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morton027

‘Weather box’ vs shade

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Posted (edited)

So, I have a Netatmo weather station and have finally gotten round to completing my DIY weatherbox for it.

I’ve moved the outdoor unit from my previous ‘shaded spot’ to the box and back today for sake of experimenting, and noticed that the readings in the box are higher than the shaded spot by up to 2°C.

So I guess my question is, which is likely to be the more accurate reading, and why?

I've attempted to attach photos from iOS but unfortunately can’t get it to work.

I will be interested to see opinions from anyone on here.

Edited by morton027

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A lot depends on the construction of the box, the materials it's made from, the amount of air space inside relative to the overall mass of the box, the paint finish etc. It's also possible, if the shady spot was also a leafy spot, that a lower reading than the ambient air temperature could be obtained due to increased humidity as a result of transpiration from nearby plants.

Even the highest spec' unaspirated thermometer screens are subject to error in conditions of hot sunshine with very little wind. It's a question of constructing, or buying, something that reduces the errors to an absolute minimum. 

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I think the problem lies with the construction of the screen. Firstly a single thickness of board will be susceptible to admitting secondary radiation from objects nearby such as the fence, as well as radiating some heat itselt as it heats up slightly in the sun, despite being painted white. Secondly the holes drilled in the body of the screen will also admit secondary radiation and will not provide sufficient ventilation.

Ideally the screen needs to be constructed of some sort of louvre system. If it's a wooden box it needs to be louvred on the inside and outside to provide adequate ventilation and to prevent secondary radiation, also the floor of the screen needs ventilation slats and the roof should be double skinned with an air gap between the two layers.There are number of plastic screens on the market in the form of a 'pile of white plastic saucers' which work well ( generally speaking the larger the better) but I don't know if one is available large enough to hold your system.

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Sun will be striking through the holes and warming the interior, louvres would help, I have seen plastic grill covers might work, and could be attached over what you have now.
This sort of thing
https://www.amazon.co.uk/White-Plastic-Louvre-Grille-Flyscreen/dp/B00GOTZH4U

A shady spot can receive quite a bit of  reflected heat both from warm surfaces nearby and light bouncing off bright areas.
But you might find a place almost good as a screen can do.
 

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The sides are double walled with offset holes and an air gap, that’s probably not clear from the photos. I built the box from a design found online and modified it slightly to fit the space I had.

the Louvres are an interesting idea, I’ll give that some thought thanks.

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On 21/04/2018 at 09:59, morton027 said:

The sides are double walled with offset holes and an air gap, that’s probably not clear from the photos. I built the box from a design found online and modified it slightly to fit the space I had.

the Louvres are an interesting idea, I’ll give that some thought thanks.

Yes, looking at your photos more closely I can see now that you have a double wall to your screen. However I still think that the problem of overheating lies in the fact that the overall mass of wood used in the construction is too great relative to the air gaps and ventilation holes, particularly as the back, top and bottom of the screen are solid, and that if you add louvres to the existing screen it will make matters worse.

It might seem a bit of an ask, when you've obviously put in quite a bit of effort to build your screen, to start again, but if you're relatively handy at woodwork you can buy off the peg louvred doors of a suitable size and contruct a double louvred box with a double boarded roof, the inner one ventilated, and a ventilated floor. If possible fasten your sensor system to a piece of wood in the middle of the box, or suspend it in free air, so that there's no chance of it being subject to  direct radiation from the box itself.

Having said all that if you're happy to live with the fact that on sunny days your recorded  temperatures are couple of degrees higher than you suspect they should be then stick with what you've got as your readings will always be comparable to themselves and you can make allowances if comparing with official Met Office sites etc. The enjoyment of taking your own readings shouldn't be completely overshadowed in a striving for complete accuracy as very few, if any, sites, even official ones, can claim to be perfect.

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Posted (edited)

I've had my Netatmo for 4 years or so. I just use it for fun alongside my Davis. I did build a box similar to yours, but temperatures still read high. In the end, I just bought a 4" PC fan which runs at 6V and added it to the bottom of the box. The temperatures have been pretty much in line with my Davis since adding it. The box is always in direct sunlight for 90% of the day.

Edited by Mapantz

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Thanks for the advice TM. It is all about the interest factor having my own readings to check on, and a little fun with projects along the way.

Mapantz, how did you power the PC fan? Battery, solar? What is the optimum placement and airflow for the fan?

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Posted (edited)
22 hours ago, morton027 said:

Thanks for the advice TM. It is all about the interest factor having my own readings to check on, and a little fun with projects along the way.

Mapantz, how did you power the PC fan? Battery, solar? What is the optimum placement and airflow for the fan?

