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DIY Stevenson screen

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I have a small DIY Stevenson screen, which is made of 17mm thick wood painted white outside and black inside. There are vent holes in the wood and on the outside there are white plastic louvres to add protection from sunlight.

 

The box doesn't get much direct sunlight due to shade from my house and nearby trees. However when it does, usually in the afternoon, I suspect the  sun influences the temp a bit because when the sun moves away from the box, the temp  sharply drops about 0.7C in 15 mins or so.

 

Could anyone advise how I could further protect the sensor from the sun's effect? Would another layer of plastic louvres help?

 

See photos of the box below.

 

Thanks!

 

post-22407-0-20161300-1425230969_thumb.jpost-22407-0-13877600-1425230973_thumb.j

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I have a small DIY Stevenson screen, which is made of 17mm thick wood painted white outside and black inside. There are vent holes in the wood and on the outside there are white plastic louvres to add protection from sunlight.

 

The box doesn't get much direct sunlight due to shade from my house and nearby trees. However when it does, usually in the afternoon, I suspect the  sun influences the temp a bit because when the sun moves away from the box, the temp  sharply drops about 0.7C in 15 mins or so.

 

Could anyone advise how I could further protect the sensor from the sun's effect? Would another layer of plastic louvres help?

 

See photos of the box below.

 

Thanks!

 

attachicon.gifB_BypiIXAAAs0Ji.jpg large.jpgattachicon.gifB_BynuVW4AA6OCi.jpg large.jpg

 

 

paint the inside white as well

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Usually screens are double-louvered so another layer may help, but I would be more worried about its location. Under a tree may shield it from the sun but it looks like airflow will be quite poor where it is and you could well experience higher minimum temperatures where it is sited. 

 

You would probably be better bringing it out into the open and fitting a fan in the bottom for aspiration. If you can get power to it, a PC fan and a 12v DC adaptor would work a treat, That way you'll get a better response along with better maxima and minima.

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paint the inside white as well

 

I read somewhere else that the official screens are painted black inside to absorb any heat that does get in rather than reflecting it onto the sensor, but I'm not sure.

 

 

Usually screens are double-louvered so another layer may help, but I would be more worried about its location. Under a tree may shield it from the sun but it looks like airflow will be quite poor where it is and you could well experience higher minimum temperatures where it is sited. 

 

You would probably be better bringing it out into the open and fitting a fan in the bottom for aspiration. If you can get power to it, a PC fan and a 12v DC adaptor would work a treat, That way you'll get a better response along with better maxima and minima.

 

It's actually a couple of metres away from the tree - it looks closer in the photo. Airflow is actually pretty good where it is. I considered moving it further away from the tree to avoid potential climatic interference by the tree, but then I'd run the risk of the temp being influenced by the nearby house and/or shed if I moved it away. I can't have it too far down my garden otherwise the wireless signal can't reach the station.

 

On the subject of a fan, wouldn't the heat generated by the fan motor distort readings?

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I read somewhere else that the official screens are painted black inside to absorb any heat that does get in rather than reflecting it onto the sensor, but I'm not sure.

 

 

 

 

not unless the ideas have changed since I worked with UK Met, all our screens were white inside and out, double layered top and bottom, not front and back, can't remember about the ends, well it was over 25 years ago!

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not unless the ideas have changed since I worked with UK Met, all our screens were white inside and out, double layered top and bottom, not front and back, can't remember about the ends, well it was over 25 years ago!

Done a bit of digging and it seems MetO favour black inside for plastic screens, but white inside for wooden screens is still the norm although having black inside wouldn't do any harm. It's thought the reason being is that the plastic screens can have a degree of translucency. Source - http://www.uktrail.com/colchat/viewtopic.php?f=8&t=770

 

So I may paint the inside of my screen white and add some extra plastic louvres on the front. I suppose I could paint the reverse of the louvres black but I imagine that'd be pointlessly fiddly!

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The gap at the top under the roof could potentially be letting radiation in. check to make sure there are no other gaps any rays of sunlight are creeping in.

 

louveres on the inside would also help.

 

what height off the ground is it? in a sheltered location higher up will give a better airflow and i think officially anything from 1.25m to 2m is an acceptable height to measure temperature.

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The gap at the top under the roof could potentially be letting radiation in. check to make sure there are no other gaps any rays of sunlight are creeping in.

 

louveres on the inside would also help.

 

what height off the ground is it? in a sheltered location higher up will give a better airflow and i think officially anything from 1.25m to 2m is an acceptable height to measure temperature.

 

The gap at the top was designed to maximise airflow but I may have overdone it - I'll experiment with closing it.

 

Might also try putting louvres inside.

 

It's about 1.5m off the ground so a suitable height. :)

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I installed a solar powered fan which helped in bright sunshine.

However I think the amount of light in the box could be reduced although no direct rays are entering. Any ideas how without restricting ventilation too much?

Maybe painting inside black would work after all as heat from the light would be absorbed by the box rather than being reflected onto the sensor (I painted the inside white since my last post)

I considered making a bigger box but I assume greater mass means there's more to be heated!

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