I have been using the Whirley Pop and had steady success, however I started to notice that the latest roasts were starting to roast unevenly, and the same temperature settings were causing some roasts to jump from first crack to second too quickly.
Several roasts that were targeted for Full City, were skipping from first to second crack too quickly, within 30-45 seconds of each, actually. A small portion of the beans were hitting first crack much earlier than the bulk of the beans, and while the majority of the beans were still around City+ they were jumping ahead to second crack. This was causing some problems with flavor, as some were actually overcooked at the end of the roast. All signs were pointing to a heating issue.
After some research, I came to the conclusion that the burn-in on the kettle was affecting the temperature response. Given that the roast size had not changed, I switched to using a lower temperature setting, which had 2 advantages. It allowed me to re-calibrate the roasting times, and also increased the time required to reach first crack after the beans were added. The additional time allowed the beans to roast more evenly, and with the new calibration, I was able to adjust my roast times to be more consistent.
Along with the lower temperature, I also switched to roasting with all the movable doors of the Whirley Pop closed, so I was unable to view the beans as they roasted, however, I was able to smell and hear the beans as they progressed through first/second crack. This had the added benefit of reducing the amount of smoke released, but still allowed me to gauge where in the roast the beans were. I also discovered that opening the lid resulted in a 50 degree drop in the ambient air temperature inside the kettle. This allowed me to draw out the time between first and second crack merely by opening the lid, which gave me more time to calibrate for City, Full City, and Fully City+ roast levels. Normally, given the variations in heat application via a gas stove, it was difficult to gauge a City/City+ roast level, but with the adjustments made, I have been able to achieve a consistent, even City roast!
During my research on this issue, I ran across several different roast monitoring applications, and have since chosen the RoastLogger application. It’s ability to use different heat gauge sources, and being written in Java allows for more compatibility across operating systems. It will track heat over time, measuring down to 3 second intervals, if necessary. This will allow me to create much more detailed roast profiles for my favorite beans, as well as generate nice graphs of the roast temperatures with respect to time, including graph milestones, such as first/second crack. I have already ordered a compatible multimeter to use with the application, and once I have the full configuration complete, it will definitely merit its own post, so stay tuned!