last Wed (visiting my Mum in Macclesfield, so not far from you) but left my complete WU logging system running so I saw that on Thurs 8th, mine hit 20.9C, my highest of the year so far and a bit higher than the local ref points I use, e.g. Pershore via this site.

Your chosen "ref" station setup can be readily seen via 'Streetview'. I'm not sure about the aspect of roof mounting though as what does it mean relative to the near ground temperature which is what one feels and is used for all main official weather records?. A station near to me is also roof mounted, one of those I linked in an earlier response. Anyway, I had to remove my rain guage funnel yesterday to remove a dead spider from the outlet, it probably wouldn't have affected overall rain amount measurements but may well have affected ongoing rain rates. Not an easy thing to check or do when it's on the roof.

RayProudfoot wrote:No such temps here today. The sun is now out and the high was 13.0C - identical to my reference station. :D There's a breeze which helps to keep the air moving. The airport recorded a high of 12C.

Mine got to 14.8C on the same day which was fairly close to one of my local ref points at 14.1C, again Pershore. I will be watching mine closely compared to others locally as the Spring advances and Summer comes and then deciding whether to try the DFARS mod or try something else.

RayProudfoot
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Re: Radiation Shield accuracy

Postby RayProudfoot » Sat May 01, 2010 6:35 pm

Pete B wrote:Your chosen "ref" station setup can be readily seen via 'Streetview'. I'm not sure about the aspect of roof mounting though as what does it mean relative to the near ground temperature which is what one feels and is used for all main official weather records?. A station near to me is also roof mounted, one of those I linked in an earlier response. Anyway, I had to remove my rain guage funnel yesterday to remove a dead spider from the outlet, it probably wouldn't have affected overall rain amount measurements but may well have affected ongoing rain rates. Not an easy thing to check or do when it's on the roof.

Firstly, my apologies for not posting earlier. I was expecting the pole change to have been done by now but Alan (my ref station owner) is a busy man. However, it looks like he'll be able to do it by the end of May. I was getting a bit worried about him as his station had gone offline for over a week and I had visions of him having fallen off the roof trying to maintain his VPPro. :cry: Fortunately, it's only a dodgy modem that's responsible so he should be back online soon.

I don't subscribe to the roof mounting for the ISS. Maintenance of the rain gauge will be difficult and temps are meant to be taken 4-5ft above ground level - not 30ft up. Although it was a bit galling that his station matched Manchester Airport more thans mine did! Actually, having looked at the airport data a bit closer they only record to the nearest degree C and only take readings every 30 mins so there will always be a disparity with my station. If after the change I'm no more than 1.5C warmer than the airport I'll consider that job done.

Mine got to 14.8C on the same day which was fairly close to one of my local ref points at 14.1C, again Pershore. I will be watching mine closely compared to others locally as the Spring advances and Summer comes and then deciding whether to try the DFARS mod or try something else.

That's pretty good. Having seen your setup I would say that is as perfect as you could wish for. Any differences are down to local conditions, not station inacccuracy or poor placement.

My highest temp is 20.9C on 10 April. I'm sure yours will be higher.
Cheers,
Ray, Cheshire.

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RayProudfoot
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Re: Radiation Shield accuracy

Postby RayProudfoot » Sat May 15, 2010 3:13 pm

Hello everyone,

Well it's finally happened and I'm a happy bunny! :D Alan - owner of my reference station and general excellent bloke - came round today and we swapped the wooden post for an aluminium one or rather, two posts.

The wider lower section goes a couple of feet into the ground and the smaller upper one slides down into it and holds the ISS. It's only moved 4 feet from the old post but importantly, the ISS is around 2-3ft higher and that is making a substantial difference to my temperatures. Yes, they're a little higher than the airport but the airport only measures to whole degrees C and every 30 mins so that is bound to introduce some differences.

More importantly, the BBC forecast a high of 15C for this area today and my max to date is 15.2C and that is not something I've had before on sunny days.

The post will be cemented in in a few days or next weekend once a few days recordings confirm higher station accuracy. I'd liket to publicly thank Alan (ITRAFFORD2 on Wunderground) for all his help. He's keen to use Cumulus once he has a netbook so that will give me a chance to repay his favour.

Here's a photo of the new location. You can see the old post just to the left.
Ray_Davis1.jpg
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Cheers,
Ray, Cheshire.

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Pete B
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Re: Radiation Shield accuracy

Postby Pete B » Sat May 22, 2010 10:01 pm

RayProudfoot wrote:Hello everyone,

Well it's finally happened and I'm a happy bunny! :D Alan - owner of my reference station and general excellent bloke - came round today and we swapped the wooden post for an aluminium one or rather, two posts.

The wider lower section goes a couple of feet into the ground and the smaller upper one slides down into it and holds the ISS. It's only moved 4 feet from the old post but importantly, the ISS is around 2-3ft higher and that is making a substantial difference to my temperatures.....

.....The post will be cemented in in a few days or next weekend once a few days recordings confirm higher station accuracy. I'd like to publicly thank Alan (ITRAFFORD2 on Wunderground) for all his help. He's keen to use Cumulus once he has a netbook so that will give me a chance to repay his favour.

Hi Ray

That looks a better setup than your previous one in that there'll be far less interference from the mounting post. I'm not sure how important the exact height is - I suppose you could do some comparisons at diferent heights on that pole from the "std" 1.25m to the height you have it ATM.

