Electronics → Re: A Simple Solar DHW Differential Controller Project.

The four I used came out as +.2, +.1, -.2, -1.1, degrees off of the oil bath temperature.

Now once encapsulated with epoxy into the probes they are all reading a tad bit higher due to self heating, but the amount of deviation from the actual temperature did not change.

I'll admit that I did not test to this level, my application was not that critical. I just put them on the bench and compared to ambient. I imagine if I was doing something really critical I would want to compare them in 2 different solutions to also verify tracking.

I found the easy way to test self heating is to be all set up, and then apply power to the device and watch the rise of the temperature reading. If they were somewhat isolated it was not uncommon to see a 3-4 deg F rise. Small slip on heat sinks greatly reduce the self heating.

My primary concern was not accuracy, but to ensure I didn’t have an LM34 that read high up in the panel, versus one that read lower in the storage tank, as that would cause the pump to circulate heated water into the panel, when in fact the panel was at a temperature lower than storage.

I actually pulled the 1 meg resistors out of the design as they caused too much hysteresis, the “semi-matched” potted LM34’s worked out to have a 5 degrees offset which is perfect for this application.

It’s crude and simple but will last forever.