Stevenson Screen

An unsheltered temperature sensor will read high even in the shade. To properly measure outdoor air temperature the sensor should be placed inside a device called a Stevenson Screen. One version is the commonly seen lovered box.

I found a DIY unit from down under. It is constructed of plastic upsidedown flower pot saucers with the centers cut out. The instructions for building and locating this version of a Stevenson Screen can be found here.

I wanted to try the screening thing, but was not up to building anything near a proper screen. Instead I covered a flowerpot with alu foil and inverted it.

The sensor goes somwhere in the interior of the pot. The foil reflects most of the radiation that would heat the sensor. If the pot should heat the warm air should in theory move up through the holes in the pot and out the top of the foil.

Please note that I an not worried about rain at this point.

Now a proper screen should have a proper mounting system. As you can see from this photo I stuck it in a crack between the fence and a gate. The scary part is that it seemed to be working. For the first time my reading were in line with what the weather channel was reporting.

I was so encouraged I constructed a second unit for use inside the greenhouse. In this version the sensor wire is feed through a hole in the top of the pot. The entire unit is supported by a clothespin that clips the foil to a wire on the plant hanger pipe. The results are encouraging. The greenhouse reading fell.

But it is too early to put much stock in the 'design'. I need to setup a 2nd probe at each site and compare the readings.

There are some glaring problems here. The worst is that the sensor could get rained on. The other problem is that when the sun is overhead it might actualy get a look at the sensor.

In the near term I will continue to compare the outside temperature with the weater channel. If it continues to look reasonable I will continue developing the design.