More on the Quest M3s Airflow Circuit

The Quest M3s, a simplistic and robust sample roasting machineThe Quest M3s, a simplistic and robust sample roasting machine
Within the class of table top coffee roasting machines, the Quest M3s sample roaster is about as simple as they come. There are no preset roast profiles, built in data loggers, blue tooth technology, or any other gadgetry to speak of. In this way it's much more like a larger production drum roaster, thermal dynamics controlled by two main adjustable parameters, good old fashioned heat and air flow.

Adjusting the Quest's temperature output is pretty straight forward. A range of 0 to 10 amps can be sent to twin heating coils inside the machine, the adjustment made via a dial on the side of the roaster. And while fan speed is similarly controlled, the Quest's airflow is affected by more than just a turn of the dial. Let's first take a quick look at how the Quest's airflow circuit works, and then how other factors outside of fan speed can greatly assist in making quick adjustments to the thermodynamics of your roast.

A basic outline of the Quest M3s airflow circuit - in the back of the drum, and near full circle to exhaust at rear chaff boxA basic outline of the Quest M3s airflow circuit - in the back of the drum, and near full circle to exhaust at rear chaff box

The Quest M3s airflow circuit starts just behind the drum housing via two stainless steel intake tubes. The inlet to these tubes is sandwiched between the rear of the drum and the chaff collection box, a tight space, but ample enough for pulling in the outside ambient air. The air is then pulled across the drum back-to-front, up the loading chute, across the top tube, down through the chaff collection box, and then exhausted out a vent in the back. That's a bit to follow if you don't have one sitting in front of you, and hopefully my crude drawing above helps to illustrate the air path.

Increasing fan speed pulls more air across the bean bed, and decreasing does just the opposite. The steel air intake tubes are situated between the heating coils and get really hot, warming the incoming air just a bit. We incorrectly stated in an earlier blog post that an increase in airflow actually speeds up the roast since the incoming air is warm (our first test of this theory seemed to support this, however more recent tests prove otherwise). You'll see from my test results that this isn't actually the case, and that increasing the air flow slows the temperature's rate of rise (ROR) by a few degrees per minute, a significant difference when spread over the entire roast time.

At the risk of stating the obvious, decreasing the fan speed creates an inverse effect - incoming air is all but cut off, which in turn pulls less heat from the drum, allowing the temperature to rise at a more steady clip. But as many Quest owners have pointed out, the fan in the Quest M3s is always running, and so it's impossible to shut off airflow completely. While it's true that the fan is always running, you can cut off airflow to the drum altogether by opening the cooling tray door on the top of the cooling/fan compartment at the back of the roaster. Opening this door effectively disrupts the airflow circuit, changing the air intake from the rear of the drum to directly into the cooling tray area. This one parameter change completely cuts off air to the drum and bean bed, thus creating the hottest roasting environment.

Have a look at one of two steel intake tubes positioned smack dab in the center of the heating coilHave a look at one of two steel intake tubes positioned smack dab in the center of the heating coil

To gain a better understanding of the effects airflow has on ROR for this machine, I roasted three 90 gram batches of the same coffee, one with full fan speed, one with fan speed at 0 (fan still on but at minimum speed), and one with the rear door open cutting off air to the drum. Even though my drop temperatures vary by a few degrees (I was shooting for 400f on the theramaprobe, but it's hard to hit on the money with such a small roasting chamber!), logging temperature changes for the three roasts shows a clear difference in ROR, which though small from minute to minute, equates to pretty drastic changes in overall roast times.

A couple things to point out about the graphs. First, these temperatures are in Fahrenheit, measured with a digital thermocouple in the bean bed position. I also didn't record where the temperatures bottomed out and turned around, but it was within the first 2 minutes. That's why the first two minutes are greyed out, and no ROR is listed as that data would be incorrect based solely on 1 and 2 minute temp markers. I've also marked where 1st crack occurred in the orange/brown cells, and then list exact time and temp of first snaps at the bottom of each graph.

Graph outlining my three roasts: #1 max airflow, #2 minimum airflow, #3 airflow circuit interruptedGraph outlining my three roasts: #1 max airflow, #2 minimum airflow, #3 airflow circuit interrupted

As you can see from the data logged above, the smallest ROR difference is seen between the first two roasts with fan speed dialed at 10 and 0, and the widest time spread is between roasts #1 and #3, where the airflow circuit is interrupted. That's to be expected, but just how significant are these differences? Using the temperature logged at minute 2 as a starting point, roast #1 with maximum airflow advanced 59 degrees in 4 minutes before hitting first crack (1C), whereas roast #3 jumped 58 degrees to 1C in just 2:40, a pretty huge difference. If you look at the 2 minutes following 1C, roast #1 climbed at roughly 8 degrees per minute, while #3 barrelled forward at 20 degrees per minute.

The roast progression is much closer between #1 and #2, but still worth our attention. The difference in ROR is only about 4 degrees in the first few minutes, dropping to 2 degrees by minute 4. This slight degree of difference seems almost negligible in this context, however, if 420F was your roast target, roast #2 would hit that mark nearly a full minute faster than #1. Depending on the coffee and desired results, that can be the difference between developing a coffee's sweetness, or baking the sweetness out to some degree. It's also worth noting that because each roast batch is subjected to a hot drum at the outset, ROR is much closer in all three roasts from minutes 1 to 3. After that, roasting with airflow slows the progression, whereas the roast with no air continues to take off.

These are the sorts of factors I take into consideration when roasting, in particular when transitioning from one coffee to the next, and especially if they differ in factors such as water content, density, and processing method. I might cut the fan speed to add heat during the drying phase for a wet coffee, increase the fan speed to take the edge off a potential runaway roast of a dryer coffee before 1C, or open the back door to pick up the pace of a stalling batch. Changing the amperage can be effective too, but oftentimes with electric roasters you have greater influence on roast dynamics with air flow, and are less likely to over correct the heat settings, and find yourself struggling to return to a properly warmed machine.