Since I've had my Ambex the temperature readings never seemed right to me.
1st crack beginning was almost always between 385 - 390 degrees F
2nd crack beginning was at 412 - 416
These readings were very consistent. However, the issue I see with this is there is only a measured maximum difference of 31 degrees between the beginning of 1st and the beginning of 2nd. So it's very difficult to judge if If I was at C+ vs FC except for looking at the beans. I wasn't happy with the minor inconsistencies I've been getting and assumed I was actually measuring the environment temperature near the top of the bean mass (possibly an occasional bean or two coming in contact with the probe)
Back in my sonofresco days I had a temperature probe and measured temperatures closer to:
1st crack (which I could hear) started at about 400 - 415
and I'd cool at about 438 - 450 depending on the bean
George Steinert's Degree of Roast/Temperature chart:
Degree of Roast Temp
Green Unroasted 75
Starting to pale 270
Early yellow 327
Yellow-Tan 345
Light Brown 370
Brown 393
1st Crack Begins 401
1st Crack Under Way 415
City Roast 426
City+ 435
Full City 446
Full City+ 454
Vienna (Light French) 465
Full French 474
Fully Carbonized 486
Immanent Fire 497 So at Coffee Fest I spoke to Terry from Ambex about it and he said I should be reading a higher temperature and he recommended replacing the thermocouple, making sure I get the probe into the bean mass.
I replaced it with the 4" probe Ambex sells and got the probe placed a low as I could. My readings went up about 2 degrees. I still wasn't happy so I started doing some research on probe placement and looking at it more from an engineering standpoint. I came across
a good article...... on the Ambex siteIn trying to estimate how deep the beans would be in my drum I determined that a 4" probe is not long enough to get through the compression fitting in the wall of the roaster, plus the bend radius and the down near the stir veins as per the above image. In fact with the new probe I was a good 2" above the stir veins. So I found (in my goodie box) a 6" probe. I installed it and got the probe tip approximately 1/2" above the stir veins, checked for interference and then roasted a batch. The results were what I'd hoped for.
1st Crack began 401
2nd Crack began 437
I may try to get the probe even closer so I can get more consistent readings with only 1 lb of beans in the drum. The 6" probe gives me much more flexibility in probe placement.
You don't need to buy the probe from Ambex. Any J or K or T type, 1/8" diameter probe will work. Just make sure you change the PID if you change from a J type to a K or a T type.
I'll post photos in the next week or so, I didn't get around to taking any last night.