Home built FARS

[mike]

Here is a fars that im working on

[steve]

Nice. Shame it had to be painted, really. That green is rather fetching

[mike]

That green is rather fetching

[clyde66]

Here's a nubbie question for you.

Doesnt the extra shade/covering and fan change the readings to the equivalent of "in the shade" readings.
So it would (if wireless) be easier to place the sensor under an awning or eve?

Also what happens to the humidity readings during a light rain on a stinking hot day?
ie I had 43c here on 24-dec with light showers that couldn't register on my rain sensor as it evaporated almost as it hit the ground.

[clyde66]

Oh yeah,

That green was way cool.

Woulda looked great in the garden.

[beteljuice]

Doesnt the extra shade/covering and fan change the readings to the equivalent of "in the shade" readings.

That's the idea !

So it would (if wireless) be easier to place the sensor under an awning ?

That actually gives a decent result, but you would still get direct radiation at low Sun, and depending on it's situation, some reflected radiation at other times.

or eave ?

Talking of situation, that's why the temperature sensor is supposed to be just below head height, in the open and above grass.

[Gina]

Did you get a chance to see how much difference the fan made? I've built an extra Stevenson screen along similar lines and it helps but yesterday we had very strong sunshine and no wind at all and during the middle of the day the WS reading was around 5C whereas my max/min thermometer on the back (north facing) wall of the house read -4C. So I'm thinking of aspirating it - or maybe making a standard wooden Stevenson screen.

[Gina]

I've made an extra Stevenson screen but still get increased temperatures in strong sunshine so I've decided to add an aspirating fan. I have a spare 6cm computer fan. This is the current setup.

[TNETWeather]

hmmm.. the Davis Vantage Pro2 FARS that I have does have a fan, but the fan is very very low speed. It just does a little suction pull out the bottom which keeps air from accumulating on in the unit. The sensor, combined temp/humidity is a small thing located near the top of the unit. The enclosure is white to help not accumulate heat.

If you put your ear near the unit, you can barely tell there is a fan at all and you don't really feel any air movement out from the bottom where the fan is located.

My unit is in the open, aprox 6ft above grass mounted towards the south on a wooden posted. The solar panel which collects sunlight and helps power the unit is facing south and the whole unit is out in the open with no shade. The fan runs 7x24.

AZ gets a lot of direct sunlight... with heat up to (so far) 122°F and cold down to (so far) 21°F.

BTW... it's GROUNDHOG DAY! Time to get the little er portly guy out and see if he see's his shadow. Here it would be overcast with rain expected tomorrow... so no shadow.

[Gina]

I've now added a fan and tube to channel the air flow, to my Stevenson Screen. I've yet to provide the power. I'm planning to reduce the 12v to give a much reduced fan speed. I think it only needs a slight movement of air to stop heat build-up. The tube was made from a sweetener powder plastic jar with the bottom cut off and a large hole in the screw top for air flow through the fan. A similar size hole was cut in another dish ( to replace one of the originals which had an even bigger hole). I think the attached (cross-section) diagram should be self explanatory.

[beteljuice]

I think it only needs a slight movement of air to stop heat build-up

A clockwork fan pyschrometer has a minimum airflow of 3 ms-1
(Dry) inanimate objects don't suffer windchill, so as long as the power of the fan / housing combination doesn't create a pressure differential, it is impossible to 'cool' below ambient temperature.

Wouldn't it be better if you removed the original sheild ?

[Gina]

Wouldn't it be better if you removed the original sheild ?

Yes, I think it would. I can see no benefit from keeping it on. In fact I think it would encourage an increase in temperature due to (admittedly small) heat dissipation from the transmitter circuit. And with the new tube, there is no danger of rain blowing onto the sensor unit. I think it may be better even without the fan running. I shall remove it tomorrow.

As for fan speed, I've had a thought... Even 5v runs a computer fan at a significant speed and I think it possible that the output from a garden light photovoltaic unit might run the fan in sunlight. I'm going to try it. Not that there's any problem running power to it.

Edit... Just tried another fan on my bench power supply and it needs 3v to run. So since the photo cells generate up to 2.5v in sunlight it will need two.

[werribee_au]

OK, here is my effort.
I have an array of garden lights solar panels configured for 6VDC, and a 6VDC solar panel in parallel supplying a 5VDC 30mm fan drawing about 70mA. Maximum sun the voltage is about 5.8VDC and cloud cover it is about 5.2VDC.
It has reduced the reported temp while in sunlight by about 2 deg C and now have similar figures to the weather bureau stations in my area. I'll tidy it up a bit and make it more permanent when my trials are complete. Also thinking of more solar cells with a voltage/current regulator to charge batteries and power the transmitter. Maybe a current reg for the batts and a voltage reg for the transmitter. Another project that will probably take some time while I gather parts.

Oh, by the way, I put the plastic screen on the sensor to help direct the air flow towards the sensor as it is a small fan.
If I can locate a larger 5VDC fan I'll try that at a later date. I would like to ventallate the stevenson screen with a larger 12VDC fan I have here but I want to keep away from external power and the added electronics of turning the fan off in low light.
But, you never know!

[Gina]

I'm using a 12v CPU cooler fan which works fine off 5v (min 3v, below that it stops) , I'm just about to try running it of two garden light solar panels.

[Gina]

I have an 80mm fan indoors and that will just run with 2 panels and strong sunshine through the window. I tried it outdoors on the 60mm fan in the FARS and no movement. Tried 3 panels on the 80mm and it was worse so it's the current/resistance of the panels that's the problem. Maybe 4 panels in a series/parallel arrangement would work.

UPDATE
Brought the FARS indoors and connected the fan to my bench PSU. Min volts is 3.4 and it draws 60mA on starting and 50mA once running. That compares to around 10-20mA for the 80mm fan. Evidently the 80mm fan is much more efficient. I don't think the garden light solar panels will power the 60mm fan even using 4

So now i have to decide if I can change the FARS to use the bigger fan or run power cable to the unit. I shall be running 12v to the rain gauge heater so will have power just a few feet away.