Agtron ColorTrack cropster existential crisis Javalytics Loring Nossa Familia Coffee Pobatino rate of rise roast profiles roast profiling Rob Hoos thermocouples thermometers US Roaster Corp Wilbur Curtis G4 Thermopro

Understanding the Limitations of Thermocouple ReadingsDaily Coffee News by Roast Magazine

(Editor’s observe: This text originally appeared in the September/October 2017 difficulty of Roast Magazine. Pictures by Rob Hoos and figures tailored from Cropster export information. Hyperlinks have been added for context.)

Which of the roasts illustrated in Figures 1–3 do you assume resulted in the greatest taste for the Colombian Huila coffee I was roasting?

Why do you assume it was a better profile than the others?

Which of the roast profiles appears the most acquainted to you, or is most in line with the approach you roast espresso?


The truth is, these will not be three totally different roasts, but all measurements taken from the actual similar roast. Three thermocouples have been positioned in the drop door of the roaster and measured the similar 2 cubic inches in the roasting drum. The differences between the graphs in the thermocouple readouts; plot shape; price of rise (RoR), which measures the temperature change over time; and finished temperature all occurred because there have been three totally different thicknesses of thermocouple measuring the similar coffee simultaneously (Figure Four).

While a thermometer uses the recognized enlargement of a cloth (liquid or strong) to measure temperature based mostly on modifications in quantity or measurement, thermocouples perform in a different way. Simply put, a thermocouple is a temperature measurement system that makes use of the distinction in voltage between two wires made of differing types of metallic. The temperature is measured at the junction of the two wires. These wires are sometimes sheathed in a metallic coating, which varies in thickness and material. Alternately, there are resistance temperature detectors (RTDs), which base measurements on the distinction in temperature by passing a low voltage present via the system and measuring the voltage drop, which has a recognized correlation with temperature. Incessantly in our business we inaccurately discuss with any measuring system in a roaster as a thermocouple. In this article we’re talking about precise thermocouples, specifically the thermocouple situated in the bean pile inside the roaster.


As specialty coffee roasters, we’re ceaselessly introduced with a big drawback: We’re successfully unable to talk to each other about our roasting process, method or outcomes with regard to bean temperature as a result of the measurement of the “bean temperature” is just not transferable from machine to machine. Individuals ask me questions all the time about turnaround time and temperature, most/minimum RoR, finish temperature, and so forth. Nevertheless, the drawback with these questions is the relative nature of bean probe thermocouples and how they relate to temperature readings.

In this article, we’ll assessment some experiments using three totally different roasters and varying thicknesses of thermocouples. At the finish of the day, I hope to make you question your thermocouple, perhaps experience some existential crisis over the proven fact that we don’t actually know exactly what is occurring and, lastly, make you are feeling a sense of calm as you understand our thermocouples nonetheless give us an exceptional capability to regulate high quality and produce superb coffee.


Three thermocouples of totally different thicknesses. From left: 1.6 mm, Three.2 mm and 6.Four mm.

What Are We Seeing?

I need to start by asking a seemingly easy question to help us frame the conversation: Of what exactly is the thermocouple positioned in the bean pile reporting the temperature? The seemingly simple answer is, “The mass of beans rolling around in the roaster.” Nevertheless, this reply is wrong. The reality is, the thermocouple is reporting its own temperature, particularly at and near the tip of stated thermocouple.

What, then, is influencing the temperature of the thermocouple?

That’s difficult, as a result of the thermocouple is affected by sheath thickness, probe placement relative to the roaster design, probe placement relative to the bean pile, the bean mass, the airflow filtering via the bean mass, air velocity and burner adjustments. We should continuously remind ourselves that the actions of a skinny (1.6-mm [1/16-inch], 3.2-mm [1/8-inch] or 6.4-mm [1/4-inch]) piece of metallic jammed inside a metallic machine — with scorching airflow filtering via the coffee beans and radiative heat, and crammed with natural materials — are usually not going to report with 100 % accuracy the temperature of that natural materials’s surface, not to mention its internal core. We should always as an alternative be content to debate the measurement of bean temperature by means of a thermocouple buried in the bean mass inside the roaster as being a proxy for the theoretical bean mass temperature. Maybe it is best to name the “bean” temperature thermocouple the “process” temperature thermocouple as an alternative. (In mild of this, we’ll check with it as the process temperature thermocouple from here ahead.)


Experiments in Thermocouple Readings

Now let’s begin to delve into the causes behind temperature readouts on the course of temperature thermocouple in relation to roasting espresso, and the way these may affect our understanding of the knowledge we obtain throughout the coffee roasting course of.

