An aspirated screen for the WH1080

[Repairman77]

As you know there are problems with temperature affecting the other sensors in the remote unit on the WH1080.
I think the main problem is the inadequate screen supplied with the unit; it's just too small; and the Sun soon heats it up giving higher readings than are actually present in the air. It can also have an effect on the other sensors throwing them out of track with manual sensors which are known to be correct.

I saw the Spanish site project for making a larger screen with a fan in the top and decided to give it a go.
It's not a very expensive project and should cost around £20, including a 12V solar cell.

I've pictured all the parts needed below, 7 x 6" plastic plant saucers, spacers made from 10mm plastic plumbing pipe and 6mm threaded rod to hold it all together. Even without a fan I'm sure this larger screen would make an improvement on the original and help give more accurate readings.

I found a motor from a Pioneer cassette deck from the 80's that I had salvaged many years ago. It runs on 6V so I have fitted a 7806 voltage regulator on that section so that it will run from a standard 12V solar cell. The type you stick in your car windscreen to keep the battery trickle charged. They give around 1.5 watts; plenty enough to run the fan. The advantage of running from a higher voltage is that if the 12V from the solar cell drops due to poor weather conditions there should still be enough volts to run the fan. You could, I suppose, use any small motor, even one of those hand held vanity fans that run from two 1.5 batteries but you would need to drop down the voltage with a regulator or string of Silicon diodes.

I now have to paint all the sections white.

I will keep you updated on it's progress and let you know if it makes the sensors track more accurately. I'm sure it can't make it any worse.

Mike

[Repairman77]

It's best to make a cardboard jig in able to get all the support holes lined up on the sections.
The 2" holes in the centre of most of the sections were cut with one of those 2" hole saws with a 6.5mm centre bit.

[daj]

I will keep you update on it's progress and let you know if it makes the sensors track more accurately. I'm sure it can't make it any worse.

Thanks, look forward to it. I went from -2.5 to +14 very quickly yesterday. I suspect the limited spell of sunshine had something to do with that!

[Repairman77]

Thanks, look forward to it. I went from -2.5 to +14 very quickly yesterday. I suspect the limited spell of sunshine had something to do with that![/quote]

I doubt it would have been the Sun on the screen; that's too much of an error.
Had you checked it with an outdoor thermometer?
Mike.

Sorry for the double post - please delete this one if you see it Steve. Thanks.

[Repairman77]

Thanks, look forward to it. I went from -2.5 to +14 very quickly yesterday. I suspect the limited spell of sunshine had something to do with that!

I doubt it would have been the Sun on the screen; that's too much of an error.
Had you checked it with an outdoor thermometer?
Mike.

[beteljuice]

Re. Solar panel / fan power.

Whilst the Sun is (usually) the greatest and most obvious source of radiation, it is not the only one !

The beteljuice built a stainless steel housing for his (LaCrosse) sensor, and tests have proven that it is very rare the fan is necessary ! - however, I mounted my solar panel nearly 3m above the unit so that the fan continues working until the solar panel is shaded, NOT the sensor housing.

This reduces the effect of any 'local' masses which may have been heated by the Sun still 'radiating'.

[Repairman77]

Thanks for the tip, will do that.

The fan measures 45mA at 6V to run so the solar panel I've ordered will cope with that with no problem.
I could add a rechargeable to the system which would run the fan 24/7, not sure if that is necessary as some say you only need it during the day. Not sure what the theory is behind that? Perhaps someone could explain.
Mike.

[akasonny]

That is, indeed, interesting. My tests revealed just the opposite.

When there was a detectable breeze, the sensors were "within parameters" with the most accurate results when the sustained wind was above about 10-15 mph but when the wind was calm, humidity/dewpoint/temperature all lost considerable accuracy due to direct solar heating. Of course I live in one of the hottest places in the US and this may be a bigger concern to me than others who live in more temperate climates.

Therefore, my conclusion was that if the sun is hitting the screen (or whatever container one chooses to mount the sensors in), fan-aspiration is best regardless of true wind speed. Not only that, but the air-flow across the sensors must be fast enough to dissipate any radiational heating to which the sensors are exposed be they direct or indirect. In my case, 10+cfm fan was adequate.

