Our IBTS temp has been dipping below my bean temp towards the ends of our roasts, at first we were concerned and cleaned off our IBTS probe and saw it continuing to happen so we didn’t think too much of it at that point. Today I had an abnormal roast where my coffee was taking on a lot more heat than normal, but the IBTS temp had dropped below the bean temp significantly earlier. We normally rely on IBTS for our end temp but out of fear over roasting too dark I dropped it early at what I felt would be acceptable.
Does anyone have insight as to why this is happening and whether we should continue relying on IBTS or if we should use bean probe as our primary.
have same issue
My 2 cents as a 1kg roaster. This does happen when you roast 800g to 1kg batches. With this batch size we also tend to start with higher PH temps as well. IBTS and BT temps will run a lot closer to each other at big batch size and I would not worry too much. In my 1kg batches I roast the IBTS will either overlap with BT or cross BT right around FC. Sometimes the BT will be a bit above IBTS after FC depending how dirty my IBTS is. When roasting large batches as the beans expand the BT is in more contact with the beans to give you better reading of the bean. I clean my IBTS when the BT line is a bit more above (more so than what you’re showing) the IBTS (or every 25 to 30 batches).
I agree with what @Blacklabs shared above.
If you feel that one of the roasts was abnormal and was adding more power than normal, I would go to the analyzer in Roast World and overlay a good previous roast and the roast that felt like too much power and look at the BEAN curve and see if they are the same. (This is assuming that it is the same bean, same recipe or playback or that you manually followed the know good roast)
Whenever I run large batches and the overlap happens, I will normally use the bean temp for determining my drop temp. I feel that the immersion of the Bean probe into the bean mass is better and the bean probe, although slower to react to temperature changes is less susceptable to a dirty lens error which is possible with the IBTS.
Also the lens of the IBTS seems to get dirty faster when roasting darker. More smoke, more smutz.
Completely agree with @billc on that point. I too will switch to using the BT temps once I reach FC with my 1kg batches when/how I adjust F and P setting after FC is based on BT temps - as well as final drop temp. I found I get more consistent results this way with the 1kg roasts.
To add a little to what @billc & @blacklabs said above, if the IR sensor lens (the clear glass in front of the IR sensor) is clean (!!), I would expect IBTS to be greater than B-Temp until near the end of the roast. At the end I would expect them to draw very close together, maybe even superimpose in some cases depending upon calibration of the instrumentation. While differences in calibration between the IBTS & bean probe may affect the relative position of the 2 curves as the roast progresses, those differences in calibration won’t affect the shape of the curves of those 2 measurements.
Difference between I-Temp and B-Temp is affected by batch size and calibration of the sensors. Except for a rare few, we users aren’t going to have the resources to re-calibrate e.g. appropriate meters & simulation sources plus reference standards traceable (in the US) to the NBS to establish measurement baselines (this struck home when I had to replace the bean probe).
So (imo) it becomes a case of learning your roaster’s behavior so you can repeat a good roast profile by whatever means you arrive at it. It’s all about the coffee and less about producing a perfect graph with coffee that doesn’t quite hit the mark. I guess this is a convoluted way of saying it matters less about the graph as long as you can reproduce a previous graph of something you like on the Bullet you use to roast.
Bruce