In my previous post on my weather station I included a graph of the relationship between the three temperature sensors I’m collecting data from. It turns out that the plot is incorrect because I had messed up the program that converts the raw data from one of my Arduino stations. What became clear after looking at the (fixed!) data is that the enclosure I built isn’t adequate at keeping the sensors cool when in direct sunlight. I could probably improve the design of the enclosure somewhat, but I decided to aspirate the sensors instead.
The photo on the right shows the inside of the enclosure. The sensors are sitting on a platform inside the pipe, and there’s a small muffin fan (the kind you’d use to vent a computer case) on the top. It’s a 12V fan, and at the moment it’s being driven by a 9V AC/DC converter. The plan is to replace the converter with a 12V solar cell that is sufficient to drive the fan. This way it doesn’t consume any electricity, and the fan is only spinning when it’s necessary (when the sun is out). Thus far, I haven’t found a suitable solar panel. The small ones designed to charge a cell phone battery don’t operate at the correct voltage (and probably don’t produce enough current anyway), and the big ones designed to keep a car battery charged are expensive and overpowered for my needs. With winter rapidly approaching, I’ve got plenty of time to figure something out.
The pipe is a piece of 4” sewer pipe that’s been spray painted white and has a series of holes drilled into the bottom. The fan pulls air up through these holes and over the sensor array in the middle. If I had it to do over again, I’d cut the pipe a bit shorter so it’s not so difficult to get into the enclosure. But for keeping the sensors bathed in atmospheric air, it works quite well.
The plot shows the observed temperature for each of the three sensors I’ve got. The blue line is the “west” sensor that’s inside the enclosure I built and is the subject of this post. The red line is the reported temperature from the Rainwise station that sits atop a post attached to the dog yard gate. The green line is the sensor that’s behind the house and under the oil tank. I built the enclosure for the west sensors on July 12th and installed it that evening. You can immediately see the effect of the shielding during the high temperature peak on the 13th. But you can still see the two little peaks that are present in the previous plots. These peaks come from direct sun on the station, split by some trees that shade the station for an hour or two.
On the evening of the 13th I installed the pipe and fan. It was smoky on the 14th and cloudy on the 15th, but you can see the effect of the fan on the following dates. The double peaks are now gone, and the temperature from the west sensors at the high point during the day is now a few degrees cooler than the measurement from the Rainwise station. Also notice that all three sensors are virtually identical on the 15th when it was cloudy and raining.
What this demonstrates to me is that the aspirated west sensor is now the best reference sensor for our site. The Rainwise sensor is a close second, but it’s Gill multi-plate radiation shield isn’t as effective as my aspiration system at reducing the effect of solar heating on the station.