Better screen for the FO temperature sensor box

[fractonimbus]

This may have been discussed here before, but I couldn't find anything in a quick search.

The plastic "stacked flying saucers" screen used by the FO WH1091 are definitely not good at stopping the temperature sensor from being heated when the sun is shining on it. From looking at my daily temperature traces, there's perhaps 3 degrees C added to the true temperature while the sun is on the unit.

So I was wondering if anyone had thought about it, or better, made an improved version.

I don't have the opportunity to mount a proper Stevenson Screen on my roof, so something like a larger version of the current plastic thingy would be the way to go. I suspect a series of larger concave aluminium disks made to the same pattern might be a good way to go, as aluminium's emissivity in the infra-red is poor, so it should both reflect sunlight well and not re-radiate heat inwards.

Has anyone done anything like this?

DN

[Gina]

Several of us have made screens from inverted plant pot saucers and/or inverted plastic picnic plates. There are threads in this forum about them. Before I went for a more standard Stevenson screen, this is what I made :-

Screenshot-20.png

[artworksmetal]

I would stick to plastic or wood. Aluminium (hard putting that extra 'i' in there) is an excellent thermal conductor, so it will carry heat inward. Also, I'm betting it will block the transmitter.
A thin layer of reflective paint or foil would accomplish what your suggesting, without the conductive mass. Even the metallic paint may hinder the transmitter, though.

I covered mine with 1/2 of a plastic 5 gallon bucket (with some vent holes drilled). Not sure how effective it is yet. I'll begin experimenting more after it warms up a bit. I bought a cheap solar fan to help circulate the air, but haven't tried it yet.

[artworksmetal]

Wow. That is gorgeous and ingenious. Puts my redneck plastic bucket to shame.

[fractonimbus]

I would stick to plastic or wood. Aluminium (hard putting that extra 'i' in there) is an excellent thermal conductor, so it will carry heat inward. Also, I'm betting it will block the transmitter.
A thin layer of reflective paint or foil would accomplish what your suggesting, without the conductive mass. Even the metallic paint may hinder the transmitter, though.

Good point about the transmitter! It would only fire out the gaps, so the receiver would need to be able to "see" it. OTOH, aluminium would have the benefit of stopping any interference pickup by the transmitter (though not the receiver).

Aluminium (BTW, that was the US spelling, too, until about 1905!) is a good conductor of heat as you say, meaning wind passing over it will cool it quickly (unlike poorer conductors like plastic). But aluminium is a poor emitter of infrared radiation, so re-radiates very inefficiently in the mid to far infrared, which is where the heating of the transmitter would come from. So apart from the transmitter masking, it should work well. Paint or any sort is likely to be better at emitting mid/far infrared so would transfer heat radiatively more efficiently than aluminium.

[Aside: I noticed some decades ago that the aluminium alloy engine block of my Norton Commando motorcycle (that dates me) did not feel hot when I held my fingers close to it, but was in fact very hot (after a burn up through the Adelaide Hills) when I touched it. Then a mosquito flew by, detected the "warm" engine block, landed on it thinking it was "food" and was suddenly frizzed into a dried out husk. It didn't know about infrared emissivities ]

The lesson from that, though, is that in the absence of wind, aluminium things cool slowly.

DN

[fractonimbus]

Several of us have made screens from inverted plant pot saucers and/or inverted plastic picnic plates. There are threads in this forum about them. Before I went for a more standard Stevenson screen, this is what I made :-

Nice elegant design, Gina. Food for thought.

DN

[mcrossley]

Food for thought.

Boob boom tish! I thank you.

[Gina]

[iceberg]

This may have been discussed here before, but I couldn't find anything in a quick search.

The plastic "stacked flying saucers" screen used by the FO WH1091 are definitely not good at stopping the temperature sensor from being heated when the sun is shining on it. From looking at my daily temperature traces, there's perhaps 3 degrees C added to the true temperature while the sun is on the unit.

So I was wondering if anyone had thought about it, or better, made an improved version.

I don't have the opportunity to mount a proper Stevenson Screen on my roof, so something like a larger version of the current plastic thingy would be the way to go. I suspect a series of larger concave aluminium disks made to the same pattern might be a good way to go, as aluminium's emissivity in the infra-red is poor, so it should both reflect sunlight well and not re-radiate heat inwards.

Has anyone done anything like this?

DN

Yes i have but with Aluminium tape and it works great.

[gbru316]

I ended up just using a piece of PVC pipe instead of the standard screen.

I cut a piece of 4" PVC to about 18" long, fitted the bottom with a drain screen (found a 4" slip screen that slid right into the PVC). Then for the top, I used 4" to 3" PVC reducer and a 4" PVC cap.

I mounted a computer fan into the reducer so that it draws air through the bottom of the pipe and pushes it out the top. I used screws to mount the 4" cap onto the 3" reducer so that there is a gap of about 1/2" between the 4" part of the reducer and the bottom of the cap, to allow air to exit the pipe.

Finally, I cut a small hole in the drain screen for the sensor wiring, and just sat the sensor on top of the drain screen, inside the pipe.

I was having temp spikes of 15F+ before using this, now I don't have any spikes at all. Temperature is now consistent with MADIS quality control, whereas before it failed horrible in the mid-afternoon sun.

Here's a quick paint drawing of the shield

Shield.jpg

[Gina]

Very interesting I built a conventional Stevenson Screen following experiments with a larger version of the original with a fan which were better but still giving a degree C rise. The Stevenson design is better than half a degree. Much more effort to build though - I like your nice and simple design

[mcrossley]

How about a self-ventilating screen? Along the same lines - though you could use concentric pipes to create a double wall around the sensor area. But make the central pipe quite long, the bottom white, and a section at the top painted black. The idea being to create a chimney effect, the warmed air at the top would rise out, drawing in cooler air at the bottom where the sensor is mounted. You would have to be careful about radiative effects - hence the long pipe to keep the warm section away from sensor.

I have no idea if this would work or not.

[gbru316]

How about a self-ventilating screen? Along the same lines - though you could use concentric pipes to create a double wall around the sensor area. But make the central pipe quite long, the bottom white, and a section at the top painted black. The idea being to create a chimney effect, the warmed air at the top would rise out, drawing in cooler air at the bottom where the sensor is mounted. You would have to be careful about radiative effects - hence the long pipe to keep the warm section away from sensor.

I have no idea if this would work or not.

I tossed this idea around too, but I had a few old computer fans and some 12v transformers laying around, and I figured the active cycling of air would be fool-proof, which is why I used the fan.

The real test will be in a few months when the pipe is exposed to full afternoon sun. I don't foresee any erroneous data points as the sensor is located at the bottom of the pipe, but you never know.

[Orion]

Here is the link to the one i made last year
still going stong and getting very good readings

https://cumulus.hosiene.co.uk/viewtopic.php?f=16&t=5023

[41south]

+1 on Roys design, I built mine a while back and it works great. I did break a few melamine plates getting the holes cut though Glad I bought spares