mycuppa wrote:
When monitoring your roast in the early stages you need to look at your turning point - the moment when the rate of temp drop has "bottomed out" and the beans start to heat up and your initial ramp is underway. Turning point (TP) ranges vary depending upon the roasting device, charge temp, bean mass & type, energy source (heat) and the environmental conditions such as ambient temp of beans, etc. For roast devices that use some forms of conduction and convection, e.g. a drum, etc. you should be trying to look for a turning point between 2:30 and at perhaps 3:00 mins. I find TP calcs a highly valuable indicator on whether I've worked out the correct ratios of green kg's, charge temp, energy applied, drum speed and airflow. So important is this for me that when roasting a new bean I will do some very quick sums in my head based on experience and try to run ahead of the profile curve to predict (or visualize) how the bean will behave in the chamber during the execution of the roast. It's a particularly fun game for those times when you are shackled to the roaster for 11hrs straight ???.
Since this thread had some recent activity, thought it'd be ok to comment.... The biggest influence on "Turning Point" is actually temperature probe. Think about it - the beans are at 20*C for example, ambient temp. The roaster temp probe says it's 200*C in the roaster. If the probe had no thermal mass and accurately measured the bean's surface temp, as soon as you loaded the beans in the roaster it'd register 20*C. But the temp probe's thermal mass and the fact that it's not measuring the bean temp, just the temp around the beans, means that the temp shows a decreasing trend - the beans are sucking a bit of heat out of the probe over time. A low thermal mass probe (eg thin), it sucks heat out faster, a higher thermal mass probe sucks less heat out so takes longer to get down to the same point. So now we have two probes that show a different "apparent decreasing temperature" rate or slope of the graph. You have beans that are taking on the heat in the roaster, and are coming up in temperature no matter what the probe reading says. At some point, you have these two factors coming together, which forms the turning point on the graph - but nothing, besides the reading, is "turning"; the beans have been increasing in temp all the way along. This is why comparing my turning point and your turning point is yet another "pointless" thing to do (pun intended). I do agree that all the other factors mentioned above like energy input and green weight and charge temperature, that influence the apparent TP on my roaster help me understand my roaster and improve my roasts, but the time it takes for me to reach TP is not transferrable to your roaster or your probe location or your probe thermal mass. The most important point is that TP is not real - it's just a representation of what is going on when measuring beans through a medium that doesn't transfer heat well.
