I roast professionally and have done for some time now. However, I came up from the home roasting community, a group that likes to build, innovate and general figure out how to do things better. I currently employ an engineer who also likes to tinker. Together, we have conceived and effected a number of improvements to the commercially built 3kilo coffee roaster that arrived newly built from the factory in August 2010. Those modifications have allowed us to take a very scientific approach to our roasting process. The roaster came with one thermocouple (a thermometer that a computer can read); we installed five more. The roaster came with sophisticated electronic controls for the burner pressure, drum speed and air circulation fan; we installed circuit boards that allow the computer to track the position of those controls. My engineer designed software that generates a graphical representation of these parameters for every roast we do. A typical one looks like this:
While our roaster is manually controlled, we use a computer to generate a detailed picture of the roasting process or profile.
As much information as this set up provides, somehow six thermocouples just doesn’t seem like enough. When roasting coffee there are two forms of heat applied to the beans: convection from heated air moving through the roaster, conduction from the gas flame under the drum, and transference which is heat shared among the beans. We can manipulate two of these by adjusting the size of the flame and the speed of air moving through the drum. Up until now, we have been able to get a sense of the balance of conduction and convection by measuring the air temperature before it enters the drum and when it leaves the drum. While helpful, it’s become clear that this proxy strategy doesn’t tell the whole story. So recently we began our next modification which will allow us to measure the temperature of the metal drum directly. It’s already apparent that with this latest modification, sparks will fly, literally.
Cutting a notch into the drum spindle so that we can pass the thermocouple cable through the gear box.
This is by far the most complicated thermocouple install given the fact that the drum is rotating at roughly 60 RPM. Because of this, we are using a revolving connector that will allow the thermocouple to spin with the drum, but still supply a consistent signal to our computer tracking system. The first step, as seen here, was to cut a channel into the drum spindle so that the thermocouple cable can pass through the motor gearbox.
Next we drilled a small hole in the drum (easier typed than done) and bolted the tip of the thermocouple to the inside surface of the drum as close to the center along the axle as possible.
The thermocouple has been securely fastened to the inside of the drum.
Then we cemented the rotating connector to the end of the drum axle.
A rotating thermocouple connector allows the computer to read the drum probe as it spins.
The other side of the connector is plugged into the USB module we use to feed the temperature readings to our computer. Now the fun begins. With everything rigged up, we have started collecting data on the drum temperature. As we gather information from different size roasts, we are trying to understand what these new temperature measurements really mean and how we can best use them.
