Topic: VP2 24-hr FARS fan "chugging"?

[Old Tele man]

Question for all Davis #0613 VP2 24-hr FARS owners: Have you noticed 'slight' temperature fluctuations as clouds briefly darken otherwise sunny daylight conditions?

During cloudless days, my temperature plots are far "smoother" than passing-cloudy days, which create slight "up & down" variations in an otherwise smoothly changing temperature plot, suggesting that the FARS fan is "chugging" (slowing down and then speeding up). Such chugging can occur when sunlight ADDS voltage so the fan speeds up and then slows down when the sunlight is reduced as the cloud passes overhead...thus creating a situation where the FARS fan airflow is speeding up & slowing down as sunshine and cloudiness alternate, causing the cloudiness to "modulate" the airflow and the temperature readings.

I assume this does NOT occur with AC-powered FARS stations where the fan speed is constant and not dependent upon sunshine power.

Comments?

[CW2274]

Comments?

https://www.ebay.com/itm/80mm-25mm-New-Case-Cabinet-Fan-Kit-110-115-120V-AC-67CFM-Cooling-BB-8025-1766/380794571711?hash=item58a9200bbf:g:ed0AAOSwPc9W12ei

You didn't expect me to say anything else, did ya?

[CW2274]

Comments?

https://www.ebay.com/itm/80mm-25mm-New-Case-Cabinet-Fan-Kit-110-115-120V-AC-67CFM-Cooling-BB-8025-1766/380794571711?hash=item58a9200bbf:g:ed0AAOSwPc9W12ei

You didn't expect me to say anything else, did ya?

Take a look at my data and see if you "see" the samething NOT occurring at your station.

Doubt that would help. If you remember we discussed about temp fluctuations a month or so ago about how the hotter months tend to make the temp yo-yo like crazy here. Mines no different. Thing is, my flow is constant, reliable, no matter what, where, how, or why.

[ValentineWeather]

I'll post graphs of 2 stations (AC and DC fans) different locations but only couple miles apart (today) with occasional nimbus cloud blocking sun. The AC response shows bigger dips and it works both ways as it heats up the AC fan generally see's higher temperature peaks because sample rate of air is that much higher.

[CW2274]

Looks like top (AC) pix is less 'smooth' than the bottom (solar DC) pix...opposite of what I'm thinking.

Think if no fan was being used, the air in the sensor chamber would be very stagnant (our VP2 24hr FARS design), making it very consistent, but most likely very "un-ambient". The stronger fan is pulling ambient air into the chamber faster, allowing for greater, quicker and more accurate reading of fluctuations. This is exactly why ASOS's use an averaging algorithm. When I first got my SHT31 and a/c fan, the yo-yo-ing use to drive me crazy. Now that I get the "big picture", I am perfectly good with it.

[ValentineWeather]

Yes what I was trying to say is AC fan is even more gagged, because of faster response time.
Where I really see issues is dewpoint on hot days especially over grass the AC fan will often pull higher samples of moisture causing abnormally high spikes in dewpoint. The DC fan does it somewhat but not near as drastic.
 
I did some testing last summer raising shield higher, about 15' and was able to smooth out the dewpoint spikes but it caused other issues, abnormal cooling of ambient temperature negative (1-2°) so decided not to continue with shield positioned 15' off the ground.

Oh yea the 5 minute ASOS averaging algorithm would solve my issue for sure.

[ValentineWeather]

I've been kicking around the idea of returning to the Cotton Region Shelter I used for years. This would give the equivalent of 5 minute averaging.
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[ValentineWeather]

...called a "Stevenson" screen shelter?

By some, NWS calls them Cotton Region Shelter. 

Novalynx carries them and they aren't cheap.

[ValentineWeather]

The one I had was painted with whitewash, I was reading they use some white latex paint now and run warmer inside because of it.

[CW2274]

Q: Does DAVIS apply "averaging" to both DP and Temps?

Neither, and frankly, I don't think any PWS's do. What is displayed is the raw reading at each tx cycle. However, in WL you can select to average the temp over a 5 minute period, but that average is not relayed for on-line data.

[ValentineWeather]

This was a long term study of FARS vs Stevenson screens...Stevenson screens faired well.
http://www.wmo.int/pages/prog/www/IMOP/WebPortal-AWS/Tests/ITR649.pdf

Whitewash paint test also was done by someone else but it was a 1 day study so not totally convinced.
Sensor placement inside these shelters matters too.  The study above was done in Australia so there hot side would be north side of enclosure.

Here was the paint test:
https://wattsupwiththat.com/2008/01/14/a-typical-day-in-the-stevenson-screen-paint-test/

[CW2274]

I think a Stevenson would fair favorably in our desert landscape. I remember the Desert Museum had(s) one, I think it reported onto a rotating drum. Guess I'll stick with the little cylinder and a blower.

[ValentineWeather]

SUMMARY:  FARS vs. Stevenson = Vertical versus Horizontal air motion measurements.

The larger volume of the Stevenson acts as a real-world mechanical "integrator" when contrasted to the typically smaller chamber(s) of FARs. Also, the solid/flat bottom of the Stevenson blocks all vertical airflow so that only horizontal airflow is possible; conversely, only vertical airflow is possible with most FARS. This difference creates detectable measurement differences, especially when ground surfaces below the sensors are different.

I wonder now what version of stevenson screen they had. All the ones I've seen including mine had slated bottoms and definitely not closed. Air could easily move up through the bottom.

[CW2274]

As I search around for pics, when you do see the bottom, it's solid, but raised up an inch or so like a shelf, so air can still circulate underneath.

[CW2274]

As I search around for pics, when you do see the bottom, it's solid, but raised up an inch or so like a shelf, so air can still circulate underneath.

Airflow 'around' a flat plate is far more impeded than it is 'through' slatted louvers.

No doubt, and I would think a purposeful design. I didn't spend alotta time looking, but I didn't see any that had louvers on the bottom, just solid and raised.

[ValentineWeather]

The Cotton Region is the USA name, this is exactly what I remember with slatted openings on bottom.
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[CW2274]

Theses things I think would be a nightmare to keep bugs out of.

[ValentineWeather]

I read where NWS was actually giving there surplus away when they changed over to the MMT stations. IN UK they are still popular and make an all plastic version (Long term no louver warping) painted black on the inside.

[CW2274]

I read where NWS was actually giving there surplus away when they changed over to the MMT stations. IN UK they are still popular and make an all plastic version (Long term no louver warping) painted black on the inside.

Our 24hr FARS design has the bottom of the senor chamber in black. Can only assume it helps with keeping things more ambient inside.

What's MMT?

[WxLover16]

CW2274 provided the link to his AC fan and he's right. If your setup is at all possible to have an AC fan, then do it. You don't have to worry about clouds moving over the sun, fan voltage dropping, then the cloud moves away and fan voltage rises again (causing possible spikes in your temps). AC fan, completely constant airfow 24/7. No ifs ands or buts. THE way to go, especially in very hot climates. I'll never look back.