Solar Thermal Tube testing

I have 40 solar tubes on the roof of my house. I’ve always wondered how well each tube was functioning. I wrote a monitoring system built around Zigbee for my company, Bejouled. I wanted to see if I could build an open-source monitor or even a full controller around the Arduino platform and the cheaper RF12B wireless modules.

I took one of the new Ruby v1.0b boards and connected it up to a simple PCB I made for one-wire devices. Actually, I connected to a stack of four of them. It looks like this:

That is 20 DS18B20 temperature sensors with 2m of cabling connected up to 4 of my little one-wire bus PCBs.

The head unit looks like this:

As you can see, it is a simple board that allows you to build a linear network of 1-wire devices. At each end are simple 0.1″ pin holes to make spacers between the boards and to provide some rigidity when they are stacked as they are above. An interesting side note, I had a 4.7k resistor for the pull-up for the one-wire network. It worked fine for 10 sensors. When I stacked on the next 5, it didn’t work at all. As suggested here, I changed it to a 2k resistor and now all is good. Cleanly reads all 20 sensors, once per minute and sends the data to the net.

Incidentally, the DS18B20 chips are epoxied into a stainless steel hood to make them waterproof. They are sweet:

I have tweaked my code and am now receiving all the data. I already knew that there was some variability between tubes, but it can be > 10°F which was surprising. Here is an early graph of this afternoon.

Not too surprising, the first and last tube in the manifold get colder as they are near an opening in the manifold. When we get a nice sunny day, it will be interesting to track how the temperatures sync to the irradiance as well as to each other. Stay tuned…