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IR Thermometer for roasting - emissivity of coffee beans?

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  • IR Thermometer for roasting - emissivity of coffee beans?

    I've been using an IR Thermometer for roasting for a while, as suggested by several people on this forum. It gives reasonable repeatability for a cheap gadget, no more than 2-3°C difference on repeat sampling. The main issue is accuracy - 1C starts at 220°C and 2C at >240°C depending on the bean, which clearly doesn't correspond with the usual parameters. The IR thermometer is one of those where the emissivity can be set, it comes pre-set to 0.95, so I suspect this is the issue. I can't find a reference for the emissivity of coffee beans anywhere though. The emissivity for wood is 0.7 - which seems to be as close as I can get. Any suggestions?

  • #2
    My suggestion would be to use the CS USB Datalogger with probe CoffeeSnobs - BeanBay

    The IR non-contact method just won't work as well as a contact one with logging and live graphing using our free software.

    Not much help to your actual question though.

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    • #3
      Thanks Andy, I explored the thermocouple/datalogger option a while ago and general consensus was it wouldn't work with the "brikkie's mixer" style of roaster I've built. The other issue would be the Mac compatibility - I'd have to run a microsoft parallel on the Mac.

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      • #4
        Originally posted by habahabanero View Post
        Thanks Andy, I explored the thermocouple/datalogger option a while ago and general consensus was it wouldn't work with the "brikkie's mixer" style of roaster I've built. The other issue would be the Mac compatibility - I'd have to run a microsoft parallel on the Mac.
        You might want to try Roast Logger. It's also free to download and use and is multi-platform compatible... including Apple OS-X w/64 bit processors. You can easily find it via Google

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        • #5
          Hi

          Are you measuring through glass? That will effect your result depending on what how your IR measurement unit works (IR diode or bolometer etc) and what wavelength (short or long) is being used. I'd suggest manually calibrating your device by using a thermocouple in the bean mass sorta like this:
          Code:
          your IR detector looks 
          at pan trough glass 
             |
            \/
          
          ----------- glass plate
                  /------------------thermocouple in pan  
          \______/_/ pan
           ^^^^^
           heat
          Mike

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          • #6
            Hi Mike, I'd thought of calibrating the IR thermometer in a similar way, but the drum is a rotating billycan over an open gas burner. It has its benefits. The drum is open to allow for direct visualisation of the beans, which makes the IR thermometer useful. Due to the extreme heat spilling past the drum getting a thermocouple into the beans is unlikely without frying it - I've tried and got my hand singed in the process. There is also the issue of a constant torrent of beans hitting the probe.

            The roaster was built on the cheap, and gives me excellent bang for my buck. I posted some pics in a previous thread, http://coffeesnobs.com.au/roasters/3...y-roaster.html when I initially explored the thermocouple option. I've no doubt that I could overcome the challenges of installing a thermocouple with some industrial, fire retardant insulation around an angle poise type arrangement to keep it in position, but it will be an inelegant and costly solution, and I'd rather stick with the IR Thermometer and save the money to eventually buy a Strega.

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            • #7
              Re: IR Thermometer for roasting - emissivity of coffee beans?

              I wonder if the open flame might affect the reading?

              Petroleum production facilities use IR flame detection, and one of the drawbacks is false triggers from things such as sunlight reflecting off pipework...

              Different kettle of fish, but who knows?

              I also wonder if the emissivity might differ as the beans roast?

              Failing that, perhaps you could calibrate it in someone else's roaster who has a thermocouple?

              In any case, repeatability is ultimately probably more important than accuracy.

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              • #8
                Hi habahabanero

                Having seen the link to your roaster I can see why you are using IR. Neat roaster, very innovative. What Mr Jack says is spot on - just roast at a temp that gives good results and use the IR temp to make that reproducible. Doesn't matter if the reading is not too accurate. I'd Good luck saving for a Stega :-) wow.

                Mike

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