The first variable value considering is the thickness or measurement of the sheath on the thermocouple. The important thing consideration here is responsiveness. Sometimes talking, thinner thermocouples are extra aware of temperature modifications, whereas thicker thermocouples have higher thermal inertia and tend to respond extra slowly. This isn’t solely true in espresso and occasional roasting; it is true all through the temperature measurement business.

As another example, Determine 5 exhibits three widespread thicknesses of thermocouple shifting from boiling water to ambient air temperature. As you’ll be able to see in the diagram, there are vital variations in the price of cooling between the three thermocouples. The 1.6-mm thermocouple cools the quickest, with the 3.2-mm thermocouple shut on its tail. The thickest thermocouple (6.4 mm) is remarkably sluggish to return to room temperature, and after almost 10 minutes nonetheless registers 10 degrees F greater than the different two.


Turnaround Time/Temperature as a Perform of Response Time

One of the first places we see the response time inflicting a big difference is in the turnaround time and temperature. Turnaround is how we describe the thermal equilibrium between the thermocouple and the bean mass at the starting of the roast. In most roasting techniques, there’s a requirement to preheat the metallic and the air circulating via the machine as a way to construct a thermal capacitance or charge to help the roast move by way of the earliest stage, where the steepest RoR is required. Subsequently, the thermocouples are heated significantly as the roaster is preheated.

When the beans are dropped in at room temperature, this causes the thermocouples to cool shortly as the beans are being heated quickly as a consequence of the thermal gradient (the differences in temperature throughout the substances in the roasting drum). Where the thermocouple reaches a zero-degree-per-minute RoR, we’ve got a theoretical thermal equilibrium between the bean mass and the thermocouple. That is the second we confer with as turnaround.

Turnaround, then, is just a perform of the thermal charge of the roaster, the thermal mass of the espresso beans and the responsiveness of the thermocouple. For the roast pictured in Figures 1–4, we noticed three remarkably totally different turnarounds for the similar roast. (See Determine 6 for a better take a look at the turnaround factors.) The thinnest thermocouple (1.6 mm) experienced turnaround in 47 seconds, the Three.2-mm in one minute 43 seconds, and the 6.Four-mm in two minutes 46 seconds. Effectively, there was a one-minute difference between each of the thermocouples. Word that this does not mean the coffee skilled something totally different, merely that the temperature studying gadget was totally different and thus gave totally different results.

The Spread Throughout and at the End of the Roast

We additionally see this difference in thermocouple response time throughout the roast and with the last temperature. Some might presume the thermocouples should improve in alignment as time progresses, and ultimately should all report the similar temperature. This is not the case, nevertheless, as we are always including warmth power into the system, and the thermocouples are continuously studying slightly in another way as the actual temperature (and the point at which they might all ultimately reach equilibrium) is consistently climbing. Had we plateaued the heat software totally, we might anticipate them ultimately to align, however the roasting process is one through which we’re fairly continually making use of some heat and experiencing an RoR larger than zero levels per minute. Subsequently, the thicker thermocouples will all the time be in a state of lag in comparison with the thinner thermocouples.

Keyhole 17

What Does Response Time Imply for Roasters?

This leads us to the question: Which thermocouple’s response time is most consistent with that of the bean mass? I wouldn’t say it’s unimaginable to know the actual response fee of the bean mass. Scientifically, we might figure it out (as some researchers have) by implanting a thermocouple inside a bean and holding another thermocouple on the surface of the bean to watch the temperature gradient and the bean temperature. But this isn’t sensible for the manufacturing roaster.

The problem in determining a “most accurate” response time is that the price of heat power absorption of the bean modifications, and the chemical and bodily properties of the bean change — along with the undeniable fact that beans of totally different sizes, moisture content material, water activity, densities and cultivars probably will react slightly in a different way. As the espresso dries out, as it loses density, because it goes by way of the glassy transition, all of these reactions cause modifications to the price of power absorption. Subsequently, it is doubtless that no single thermocouple will give us an ideal view of the bean’s temperature, and even the direct impact on the bean pile of modifications we make throughout roasting.

That being stated, thermocouples are fantastic proxies that assist us perceive the internal workings of the roasting mass higher than we ever would without them. Moreover, the process temperature thermocouple is a tremendous device for consistency. Not that it’ll essentially be constant from machine to machine, but inside the particular person machine it provides us the capacity to pretty intently replicate roasts. I might offer you instance after example of a coffee having an analogous roasted colour (whole-bean and floor, within 1 to 2 points on the ColorTrack and Agtron scales), where the roast was ended using only the bean temperature thermocouple to find out the endpoint and drop time into the cooling tray.