I used an old adaptor that I had in a drawer. It has switches to choose from 3,6,9 and 12V. I just spiced the wires on it, and the fan, and joined them up and covered it up with heat shrink wrapping. The PC fans are designed to speed up and down via a higher or lower voltage, so it is perfectly safe. My sensor is attached to top of the box, which doubles as a lid. The fan is centered directly at the bottom to draw air up and over the sensor, and exhaust through the holes on the sides, and the ventilation around the top.

The fan hasn't missed a spin in 4 years!

Both my Davis and Netatmo sensors are attached to the same post, at the same height, out in the open.

Here's the Netatmo temp & graph:

5addd4027c734_2018-04-23(3).thumb.png.231fac2a9cd2c68e140ba390c8b9d682.png  5addd43138936_2018-04-23(4).thumb.png.15d27085bf623d2f7d81768c5460d616.png

Here's the Davis at the same time:

5addd447ca890_TemperatureDay(1).thumb.png.d912334ce906b76ab0f4b0c1c11d5378.png  5addd48b4e80a_2018-04-23(5).thumb.png.e582681f577b99c4d79c4f32f57e150b.png

Quite happy with that.

Edited by Mapantz
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Posted (edited)
On 23/04/2018 at 14:34, Mapantz said:

I used an old adaptor that I had in a drawer. It has switches to choose from 3,6,9 and 12V. I just spiced the wires on it, and the fan, and joined them up and covered it up with heat shrink wrapping. The PC fans are designed to speed up and down via a higher or lower voltage, so it is perfectly safe. My sensor is attached to top of the box, which doubles as a lid. The fan is centered directly at the bottom to draw air up and over the sensor, and exhaust through the holes on the sides, and the ventilation around the top.

The fan hasn't missed a spin in 4 years!

Both my Davis and Netatmo sensors are attached to the same post, at the same height, out in the open.

Here's the Netatmo temp & graph:

5addd4027c734_2018-04-23(3).thumb.png.231fac2a9cd2c68e140ba390c8b9d682.png  5addd43138936_2018-04-23(4).thumb.png.15d27085bf623d2f7d81768c5460d616.png

Here's the Davis at the same time:

5addd447ca890_TemperatureDay(1).thumb.png.d912334ce906b76ab0f4b0c1c11d5378.png  5addd48b4e80a_2018-04-23(5).thumb.png.e582681f577b99c4d79c4f32f57e150b.png

Quite happy with that.

So you take power from your Davis station? I'm sorry I do not quite understand. I'm building a diy version of a Stevenson screen and maybe I will add a fan (considering to use the Netatmo). What would be the best way to power the fan without having to run power cables into the yard?

Edited by Ruben Amsterdam

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Just now, Ruben Amsterdam said:

So you take power from your Davis station? I'm sorry I do not quite understand. I'm building a diy version of a Stevenson screen and am considering adding a fan (considering to use the Netatmo). What would be the best way to power the fan without having to run power cables into the yard?

I don't run it from my Davis station. You have two way's to do it, basically. You could wire up your own solar panel for it without the need for a super-capacitor/battery to store power for use at night. It's just run the fan during the day (all that's needed in this country, for example)

I bought a 4 inch PC fan: https://www.amazon.co.uk/gp/product/B0037AQLLM/ref=s9_acsd_top_hd_bw_bT8Osx_c_x_6_w?pf_rd_m=A3P5ROKL5A1OLE&pf_rd_s=merchandised-search-3&pf_rd_r=9YY2Y9GVPXXC4PHXA4K9&pf_rd_t=101&pf_rd_p=0cc10d00-b188-58e9-8704-e09c86827d04&pf_rd_i=430516031

I also bought a cheap multi-voltage adapter, similar to this: https://www.ebay.co.uk/itm/30W-New-Wall-Plug-in-Switchable-Multi-Voltage-Adapter-Power-6-DC-Tips-UK-stock-/322756212548

It allows you to switch between 3, 6, 9 and 12V, so a 4 inch PC fan would work with all of those voltage's. I cut the plug off of the adapter, and the 3 pin plug off of the fan and then soldered the fan and adapter wires together, and then used heat-shrink wrap around it. Taped it all up, and plugged it in to my shed's power supply. The distance from my shed to the station is around 10ft, so I ran the cable through some hose pipe and buried it under the edge of the lawn.

It sound complicated, but it isn't. You're only dealing with low voltages as well, so it's fairly safe. The fan has been going day & night since 2014. The only maintenance I do is t the fan blades, they get a bit mucky.

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Posted (edited)

I wonder how good this is https://www.metcheck.co.uk/products/metcheck-met-m14-mini-polycarbonate-stevenson-screen

My netatmo sits in same Stephenson screen as the main thermometers. Interesting that today is reading much lower than my Davis which may mean the batteries in my Davis shield need taking out to charge up.

Just noticed the netatmo hasn't updated in an hour. So that will explain the difference.

Edited by The PIT

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