Posted by kaffelogic on March 5, 2021

When it comes to roasting coffee, time is of the essence. Variations of plus or minus ten seconds can affect the taste, so when you are aspiring to get the peak flavour out of the beans you need to be confident of your timings. This is where the naked thermocouple comes in. It is a feature of the Kaffelogic system that helps with the acclaimed responsiveness of the Nano 7.

What does this mean for an automated control system such that the one at the heart of the Kaffelogic? It means two things:

  1. when unexpected changes happen, the system needs to respond within 10 seconds, and
  2. when expected changes happen, the system needs to allow the roast designer to programme the response in advance so the response can occur right on time.

The Kaffelogic system does both of these things. In this article I discuss the first: how does the Kaffelogic system respond within 10 seconds to unexpected changes? The second, programming responses in advance, will be covered in a subsequent article.

So, how do we go about designing for the criteria: 'must respond within 10 seconds'?

The problem

A typical sheathed thermocouple
A typical sheathed thermocouple probe​​​​​


The usual way of measuring temperature in a coffee roaster is with a sheathed thermocouple probe. A thermocouple is often the preferred technology for this kind of temperature measurement. It consists of two pieces of different types of wire welded together. The point where they are welded together is the point at which the temperature is measured. It is a fascinating feature of the natural world that when you connect two different metals they generate a tiny electrical voltage. It’s a pretty small voltage, but with the right amplification it can be used to detect temperatures to within a fraction of a degree. Thermocouples are accurate, resilient, and can survive high temperatures. That said, the wires, and especially the weld that actually measures the temperature, can break. For this reason most thermocouple probes are sheathed in a metal tube of some sort. This makes them much more robust, but it also has a major effect on time. The sheath must warm up or cool down when the environment temperature goes up or down. This takes time and can delay the temperature measurement by between 10 and 30 seconds. The roast control system also requires time to analyse the incoming data and compute the response. This is not a matter of raw computing power, but a matter of data collection and smoothing. With a sheathed thermocouple probe it becomes difficult or impossible to meet the 'response within 10 seconds' criteria.

The solution

Kaffelogic probe
The Kaffelogic thermocouple probe with a naked thermocouple tip

What does the Kaffelogic do different? We redesigned the probe so that the welded tip is unsheathed. That’s right, we went naked. We didn't care that the general population of engineers find the notion of thermocouple nudity to be offensive, to us it was liberating. We still have a metal tube, but the tube does not have a closed tip and the thermocouple weld (remember that’s the bit that measures the temperature) sticks out into space and directly contacts the beans and the air surrounding them. This revolutionary design was made possible by two things:

  1. We have found a manufacturer (Labfacility) who makes thermocouples with low profile, strong welds that do not break in normal use. We now have an installed base of many hundreds of these thermocouples, and, of all the parts in the Nano 7, this is one that has never failed or been broken by a user. That’s tough enough for us. (Our initial model had a different thermocouple with a wide profile weld which was prone to damage, but that component is just a bad memory now!) 
  2. The thermocouple is in contact with beans that are in transitional fluidization, and they are actually very gentle when they touch the probe. This means that for most of its service life the thermocouple is subject to very little strain.

So what do you get when you have a component that is strong, and you don’t stress it much? You get a reliable component.

The bottom line

So it’s reliable, but why fuss? It’s all about the time. A naked thermocouple responds to temperature changes within a couple of seconds. This means the roast control software has up-to-the-second data and can begin responding to changes almost immediately. There is still the need for smoothing and this introduces an inevitable delay, but the Kaffelogic system uses a statistical analysis that allows the response to begin within a few seconds, and gradually strengthen up over the first 10 seconds.

The quicker you start a response, the less lurching there is. Do you remember driving your car when you are little tired (something you should never do) and suddenly realising you are driving off the edge of the road? When this happens you lurch suddenly in an attempt to get back on track. What else can you do? Well you could just stay alert and respond faster to smaller deviations, and that’s basically what the Kaffelogic system is doing. 

What effect does this have on taste? It is part of why professionals are so impressed by the roast quality coming off the Kaffelogic. The results are evident in the cup.

When we designed the Kaffelogic we knew that sudden lurches in temperature damage flavour, and unless we worked to avoid this we would never have a system that could produce quality roasted coffee. That is why we were prepared to get naked. It’s all about the coffee.

Author: Chris Hilder

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