One other essential consideration is that thicker thermocouples are likely to naturally filter the knowledge, or clean the line, because they don’t seem to be capable of react as shortly. This could make it appear to be the curve being graphed (or the espresso being roasted) is progressing extra desirably, however it’s really just a smoothing perform of the thermal lag (the delay in response time). Although sturdy, they will typically be too sluggish to react and should not give an correct enough indication of what is occurring in the drum. Typically this could result in unintentional variations in roast that you could taste on the cupping table however not see on the knowledge logger.


Loring’s new miniature demonstration roaster in the testing bay at Loring Sensible Roast in Santa Rosa, California. Here you’ll be able to see the return air and bean probe thermocouples.

Bean Temperature and Price of Rise (RoR)

One other question is, how does this theoretical distinction between the response fee of the bean mass and the response price of the thermocouple affect our understanding of RoR measurements?

If we’re not solely reading the bean temperature, and if the response time of the process temperature thermocouple is totally different from that of the bean mass, what can we understand from RoR measurements and how can these assist us?

Taking note of RoR is helpful in trendy roasting, in that it acts as a leading indicator of the bean mass temperature and helps tremendously in our quest for consistency. It isn’t mistaken that roasters are taking a look at it with a brand new enthusiasm and interest. In reality, I want roasters would pay extra attention to the temperature of the air getting into the roasting drum, then RoR, then process temperature in relation to monitoring warmth transfer for efficiency and consistency.

That being stated, I consider it is very important contemplate rigorously what RoR is telling us. As said previously, a thermocouple tells us its personal temperature, not necessarily the temperature of the substance it’s immersed in. Likewise, RoR for the course of temperature thermocouple shouldn’t be displaying us the fee of change of the bean mass per se, but slightly the price of change of the thermocouple caught inside the bean mass, as affected by its response time. Whether at any given level the beans are absorbing warmth more shortly or slowly than the thermocouple is up for debate and must be critically investigated in future scientific research; nevertheless, we should always not assume they’re the similar, merely that the process thermocouple’s RoR is a proxy for that of the precise bean mass.

Figures 7, 8 and 9 show multiple thermocouples on three totally different machines throughout three totally different roasts. Particularly, let’s take a look at the end of the roast, where I consider you’ll agree we see some fascinating results.

I need to draw your attention to the numerous shapes the RoR curve can take during the similar roast. In all the roasts, we see the 1.6-mm thermocouple react strongly to the burner adjustment around first crack. It even exhibits an fascinating phenomenon where it dips after which comes back up in terms of the RoR. Meanwhile, the thicker thermocouples do not experience this in the similar means. They expertise a continuing decline (with the exception of the US Roaster Corp roast, by which the Three.2-mm thermocouple rises 0.Four degrees F from its bottom-out point to the finish of the roast).

Theoretically, the temperature of the thinner thermocouple is probably going somewhat greater than the actual bean temperature, and thus decreases with the removing of warmth software back toward the precise bean temperature, and begins to rise with it after the reality. As soon as once more, this illustrates that thermocouple measurements are utterly depending on the probe you’re utilizing to measure. It additionally attracts into query, with such a spread of barely different-sized items of metallic, how totally different is the actual bean mass in comparison with what we’re measuring?

I might additionally like us to think about this in terms of defining or prescribing a most RoR at the starting of the roast to avoid defect, and a minimum RoR at the finish of the roast to avoid what many seek advice from as “stalling” or “baking.” As a result of thermocouples are so totally different, they demand us to have totally different expectations in phrases of each a most and minimal RoR, that are depending on machine, probe placement and probe thickness.


Thermocouple Placement

One other cause of measurement differences is the placement of the thermocouple, which impacts the quantity of publicity to airflow and radiant power, amongst other issues — and these variations could be amplified depending on batch measurement. Desk 1 illustrates the effect of thermocouple placement within the roaster.

Word that the 6.4-mm thermocouple positioned in a special location provides us a fair much less responsive studying relating to modifications in the bean mass. As we close to the finish of the roast, we see that the variance between the totally different placements is more and more expansive. To point out how dissimilar these measurements could be, let’s take a look at the widest potential vary aspect by aspect. Determine 10 exhibits a roast of a Kenyan coffee with a 1.6-mm probe situated in the drop door and a 6.Four-mm probe situated to the prime left of the drop door.

Similarly, Determine 11 exhibits a roast profile match carried out on two totally different roasters, a Probatino and a Loring, during which the similar primary profile was accomplished to the similar whole-bean and floor end colours (Probatino 60 WB, 55 GR; Loring 61 WB, 54.54 GR according to the ColorTrack bench mannequin | Probatino 68 WB, 93 GR; Loring 68 WB, 92 GR, in response to Javalytics using the Agtron Gourmand Scale).

