Roasting with Ramp/Soak PID Controller

Since December ’09 I’ve been using Auber Instrument’s SYL-2352P controller on my coffee roaster.  This type of controller lets you program in your desired temperature at any point in time.  I’ve settled on a roast profile that seems to work well enough for the varieties that I roast.  I’m sure that I ought to have different profiles for each bean variety but I haven’t had any really bad results yet.

Here is the program, which will make sense if you’re familiar with the syntax:

Step Value
C01 85° F
T01 1 min
C02 85° F
T02 10 min
C03 330° F
T03 5 min
C04 406° F
T04 3 min
C05 406° F
T05 2 min
C06 430° F
T06 0 (Hold)

And to better illustrate, here is a graph of the same thing:

The primary driver for the evolution of my profile was avoiding tipping.  My first profile was to simply heat the beans to 405 for 12 minutes, but I found that the heat gun had to stay on too much causing somewhat scorched beans (tipping) and/or dark chaff.  This made me lengthen the roast and to add the shallower 330 -> 406 phase.  The second was was added because during the high 300’s the roaster seems to require more heat for the same increases as compared to lower temps.  I didn’t like how the heat gun’s duty cycle increased during this period so I made its slope more shallow.  Now the chaff from my roasts is consistently medium-to-light in color, as seen in the accumulation in my “chaff bucket” at the bottom of the post.

The 1-minute 85 degree period at the start is a pre-heat phase while the 16 -> 19 minute period is the post-1C “bake” that I’ve read about on the Coffee Snobs forums.  And finally, the last phase is the ramp to 2C followed by a hold @ 430.  I often don’t make it to 430 because 2C will happen earlier.

What a few months of chaff looks like