24 Hour Fan Aspirations

Has anyone made a 24 hour fan aspirated system for the Vantage Pro 1?

I’ve read several accounts of people making a solar powered shield out of plant saucers, but I need something that works at night too. My daytime temperatures are actually pretty close to perfect, but at night my sensor seems to trap a lot of warmth, especially in the evening where it is very slow to cool down. The official metar nearby is often 2-3 degrees cooler than mine, and the air definitely feels colder than what mine is telling me.

I have some ideas, but most involve running an electrical cord to power some sort of fan. I’m not sure about using rechargeable batteries, and I’m not very good with electronics, so anything very complex is automatically out of the question.

That’s a slightly unusual finding. In general, the daytime max would tend to be a little high with a passive shield, especially on very sunny days of course, and there is also inevitably something of a lag in responding to changes of temperature, eg after dawn and dusk. But there’s usually plenty of time overnight for the shield to equilibrate tolerably well with its surroundings and so to record a representative minimum. I’d be more inclined to suspect that there’s something different about the environment of your station that accounts for the differences from the location of the official metar. But do you notice for example any better agreement of minima on nights when there’s some wind and hence faster equilibration of the passive shield compared to still nights?


Can you describe the current location of the shield, or maybe post a picture?

@ prodata & niko

I had major problem with the sensor in full sunlight (often reading 2 C too high), so I took apart the ISS and moved the sensor into the shade of a tree. It is in shade basically all day, with only a small amount of filtered sunlight (not enough to cause spikes in temperatures), hence, the daytime high seems pretty accurate (maybe just 0.5 C too warm on very hot summer days).

On cloudy or windy nights there is little difference between my station and the metar. It is only when the temperature drops quickly under clear skies that my sensor lags behind. This would not be a problem, however the temperature does not always have time to reach it’s low point before the clouds move back overhead. On a completely clear night, the temperature does “catch up” to the metar quite often, but usually not until near sunrise. And if the sky is variable, it simply can’t adjust to the changes quickly enough.

Is the tree canopy actually over the sensor? If so, on a clear night it is possible that it would help to retain some of the daytime heat while an open area would more rapidly radiate heat out to space. Of course it would be more pronounced in the warm months if its a deciduous tree.

Yes, it’s an evergreen tree, but there is plenty of open airspace around the bottom. I honestly didn’t think it could have much effect, but you could be onto something.

Should I move it back into the sun and use a daytime fan?

Optimal placement (with a radiation shield) would be in the open. A daytime fan would probably be your best bet. I have one and although not as good as a 24 hr fan, it does help.

I think I may have figured out what was causing the problem: earwigs.

This morning I opened the radiation shield to clean it and found them packed around the temperature sending unit like sardines. I think their body heat must have been insulating the sensor and not only slowing it from cooling, but keeping it one or two degrees too warm all night long.

But this discovery comes a bit too late, as I’ve already gone to the trouble of building fan aspirated radiation shield. #-o

I might never know for sure if the earwigs were the true cause of the problem, but I expect they were.

I believe that earwigs are active at night and find a shady moist place to hide during the day. Although they certainly could affect the airflow, I’m not 100% sure that they would be the main cause of your issues. So building the aspirated shield is still a good idea. :slight_smile:

Not impressed at all with my FARS so far. Daytime temps are running noticeably higher now and nighttime has improved very little, if at all.

All things considered it was better in the old location without FARS.

I’m running a PC fan 24 hours on a 9V DC adapter. I may increase that to 12V but I don’t see it making a huge difference. I may also remove some of the plates on the shield to increase airflow. Ideally, I think air needs to be flowing right over the sensor itself.

Looks like you would be better off putting slats in the side of that box…all I see are 4 holes in the bottom for airflow. I bet it gets very warm inside that box when the sun hits it…

Yeah, probably. I was hoping the fan would provide sufficient airflow to keep the air inside fresh. I wanted to keep the design simple, but I guess that was a mistake.

My ducting opens right above the sensor itself and all around is fully insulated - an upturned bucket - yes the airflow over the sensor has to be channelled and you don’t want any heat from the box itself passed through or radiated through onto the sensor device.

Not pretty but it works well.

more here…


I assumed the fan was above the sensor…seems someone one here did some tests and found it better to draw the air over the sensor rather than blow the air over the sensor. If you search the forum there have been some lengthy discussions on stevenson screens in the past, some with pictures, and lots of ideas that have been tested…

Yes Dan, it is a PC PS fan working as an exhaust fan pulling upwards over the sensor, grass field all around.

TokKiwi, I used what your ideas as a rough guide for building my shield. There is very little difference, except that mine is inside a box.

I’m wondering if my problems have more to do with the area surrounding the station than the affects of the sun itself. The house is only 15 ft away, and must radiate some heat when the sun is shining. There is also the gray painted fence, and there is the patio, both of which likely radiate some heat. I can touch the fence and it feels warm when the sun is shining on it.

I’m very limited in where I can put the sensor. The only other option would be a post in the middle of the lawn, but there’s no way my mother would approve. I’m already pushing my luck the way things are now. :lol:

So… I’m going to add some vents to the sides of the box, and increase the voltage of the fan from 9V to 12V.

My daytime high today was 2.8 C warmer than the metar. The overnight low last night was 1.1 C warmer. I’d like to have both within 1 C.

I’ve attached a couple more pics…

Fluffy Alien - you made me laugh! Copying is the purest form of flattery! I’m sure you will get good reliable measurements soon. Its funny the way those fans just fit perfectly inside that elbow when cut down - those plumbing parts were expensive here though - I was shocked when I bought the bits! What you have done is very tidy - I AM the adult here (one might wonder :roll: ) and I still have things to tidy up - wires and all that! (well just the one…)

TokKiwi, I paid $1.39 each for the elbow pieces here in Canada. I’m not sure if I was getting a good deal or not, but it was the best I could find.

What voltage are you using for your fan? I’m using 9V DC but it doesn’t seem powerful enough. Is the amperage important? My fan is rated for 0.1 amps, so I used a power supply with the same output. I’m not very good with electronics. :?

I’m glad you like my craftmanship. :lol:

Hello Fluffy Alien.

What sort of distance is it from your station location and the location where the metar comes from?

It can be quite normal to have a few degreese temp differance from one location to another, and as you have discovered there are many factors that can influance local temperatures.

One way to check your readings would be to use a ‘mercury in glass’ thermometer to compare with your station readings. You may well find that your readings are corect and your location is slightly warmer than the metar location.