knowledge of the equipment used, a high degree of technical excellence, continuous practice and an open mind. Accepting
technological change takes an open mind. To test and set our PIDs, we did extensive research on a 15-kilo Ambex coffee roaster retrofitted with the Profile Plus DCQ system. The equipment operates on natural gas, has a total exhaust length of 27 feet with (1) 90-degree angle at the base of the roasterand ending in a “no-loss” stackhead. All roasts were conducted in a hands-off manner (that is, once PID settings were made and the roast started, there was no human intervention). All
roasts were subsequently cupped for quality. All data (including all roasts conducted since October 1, 2004) have been kept
and are reviewed against subsequent changes in atmosphere, green coffee crop and periodic roaster maintenance. Much of the initial PID research was compiled by Paul Ribich for the SCAA’s upcoming Coffee Roaster’s Handbook. The graphs used in this article are actual roasts taken from the data log files and shown in the log reviewer format. Graphs 1 and 2 are actual examples of what happens when PI settings are incorrect. Graph 3 (page 60) is an example of correctly set PI values for a roasting process. The following graphs display three data lines: desired profile (set point) in blue, environment temperature path in orange and actual bean temperature profile in green (graph legends on right of graph). In Graph 1, it is easy to see what occurs when a P value is too large. The overly aggressive proportional function causes theactual temperature (bean temperature) or green line to overshoot the desired profile/set point curve (blue and purple) three
successive times. Not only is the bean temperature generally above the desired profile curve, it has produced its own distorted (and undesirable) profile curve. In fact, the green line appears to almost “bounce” from line to line as time progresses. The correction to this problem is to reduce the P value until overshooting is alleviated. In Graph 2, the gross
overshooting or bouncing of the green line has been virtually eliminated. However, upon closer inspection, it is clear that
the bean temperature, while maintaining the shape of the desired profile curve, consistently tracks below the blue and purple of the desired profile. While many roasters would be very pleased with a time and temperature curve as close to target as this, it is actually possible to shift or “gain” this droop away by adjusting the I setting. What is needed in this example is a more reactive I. A faster integrating action can be acquired by using a smaller I value. The correct I value will, in effect, offset the droop of a correctly set P.Eureka! Graph 3 shows what happens when PI values are inputted correctly. Set Point and Bean Probe lines are married up from start to finish. The Profile line is acquired at Hold Temperature, and all three lines track consistently from that point forward. Not only is the desired roast time and temperature reached (within a five-second window), but the integrity of the desired profile is maintained throughout the entire roasting process.