by Doug Coulter » Wed Jan 07, 2015 12:32 pm
Well, I've had some practice at that for an earlier system I had. Hot water is harder, in particular if the sensors have any contact with the water/steam. For one thing, there are zero reliable solid state sensors for the temperatures you'll be seeing, they will all die quite young (sometimes the first day). You therefore must use things like thermocouples and interface to them in some way (tiny millivolt signals). That said, it's definitely do-able, something off the shelf like an Arduino Uno can easily control some sensors and possibly some valves/motors etc with the right code. It will do a lousy job of logging (they have a SD shield and a kind-of file system but very limited) so in general you interface the thing with either a PC or something inexpensive like a raspberry pi for logging and any heavy lifting math-wise. The strategy here is to use the Arduino (or other small uP without a fancy opsys) to do anything time-critical, and buffer a little data for a larger machine that isn't real-time good (due to a pre-emptive multitasking opsys) but is good in other ways, while in our case...not using a lot of power. I'm testing a system right now that uses an uno and some solid state sensors for the other extreme - is my plumbing going to freeze...how's room temperature in a remote building on campus where the water system lives? I'll put up some plots as soon as I write the plotting software...
Right now, I'm using a raspberry pi with a wireless dongle to log data, and using adafruit's 10.1" LCD/LED display, and an arduino interfaced to that which actually collects the data, which is put in a file in a shared directory on the pi. The whole thing draws perhaps <10w, haven't measured it in detail yet, as I plan to run it off my 13.5v ups (separate from the house batteries) eventually, with tiny switching supplies which will cut the waste from the way I'm doing it right now - wall warts and bricks from the inverter power off the main system. I built a little UPS that keeps a deep cycle 12v battery up for just this kind of thing - it will run stuff like this for days even if the main house power goes out (it's a big trolling/deep-cycle battery with a temperature compensated charger that runs off a big 15v power supply used to run my water pump already in existence). Obviously, your details will vary.
Don't believe the high end of the temperature ratings on these solid state sensors, they are assuming a very short life at those numbers, and I've even had thermistors just plain fail. Depending on your system, your collectors might get VERY hot if there's no water circulating through them as well. If there's any steam blow-off anywhere it will eat wires, much less anything "delicate" so you even wind up encapsulating the thermocouples....Thank heavens I learned all that back in the day, nowadays it'd be a lot easier going in - uP's are cheap, simple, sensors are better, and so on - and I know what to not use, most important of all.
In your case you might not need/want super high time resolution and get by with just a raspi and some interfaces and of course, the usual UI stuff (keyboard, display etc) even if those in parens are only used initially to get it working. Right now, mine isn't fancy enough to do things like roll over logs, I'm forced to do that manually over the network and in person at the pi, but I plan to add code to do that sort of thing once I no longer care how it worked last year etc - else the sheer amount of data one collects simply won't fit on things like a small single board computer. Not sure where I'm heading vs details, but in general I'm trying to set up whole-campus data aq and storage on a lot of parameters around campus, solar stuff, weather stuff, temps and humidities in various rooms and crawl spaces, woodstove monitoring (we have actual !Winter! here - 3f + 25 mph winds tonight...), water collection system details and control...and so on. Some of that needs "right now" control, as in "the cistern is going to overflow, or the next flush is going to empty it" kinds of issues, or the inverters are going to cut out due to low battery any second now, and quicker if you cook a glass of tea in the microwave instead of on the stove top, so I'll be using all the tools in the box. And at least one always-on display and a beeper to alert me of any troubles...
The point being, I'm not getting any younger, and the more I can automate before I get too old and weak to do it by hand, the better, and the more time/effort I can save myself running off-grid, the more time I have for playing with physics and generally having fun.
I can give more focused input if I know more about the system you want to monitor and control, of course. Mine was a little off the wall as those things go and had it's own unique set of issues. It was pretty much useless in winter, and had high maintenance issues, so I ditched it in the end, but you are unlikely to have the same issues and situation.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.