For both roasts, the espresso entered yellow, brown and first crack at the similar time. The Probatino’s thermocouple was placed in the faceplate above the door and was thicker than the thermocouple positioned inside the Loring’s drop door. In both of these situations, we see the potential for the extensive distribution of thermocouple readings based mostly on thickness and placement. It makes me marvel how a lot “different-looking” profiles may be attributed merely to totally different thermocouple varieties, thicknesses and placement, and have little to nothing to do with the approach the roaster applies heat to the espresso.


The Take Away

After completing the checks detailed in this article, I consider we will make the following conclusions about thermocouples and how you can use them most successfully:

Contemplate shifting to a thinner thermocouple positioned decrease in the drum (in the drop door). The Loring we use at Nossa Familia Coffee was the sixth machine built by Loring. When we started roasting on it, the process temperature thermocouple was close to the prime left of the drop door and was 6.4 mm thick. We roasted on that machine in that configuration for quite a while. We observed that, sometimes, the profile can be bang on however the coffee would style slightly baked. We chalked it up to one of these issues in coffee roasting that is unnecessary, and we just stored up on our production cupping to do move/fail on the beans earlier than sending them out.

Once we upgraded to Loring’s present configuration (the 1.6-mm thermocouple in the drop door), we ran both for some time. Though we have been still utilizing the 6.Four-mm as our control, we have been watching the 1.6-mm thermocouple to study how it was totally different and develop into acclimated to it earlier than absolutely switching over. What we began to see helped us make sense of what was occurring with those “off” batches. Although the thick thermocouple was smack on profile, the thinner thermocouple truly showed us going into unfavorable RoR. We had been dropping power submit first crack and “baking” the espresso without realizing it. For us, shifting to that thinner thermocouple in the drop door made a huge difference in high quality control.

Please observe that making this alteration will make it mandatory to transform your profiles either barely or totally.


A US Roaster Corp 3-kilo roaster operating with 1.6-mm and three.2-mm thermocouples in the faceplate.

Keep in mind that thermocouple measurements are relative and solely a proxy for the actual bean temperature. There are not often straightforward answers in life, and occasional roasting is not any exception. One can’t merely take a look at a computer-charted roast profile and say whether a coffee is sweet or dangerous, whether it passes or fails. You will get rather a lot of info from the profile, and if it is annotated with shade modifications, occasions, control modifications, weight reduction and finish colour (whole-bean and ground), you could even come near understanding what occurred throughout the roast, nevertheless it isn’t good. For example, it is potential to have an RoR that is adverse while the roast continues to be progressively getting darker and continuing to crack vigorously. (This occurs by pushing the roast exhausting until right before first crack, then dropping off the burner. The RoR plummets, but the beans proceed darkening and cracking vigorously. Then you possibly can end the roast before the beans truly end their ahead momentum.) This isn’t widespread follow, nor am I saying roasters ought to goal to do this — I’m merely suggesting that the trajectory the beans journey throughout roasting is way extra complicated than a graph will make it out to be.

If you want to purpose for a certain form of profile or a certain objective together with your RoR, that’s superior; nevertheless, I might encourage you to not obsess an excessive amount of over details which are related to imperfect proxies for bean mass temperature. Use the thermocouple and profiling knowledge to purpose for general consistency. Use techniques and methods to adapt the flavor of the espresso by means of roast manipulation into one thing you and your clients like, after which maintain delivering that very same great coffee. Use colour measurement info, weight loss, solubility measurements and manufacturing cuppings to double-check your self and stay consistent.


Know that everybody’s roaster, fashion, buyer base and thermocouple is just a little totally different. If one thing sounds off about the approach somebody is roasting, it probably has to do with these elementary variations in how we view our roasting world. In the event you’re simply beginning, speak to the roaster producer about how you can use its machine and speak to different people who use that very same roaster, however keep away from obsessively evaluating your self to others. Typically one thing so simple as a unique air flow setup may cause your machine to roast in another way and your thermocouples to skew slightly because of differences in airflow by means of the machine.

Finally, take pleasure in yourself, take possibilities, innovate, and relish the fantastic depth and complexity that exist inside our business. Treat coffee like jazz music. Study the rules — or tips, as it might be — then don’t be afraid to bend and break them.

It is clever for roasters to continue to attempt for consistency, and to do our greatest to know what is occurring inside the espresso roaster, however we should achieve this with each eyes open. At the finish of the day, process temperature thermocouples are extraordinarily useful for consistency, but the knowledge they report is just not as accurate as many in our business may consider.