A while ago I thought I'd have a go at making a poor-mans Stevenson's Screen.
Well, this morning I tried making one.
Currently my La Crosse 2355 sits in the permanent shade off the side of my wooden garage.
I know it's not ideal, but I think it works pretty good during the daytime, but reads too high overnight.

I found some instructions online (http://www.loganvillageweather.com/station/stevenson.html) which i based my first attempt off.
I bought a bunch of plastic 28cm pot plates and cut out the middle, drilled some holes in each of them and using some threaded bar, i made up a very simple and crude stevenson screen.
it took me a few hours to put together, and after putting it up sitting just over 2m off the ground and 1m off the side of the garage, plopped my ws2355 inside, and................it didn't work.

temps climbed rather quickly, with humidity dropping.
i left it in there for 40mins before deciding that it wasn't going to get any better, and then put the ws2355 back in it's usual spot.
temp then dropped back to where it was previously and stayed there.

So i'm wondering if anyone has any suggestions of how to improve my chances of saving anything from my hatchet job?
the colour of the pot plates were green. i'm going to buy some white spraypaint and give the thing a coat of paint.
other things I can think of that would have improved it were having bigger plates, with bigger space inside for the instrument.
but is there anything I could try using what I currently have and still get a decent result, or should I notch it down to experience and move on?

i've attached a picture of what my handywork looks like, and another showing what the temp did after i set it up - i moved it around 2.20pm, and put it back to it's usual spot at 3pm.
for reference, the NIWA site 500m away read 7.9°C at 3pm.

I've had this project booked marked on my "to do list" for a while now.

I was interested in your humidity reading drop off, I have long suspected my WS2355 is giving false high readings because of the lack of airflow around the sensor set up. How do your humidity readings compare with the NIWA site?

Tony

wow derek, thats a mightily impressive contraption. must have weighed a tonne!

and tony, yes, my humidity readings are higher than the NIWA site. during the day approx 10% higher, but the same-ish overnight.
however, my intuition tells me (probably incorrectly) that if i was getting more airflow when using my stevenson screen then my temp readings would have been more better (as in lower). i took from the performance of my screen in terms of temp, that i was not getting a good airflow.

mine has a hole in ever plate except the top one.
and as for having bigger gaps between the plates - i definitely think this would help, but i was concerned with making them too big and allowing the sun to get in at low angles - my plates don't have that great slow curve of your ones derek which would have worked a treat.

i think painting them white is a must. also, looking at the spaces between the plates and also to amount of space removed from inside the plate - i think mine was a bit small....
a few things to work on in the next few weeks.

I just found this thread - so sorry for not posting earlier.  I did something unconventional - I do have a La Crosse 2308 and i guess they all have much the same hygro unit. I just happened across a tall skinny plastic bucket in the Warehouse - most seem to spy cheap stuff in here...   All of a sudden, my thoughts saw it as an inverted bucket.  I had seen a picture of another NZder DIY wx stn with a 1-Wire setup - more about that if you ask -

I layered some foil backed foam about 20mm thick in strips (I measured the diameters at top and bottom and made some triangle pieces with top cut off (trapezoids) of the right shape to glue onto the inside.  I then cut a circle out of the top and got some (I hate the word "got") plumbing pieces to make a a pipe up and an elbow over and a pipe across and an elbow over and a pipe down which I could fit a PC power supply fan inside to pull air through all the time.

It works like a charm and has cured me of highs and flat temps - it is much more responsive than a Davis especially in calm conditions - night and heat of morning.  I will try and find a photo of it all.

I too have experimented over the years with different idea's for making screens the best of them having the top significantly larger than the individual plates/slats below it (large enough to almost completely shade them) and this seems to take the direct solar effect out of the equation. Eventually i caved in and replaced my La Crosse station with a Davis VP2 which in my opinion is the best answer. I do have a daytime aspiration kit fitted which also seems to improve responsiveness and accuracy compared to not having it.

No one has yet answered the second part of the question I posted in the Shoutbox earlier.  ". . . and why are they so deficient?"

"David commented in the Shout Box that it doesn't matter where you have located it, it's common knowledge that the radiation shields on those stations are inadequate."  Why, I thought they measured Temperature not Radiation so can someone please explain?  It seems to me that those that posted above had their stations located such that they were mainly in sheltered and sunny conditions, I notice for example that David's own station (As pictured on his website) is located on his roof with lots of other reflective stuff on or about the same pole.  I am also left wondering how much these constructions affect the Min temperature, the insulation effect?

I firmly believe that the location of a Weather Station is critical, mine is purposely well out in the open, has plenty of ventilation and is high enough off the ground to not be shadowed by any buildings, trees or other obstructions etc.  It is extremely rare for us not to have a breeze here, calm conditions are observed mostly at night or on cloudy/wet days so I fail to see why my station would not be adequately ventilated.

Based on the above posts from those who have built these shields all I can see that it did, apart from look butt ugly, was to flatten the graph, my guess is that it just averaged out the data creating a different and artificial modification to the data.  If I put that kind of structure up my pole with the wind we get here it would be all accross the paddock after the first decent SW blow.

Trying to understand what all this is about as it does not seem that logical to me.

Oh and my High Temp from today of 22.6C is consistent with my manual temp gague that recorded a max today of 21.3C and is located in permanent shade behind the building, is less than 1 metre above ground level, and was fanned directly by todays predominantly SE/SW breeze.  For comparative purposes the Max recorded Temp in August 2011 was 26.9C on 11/08/11 so 20C+ temps are quite achievable here in August.

Kind Regards.

Actually it is just the anemometer on the roof in the photos on my site, the rest is over a lawn down the back of our section 

Well i finally got round to setting up the radiation shield that derek very nicely sent me on Sunday.
it was a very nice and easy installation. just had to drill a couple holes in the top plate and it fitted very nicely on the contraption I had devised on my first attempt.

it's only been 3 full days so far, but it appears to be performing very well.
I had been tracking the performance of my station against the Reefton NIWA site over the last few months and these last 3 days have been much closer to the NIWA temps than I had previously been recording.
Average difference for daily maximums and mins are +0.3°C and +0.1°C respectively. this compares to +1.2°C and -0.3°C respectively for august up til the changeover.
I'll be keeping a close eye on it, but it's looking good.
humidity was another I was interested in, but I haven't had a chance to look at that yet.

again, many thanks to derek for sending me his handiwork.