Temperature Sensors in a Solar Thermal Mass home - requesting advice.

Hello there everybody!

This summer I will be helping friends build a Solar Thermal Mass home (We're actually building something called an Earthship) up in the prairies of Manitoba Canada (building just north of Winnipeg). We have quite harsh winters here, and I want to track the performance of the building and plot it against outdoor temps.

When this home is done the entire North side will be "bermed" - essentially it's going to be buried into the ground. An articifical hill will be built up to the back of the house to raise the frost line and assist with storage of thermal energy. I'd like to have sensors in place at the bottom of this berm. I'd also like to place some sensors in some concrete slab floors, and maybe some other locations. The thought was to install these sensors on some kind of wire. Was thinking maybe using cat5 or something similar gauge but not 8 conductors.

I have played around with arduino stuff for a couple years now - nothing super advanced, but a decent amount of tinkering. I'm not worried about the coding side and all of that at this point. I'll figure that out later. I am just wanting to get the sensors in place and then worry about building the actual home, the rest comes later.

My question, or areas I'm looking for advice are:

  1. The only temp sensors I've worked with to this point is the tmp36. Is there something out there that is going to give me better performance? More accuracy? I'd like to keep this pretty cheap, and will potentially be installing a half dozen sensors throughout the entire build - who knows, maybe more!

  2. How long of a run can I run a sensor like this off? If I had this soldered onto the end of a 40 foot piece of cat5 would I get a proper reading? How about 100 feet?

  3. How would you suggest to package up the sensor? Not thinking that simply soldering lines on, shrink tubing, and then submersing the line in concrete would be good enough - but maybe it would? Anyone have any thoughts on this? I'll also be looking to essentially bury some of these underground as well (in the berm).

  4. Might be interesting to test soil moisture as well, just for kicks. See what the soil moisture is and how that compares to the soil temp. Probably not really needed and just would push cost/complexity of the overall project up.

I'd be grateful to any advice anyone can give me on any of these points - feel free also to mention any concerns/suggestions/etc if there is something that you think I've overlooked.

Lastly - if you are interested in more details about the Earthship build going on please don't hesitate to get in touch with me and I would be more than happy to explain anything and answers questions you might have!

Bill

Hi Bill

Sounds like a very interesting project. Your environmental conditions and ours are polar opposites, so I don't have any experience with "frost line". We have put a couple of DS18B20 digital temperature probes up on roofs at a similar distance (30m in one case) from the Arduino with success. They have a slightly broader range of temperature sensing (http://datasheets.maxim-ic.com/en/ds/DS18B20.pdf) to the TMP36 and are digital, so potentially are going to be more resilient to electrical interference if there's any of that from the household supply. I used a LM35DZ (similar to TMP36) mounted on a boat, and found it needed to be moved away from power sources, though a shielded cable would likely have eliminated that problem. Others with more experience in that sort of thing will probably set us straight. I don't think it's capable of higher granularity, but then I'm happy to get +/- 0.5 degree readings over the available range and they deliver that.

One of the houses with a probe on the roof was put up there using 3 wires of a Cat5 cable, so I can confirm that works from experience. For mine I used 6-core burglar alarm cable since it's UV stabilised (again not a concern for you). Our Cat 5 will ultimately be re-run through conduit to save it from the elements. The DS18B20 is available as a sealed probe in a metal jacket - and that's what we used for the ones outside. (top listing here https://www.adafruit.com/index.php?main_page=advanced_search_result&search_in_description=1&keyword=ds18b20 is the one) Perhaps something like that would be successful buried alive, or concreted in as you're proposing. Certainly I can report (so far) they've survived 70C+ tiled roofs and torrential rain...not that this helps in your assessment particularly :slight_smile:

The other benefit of using something like the DS18B20 is that it only uses the one digital pin no matter how many you run (some caveats apply) whereas the TMP or LM style will need an analog pin for each, or an analog multiplexer if you go large scale.

I've not done anything with soil moisture testing so will leave that question for others.

All the best, Geoff

Thank you very much for the quick reply Geoff.

Ten bucks a piece is significantly more than I paid for handful of tmp36 unites I picked up for playing. But I guess it doesn't make sense to skimp on something that is so long term of a project. The size/cost of this temp tracker is basically nothing compared to the time/money/energy that will be put into the Earthship construction.

I'll probably go that route. Thanks Geoff.

If anyone else has input or suggestions I'd love to hear from others!

Hi Bill,

I should have pointed out we didn't pay anywhere near that much, inclusive of delivery. Shop around and you'll be surprised I think. We also only bought those for the rooftop ones. Those in enclosures are just the bare chips which are cheaply available from more generic components suppliers too.

Cheers ! Geoff

Yeah, I'll shop around for sure before I get to the point of ordering - just was noting the price listed there.

I'm inclined to agree about the 1-Wire sensors. Use 3 wires (data, power, ground) for maximum reliability. Test thoroughly before, ah, concreting them into place under a ton of rock. If you get suitable ones for outdoors, and take care with their installation, it should all work fine, and you only need 3 cores of wire to service a group of them.

If it was me and it was going to be impossible to get to them later I would be tempted to have two completely independent groups of sensors (different wire runs) just in case one goes bad.

Good point Nick - having a second set with second wire run and everything might be a good idea. They will for all intents be impossible to get to in the future.

There's a possibility of me mounting some LEDs in some glass bottles that will be cemented into walls (again - basically impossible to ever access in the future) and I was for sure already thinking that I'd like to run double lines to those to have a "spare" if something goes wrong or an LED dies somehow. It makes sense to do the same for temp sensors.

Hi,

I agree that DS18B20 "1-wire" sensors are best.. The cable length (within reason) is not a problem.

$5 waterproof versions in a stainless steel tube are widely used in Aquariums, Greenhouses etc. Example: http://goo.gl/1RR4T

How-To information on the sensors here: http://arduino-info.wikispaces.com/Brick-Temperature-DS18B20 (see the link to: "HOW-TO Connect several DS18B20 on same wires")

I would personally use jacketed shielded twisted pair cable for burial in soils or concrete. This is not a place to skimp. CAT5 (especially if it is high-quality plenum-rated) would be a second choice.

Splices to sensors I would do by putting 3/8 inch clear plastic tubing about 6 inches long over the splice, and injecting silicone compound in from both ends, alongside the cable, until you can see that it is well filled on both ends. I have splices like that in sensors buried and in the weather for over 12 years now and they are fine.

Another thing to think about is installing plastic conduit from a good location in the house to several potential monitoring points,. It's cheap, and you can also use flexible water tubing. You will never know what sensor or actuator you will want in 10 years or more. The future fibre-optic laser powered sensors for $5 haven't been invented yet, but if you have conduit in place, you can add them later.

Let us know about the progress of this, and what you decide to do...

DISCLAIMER: Mentioned stuff from my own shop...