So now that I have a faster responding thermocouple I’m finding that the sensor works better closer to the edges of the roaster. When thinking of the bean mass on the SR300 and SR500 you probably need to consider the bean mass as if it is a donut at the bottom of the chamber. You want to get the tip of the probe centered in the middle of the donut and not the middle of the chamber.
Most modifications people have shown put the probe drilled straight down the middle. This has the advantage of clearing the “chaff grate” on the sides down a natural hole in the roaster. The problem with this is that the very center is actually a sort of “fountain” of beans and hot air. The probe has very little contact with any of the roasted beans and is mostly picking up the air put out by the roaster.
Many people who have sensored this middle area report temperatures in the 420 to 425 degree range. At the very end all of the beans are very fluid and filling almost the entire chamber area moving freely. Once this finally happens the temperature readings then reach the 440-450 range showing a typical second crack area that coincides with actual second crack occuring. Often second crack ACTUALLY happens while the probe is showing 400 degrees when placed in the middle. This is likely due to the bean mass thermic reaction being hotter than the air rushing up from below and only a few minutes later does the air column catch up.
I’m starting to get low on beans from the samplers with only a pound or two left of all the varieties. I’m feeling that the probe placement I used initially in the middle helped stall a few roasts due to inaccurate temperatures of the bean mass that I responded to. I’m testing a roast where the probe was moved closer to the “donut” range to see what sort of tastes develop but this one is pretty close to a standard roast.
My feelings are there is a “donut” sweet spot. In the diagram below there is a Orange region [1] that is the normal plume of bean being pulled from the middle area on the bottom and [2] tossed up on top to the left and right. As you descend into that area you enter part of the sweet spot [Green]. In the far left and right corners [Purple] you have a “burn” zone due to very low circulation of the beans. This is caused by the more central area of the donut [3] falling towards the middle and then being jetted to the top [1] repeating the cycle. Only when the beans have sufficiently roasted does the bean mass begin to rise above the initial [Yellow] fill line. As the mass makes it above the silver metalic band there is enough movement to cause beans to leave the “Burn Zone” and make it easily into the column. This initial burn area is where the stray first cracks are likely to originate and the uneven roast occurs.
Freshroast "Sweet Spot" created in roasting chamber.
I would thus interpret the space where the D on the left donut and the t on the right donut would be the approximate ideal locations to sample bean mass temperatures due to the piling on of beans from the fountain [1] above it.