I, too, mounted the solar panel higher to take advantage of the sun for a longer part of the day even during times when the sun isn't hitting the sensor "package" directly for the same reasons you did.

Re. Solar panel / fan power.

Whilst the Sun is (usually) the greatest and most obvious source of radiation, it is not the only one !

The beteljuice built a stainless steel housing for his (LaCrosse) sensor, and tests have proven that it is very rare the fan is necessary ! - however, I mounted my solar panel nearly 3m above the unit so that the fan continues working until the solar panel is shaded, NOT the sensor housing.

This reduces the effect of any 'local' masses which may have been heated by the Sun still 'radiating'.

[Repairman77]

Sonny; that's what I found; in low wind conditions the Sun quickly heats up the little screen and the readings go out of calibration.
The worse is the temperature reading which can be +2 degrees or more in those conditions.
One of the reasons for building this project.

However we would never see such high temperatures as you would at your location in Arizona; the most we get in a heat wave is just over 30 degrees C. What are your maximums?

Mike.

[akasonny]

Mike,

Extremes are -1C to 48.8C but averages are closer to 1C to 42.2C or so.

I placed my sensor package in a length of 76mm PVC pipe, wrapped the pipe in heat-reflective solar film and fan-aspirated with 12vdc muffin fan and 600ma solar panel. Really happy with the results.

Steven

[Repairman77]

Steven; I bet that mod was a real necessity at the sort of temperatures you get there.
Just how far out were the readings with the original little screen?
Quite a few degrees I should think.
Mike.

[keithatrochdale]

Repairman77

Did you read the mod for the humidity sensor?

IIRC drill more holes in the case around the sensor.

[akasonny]

Mike,
There was a range of error readings depending on whether the sun was directly hitting the sensor and depending on other radiational heating in the surrounding area. Average area was somewhere in the vicinity of 3-6 degrees F as I best recall. I don't remember the errrors I had on humidity and dewpoint but they were in direct relationship to the temp error.

I'm convinced that ANY installation which experiences direct solar exposure would benefit by some sort of fan-aspirated "contraption".

Steven; I bet that mod was a real necessity at the sort of temperatures you get there.
Just how far out were the readings with the original little screen?
Quite a few degrees I should think.
Mike.

[beteljuice]

Well first all, you've proved the need for a suitable screen, or more precisely something to shield and / or dissipate and / or reflect radiation.

Although everyone keeps chanting Stevenson screen, serious kit is protected in a different way within or without that enclosure !

.. fan-aspiration is best regardless of true wind speed. Not only that, but the air-flow across the sensors must be fast enough to dissipate any radiational heating to which the sensors are exposed be they direct or indirect. In my case, 10+cfm fan was adequate.

Sling psychrometers (like a football rattle) are recommended to spun so that windspeed >2 m/s.

The all time great Assman Aspirated Physchrometer (still being made today), essentially has the thermometer bulbs in a double polished metal sleeve (stainless steel now) and air drawn up both the inner tube AND the surrounding sleeve by a clockwork motor at 3 m/s - this is still regarded as an extremely accurate device even in tropical situations.

I can't find a 'cut-away-drawing' now but these give you an idea:

http://www.photolib.noaa.gov/htmls/ship4494.htm

http://www.imdpune.gov.in/surface_instr ... ssman.html

Strictly speaking, things should be freely ventilated OR aspirated, but everybody is mix and matching now.

The beteljuices accidental stainless steel double tube (which is out in the open) only gets fan aspirated up the core tube, but has been excellent - There were several pages of a forum telling me I couldn't put it in a 'tin can'

I think they all paid good money for Davis screens .........

[Repairman77]

Repairman77

Did you read the mod for the humidity sensor?

IIRC drill more holes in the case around the sensor.

Keith, I did see that mod but decided to delay that one until it's out of warranty. I had to have a replacement console under warranty (nobody's fault, just a duff one) so I'm being cagey in case I should have trouble with the remote transmitter unit.
The aspirated unit should be sufficient for now when it's finished; if the gloss paint on the plastic pot saucers ever dries, LOL.

Mike.