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  • Thermocouple placement in Corretto

    I have done three roasts with my BM/Heat gun and I dont seem to be getting accurate readings of the tempurature on my DMM. In the three roasts done I have reached FC at 184/162/150 degrees. The first one may have been second crack because I unloaded the beans shortly after as they were starting to get very dark. Either way it still seams very low for FC.

    My questions are: Which type of thermocouple is better, the probe or the flexible one with a bead on the end? I have the second one. Best place to put it in an unmodified BM (still used for bread)? I currently have it attached to a metal pin and position it in the opposite corner of the pan as to where the heat gun is aimed and at the bottom of the pan. I only roasted 250g in each of the roasts if that makes a difference. Thanks!

  • #2
    Re: Thermocouple placement in Corretto

    Placement sounds OK.
    I use the bead type and did the same with no problem.

    How far from the bottom of the pan does the bead sit?
    Were the beans the same type each time?

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    • #3
      Re: Thermocouple placement in Corretto

      The bead on the first two moved around a bit along the side but on the third one it was pretty much stable at 0.5cm from the bottom of the pan. Is it possible to put this type of thermocouple in water. I was thinking about testing the DMM by putting it in boiling water and see what it reads.

      The beans were three different one from the starter pack.

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      • #4
        Re: Thermocouple placement in Corretto

        Originally posted by Obie link=1219689837/0#2 date=1219718871
        Is it possible to put this type of thermocouple in water.
        Yes you can. Ive used one of these to measure the water temperature in the group head and did exactly that, tested it in boiling water first.

        This is how I my hold my t/c

        http://coffeesnobs.com.au/YaBB.pl?num=1216735141/17#17

        Cheers

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        • #5
          Re: Thermocouple placement in Corretto

          Testing is a good idea and even though the 2nd and 3rd readings seem way out, you dont know raelly by how much because they were 3 different beans.

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          • #6
            Re: Thermocouple placement in Corretto

            We too like to bake bread in our BM, so TC has to be hooked over the top -- placement sounds much like yours.

            However, the 1st time we tried this the TC was very close to the side of the pan, and read low (FC at ~185*C indicated reading). One suggestion as to the reason for this was that in our efforts to make sure the bead was indeed in the beans, we ended up with it touching, or very close to, the metal of the pan, so causing it to read "cool".

            Weve since improved the setup by sliding the TC lead plus a bit of fencing wire down some fibreglass sleeving, tying the lot together with some copper wire (otherwise the fibreglass frays away into your beans) then bending the wire into an artful "U" shape to suit, and bent up at the end to get away from the pan. Seems to work OK -- FC now occurring at readings of ~200-205*C (this is a different variety from the 1st batch tho, so maybe not strictly comparable)

            Also roasting larger batches might help -- more beans to cover the bead without getting too close to the pan

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            • #7
              Re: Thermocouple placement in Corretto

              Cant speak for the corretto as I use a mixing bowl and spoon ... but I use a bead type thermocouple, and have also had inconsistent and not quite accurate readings (FC ranging from 165 to 185).

              The small bean volume (relative to pan size) could be a factor, as it has been for my 230g roasts. I tried a probe type thermocouple but that was even less consistent, probably because of the amount of the probe not covered by beans. At least with the beads you can pinpoint the spot you measure temp from, as long as you can fix the thing in place.

              My t/c bead is inserted up through a hole in the bowls bottom, and now that Ive got it fixed at a good height (by lots of trial and error) the temps are consistent - FC at about 185, SC at 205. I know the true temps should be higher, but the main thing is for the readings to be consistent enough so you know what to expect. If my probe says 170, I know Im 15 away from FC and Im happy with that.

              Comment


              • #8
                Re: Thermocouple placement in Corretto

                Did another roast yesterday and it seemed a little better. I did roast 500g instead of 250g so I think the bead was in the bean mass more. I heard FC at 195C which seems more like normal..

                Now all I have to do is not panic that I am going to burn the beans and let them get a little closer to SC..... Unloaded a little early yesterday. :-[

                Thanks for all the help and suggestions.

                Obie

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                • #9
                  Re: Thermocouple placement in Corretto

                  Hi all,

                  Just thought Id describe a problem we had with a less-than-ideal thermocouple (TC) placement. We have a bead-type TC which we tape onto the breadmaker so that it projects into the opposite corner to where the heat gun (HG) is aimed. It sits just ABOVE the beans (ie. so that it gets bumped by the bean mass every rotation). We now realise that this is not ideal as it is measuring the air temperature, not the bean temperature.

                  Like Oily said, temp readings are really only useful for comparison with YOUR other roasts (not other peoples temps). I would add another caveat to that, which is that you need to have your Corretto and TC set up the same way every time for the readings to be comparable. Heres why:
                  - Our original setup had a fan blowing over the Corretto (aimed at the HG, but blowing over the bread maker bowl too). With this we got first crack (FC) around 200-210ºC.
                  - We then removed the fan (thought it was taking too much heat away from the bowl, causing us to move the HG closer, which in turn was causing tipping/scorching) and this changed FC temps to 230-240ºC (air temp, remember).
                  - Now, were using a foil "lid" to keep more heat in the bowl (so that we can apply less heat) and we get FC anywhere between 260-280ºC. [Our last roast had FC @ 268ºC and second crack (SC) @ 244ºC - as were measuring AIR temp, these weird temp readings actually nicely illustrate the slow-down after FC in order to delay the onset of SC.]


                  I guess my point is that if you want your temp readings to make any sense at all, you need to measure it consistently! And dont bother looking at other peoples temperatures unless your setup is similar - if I tried to get FC to happen at a reading of 205ºC on our Corretto, it would be 30 minutes before I got there! So while I was thinking of setting up a clip so that our TC was within the beans, as long as I understand why our temperatures are so different, Ill be fine. [As an aside, our only roasting problem has been a little bit of tipping - burnt(ish) edges of the beans caused by increasing the heat too quickly. Our set up should be ideal for eliminating this problem since we can know how hot the roasting bowl is at any time - and try not to let it get too much hotter than the beans.]

                  HTH
                  Cheers
                  Stuart.

                  Comment


                  • #10
                    Re: Thermocouple placement in Corretto

                    One early method mine for bead placement was to tie it (with wire) to the handle of a wooden spoon.
                    This kept it away from the pan side and always in the same position with the ability to change the height of its location if I did a double sized batch.
                    The spoon handle I figured wouldnt soak up heat like my original metal tube did and radiate it towards the bead (I dont know that it did this but changed to play safe).

                    The lead was threaded through heat resistant fibreglass tubing.

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