Connecting a Pyranometer to the WEL
June 24, 2011
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Getting a pyranometer to work with the Web Energy Logger (WEL) was a long process for us in the lab. A pyranometer is a device that measures the solar intensity in watts per meter squared. We used a 0-5V pyranometer from Apogee (http://www.apogeeinstruments.com/pyranometer/). This was a challenge because the WEL does not have a 0-5V input. We needed to convert the signal to 4-20 mA so that it could be read by the WEL. There is a benefit to 4-20mA though since it is a current signal and not voltage it doesn’t drop off when you use a long wire run. Originally we had an electronics student design us a signal converting circuit board because we had the tools to design and mill circuit boards here at the college. We found a company that manufactures boards like this (http://controlsignalconverter.com/) and it is much more practical to purchase them then make them in most cases.
When testing the pyranometer we noticed that the 4-20 mA signal would drop off around the 12 or 13 mA mark and saturate. The WEL would never receive the full 20 mA. We ran numerous tests with our equipment and determined that the problem was within the WEL itself. We spoke to the manufacturer of the WEL and he explained the problem and how to fix it. It requires some cutting to fix it. There are two zener diodes inside the WEL that need to be snipped off. To do this you need to remove the rabbit board network chip by wiggling it and lifting it at the same time. It is fairly snug so you will need to work at it to get it off. Once it is off you will have access to the zener diodes and you can snip them out with some wire cutters. You will see them next to the green 4-20 mA plug on the WEL sandwiched between two resistors. I took a picture of our board with them removed below and have identified where they used to be with a red circle. Once we removed the zener diodes it worked perfectly.
You need to scale the numbers in order to get the proper engineering units. We used the formula f(x)=78.125x-312.5 to convert the 4-20 mA signal into watts per meter squared. This was specific to the equipment we used so if you use a different pyranometer you will need to do the math yourself.
Pyranometers are important for research because they allow you to see how solar panel output changes with solar intensity. We are particularly interested in how solar performs in the winter time in our climate. It is cold but often the sun is shining. With a pyranometer in our system we will be able to draw more meaningful conclusions from our data.