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Green Building Bible, Fourth Edition
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  1.  
    Hello GBF. I'd like to wire up some low cost temperature probes. The main reason is to check that the rainwater stored in the roof stays cool enough not to grow Legionella. I'd like to wire it up to something simple like a Raspberry Pi. In the future I might consider using them to monitor the temperature in the house.

    Does anybody have any experience of this kind of project, and suggestions for the type of sensor that might be a good idea?
    • CommentAuthorEd Davies
    • CommentTimeJan 28th 2016
     
    A very common way to do this sort of thing is using DS18B20 1-wire [¹] temperature probes. They're cheap and give a reasonably stable output and work over longish length cables so long as you're moderately sensible about not having too many connections, etc. There are a lot of tutorials and so on around the net with a variety of ways to connect to them - directly bit-banging a Raspberry Pi GPIO pin or using a USB adapter are typical.

    [¹] so called because they typically use three wires :wink:
  2.  
    Thanks Ed! Is this the sort of probe that you could also bury in your floor to measure the temperature of the underfloor heating?
    • CommentAuthorEd Davies
    • CommentTimeJan 28th 2016 edited
     
    You could but you'd need to encapsulate the wires well - the actual device looks like a little plastic transistor with three wires coming out the bottom. Better perhaps to bury a bit of plastic pipe in a discrete corner or something to allow the probe to be pushed in with some insulation stuffed in behind.
    •  
      CommentAuthordjh
    • CommentTimeJan 28th 2016
     
    Or you can buy the sensors already encapsulated. But Ed's technique is probably better, because the sensors sometimes fail, so you either need to be able to replace them or else put two in from the start.
    • CommentAuthorskyewright
    • CommentTimeJan 29th 2016
     
    +1 for embedding a pipe to allow replacement. I've used 1-wire sensors in various remote places (e.g. soil temp & under the house floor) and even with the sensor and head of the lead embedded in epoxy I've seen a high percentage of sensor failures after a few years.
    • CommentAuthorsnyggapa
    • CommentTimeJan 29th 2016
     
    • CommentAuthorborpin
    • CommentTimeJan 31st 2016 edited
     
    If you want something off the shelf http://openenergymonitor.org/emon/ has a solution. Designed for energy monitoring it will easily capture any one wire sensors. It is relatively expensive but is a solution in a box as it were.
    •  
      CommentAuthorSteamyTea
    • CommentTimeFeb 1st 2016
     
    I used the DS18B20 attached to an RPi.
    Works well, is simple to set up (was one of my first ever attempts at writing code).
    I too have had sensor failures, they just don't seem to like the wet at all.
    I also use DHT22 RH and Temp sensors, they are much better but cannot be anywhere damp for long (had two failures in a year in my roof space).
  3.  
    Thanks everyone! This is really helpful. I'll have to be careful to get the right sensor as it'll be in water. Presumably the off-the-shelf waterproof probes will be best. At least they would be easy to replace (unlike one buried in the floor).
  4.  
    Hi @steamy, did you wire your sensors in series? If so, could you share the diagram please?
    •  
      CommentAuthorSteamyTea
    • CommentTimeJun 6th 2017
     
    The DS18B20's are wired in parallel. They even share the resistor.
    https://cdn-learn.adafruit.com/downloads/pdf/adafruits-raspberry-pi-lesson-11-ds18b20-temperature-sensing.pdf

    The DHT22's are wired seperately, though you can share the power and the ground. They each need their own resistor and a dedicated GPIO pin.
    http://www.instructables.com/id/Raspberry-PI-and-DHT22-temperature-and-humidity-lo/
  5.  
    Thanks @steamy! It's a bit confusing that they are wired in parallel. The 1-wire description implies in series.

    I've wired 6 waterproof sensors and will buy them in the screed. No time to test them, so I hope I can work out which is which in the future!
    •  
      CommentAuthorSteamyTea
    • CommentTimeJun 7th 2017
     
    You can work out which one is which if you wire them up individually at first and note the ID number, you need that number anyway if you are wiring them direst to an RPi.

    The parallel/series terminology is a bit strange, but I think that parallel is the right term as the data comes down just one wire and does not need to pass though other sensors.
      Wiring.jpg
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