Regarding my local "reference", i.e the station round the corner that uses a conventional Screen with max/min instruments, for both March and April, his max for the month and mine have been within 0.4C of each other and these occurring on the same day. It will be interesting to see how the current spell compares when Mays "COL" results are published. I could of course write to him direct to make contact and therefore make direct comparison of results in near real time. My max yesterday was 27.5C, today (Sat) it was 28.2C, a figure that you have beaten @ 28.6C.

The only thing that is really skewed on mine regularly is early morning Sun due to excessive greenery (at least it's not man made buildings) but until Davis come up with a system where the Sun sensor can be remotely mounted away from the ISS along with the anemometer & therefore above other nearby obstructions, there's not much I can do :( .

RayProudfoot
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Re: Radiation Shield accuracy

Postby RayProudfoot » Sun May 23, 2010 9:16 am

Hi Pete,

Woodford was the official hot spot yesterday at 27.1C. My max of 28.6C still concerns me given I'm only 2-3 miles from that station. But it's in the middle of a huge aerodrome in perfect conditions. My 'reference' station is mounted on a chimney 30ft up with 24HR FARS and his max was 27.8C. I'm seriously considering adding a daytime FARS in the hope it will lower my max's by around 1C which would put me very close to the official station at Woodford and Manchester Airport.

I don't feel inclined to lower the ISS to be honest. I think it would just raise the temperature. Although it's higher than the recommended height the fact that my 'reference' station gets more accurate results 30ft up suggests height and air circulation is least worse than a lower station in an unfavourable environment. Back gardens will never be perfect so I have to accept that or I'll drive myself barmy!

Your siting looks perfect to me and yet your max of 28.2C yesterday is higher than the UK max but how else can you improve your ISS location? You can't but as your readings are so close to a Stevenson Screen readings I imagine you're more than satisifed. If I were you I would make contact and see if you can get instantaneous readings. It was Alan (reference) who contacted me when I was seriously contacting him so I'm sure you would both benefit from a meeting.

Hopefully Davis will change things so the solar and UV sensors can be placed with the anemometer. I still cannot get my head around why they decided to lock them to the ISS.

It's already 2C warmer here than this time yesterday and yet they're forecasting a lower max. :?
Cheers,
Ray, Cheshire.

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mcrossley
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Re: Radiation Shield accuracy

Postby mcrossley » Sun May 23, 2010 10:58 am

Ray, doesn't it also matter what temperature you are trying to record? ...

My high temps are quite a bit higher than MCR, but they are recording in an open field with miles of farmland to the S. I on other hand am in the middle of a suburban area with lots of trees in the immediate area to slow the surface wind considerably. I am happy that what I am recording is a accurate reflection of the temperatures around my house - the sensor is mounted about 6ft agl in a spot that never receives direct sunshine, is open to 'reasonable' airflow, and matches what I get on other thermometers that I can move around. If I want the 'official' temperature I can look at the Met office web site :) but if I want to know what actually happened here I consider my readings more definitive.

Anyway wont the readings from any one 'official' station be subject to corrections to the raw readings depending on location before they can be compared with others and be used in climate models etc?

I guess I'm just saying, yes try to make your readings subject to as few systematic errors as possible, but for me chasing the readings of other stations isn't necessarily the right thing to do.

Discuss :lol:

RayProudfoot
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Joined: Wed May 06, 2009 6:29 pm
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Re: Radiation Shield accuracy

Postby RayProudfoot » Sun May 23, 2010 12:16 pm

mcrossley wrote:Ray, doesn't it also matter what temperature you are trying to record? ...

:lol: Good point. My main concern is influence from nearby objects that falsify the reading. Perhaps I should visit the EGCC station in the early hours and 'borrow' their kit so I can make a meaningful comparison. ;)

My high temps are quite a bit higher than MCR, but they are recording in an open field with miles of farmland to the S. I on other hand am in the middle of a suburban area with lots of trees in the immediate area to slow the surface wind considerably. I am happy that what I am recording is a accurate reflection of the temperatures around my house - the sensor is mounted about 6ft agl in a spot that never receives direct sunshine, is open to 'reasonable' airflow, and matches what I get on other thermometers that I can move around. If I want the 'official' temperature I can look at the Met office web site :) but if I want to know what actually happened here I consider my readings more definitive.

I have noticed your maximum temperatures are nearly always higher than mine and your minima are lower too. That's the effect of a sheltered location of course. I hope I'm not becoming paranoid about this but it is difficult when one sees such large temperature differences in such a small area. I'm sure many WU sites don't have the station setup very well but from your description of yours it's as good as you can achieve and therefore the readings are pretty accurate.

Anyway wont the readings from any one 'official' station be subject to corrections to the raw readings depending on location before they can be compared with others and be used in climate models etc?

Yes. Looking at Woodford yesterday the max wasn't 27.1 on their graphs so I'm not sure what factors they use to correct it. But like EGCC Woodford is also very open and untypical of our surburban environments.

I guess I'm just saying, yes try to make your readings subject to as few systematic errors as possible, but for me chasing the readings of other stations isn't necessarily the right thing to do.

Discuss :lol:

:) You're quite right Mark. I am a stickler for accuracy and I perceive any temperature differences with EGCC and Woodford to be a problem with my station. I think I'll just have to accept that comparisons with official sites is impossible in an semi-urban environment.
Cheers,
Ray, Cheshire.

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