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Understanding the Limitations of Thermocouple ReadingsDaily Coffee News by Roast Magazine



(Editor’s word: This text initially appeared within the September/October 2017 difficulty of Roast Journal. Photographs by Rob Hoos and figures tailored from Cropster export information. Hyperlinks have been added for context.)

Which of the roasts illustrated in Figures 1–Three do you assume resulted in one of the best taste for the Colombian Huila espresso I used to be roasting?

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

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


The truth is, these usually are not three totally different roasts, however all measurements taken from the precise similar roast. Three thermocouples have been positioned within the drop door of the roaster and measured the similar 2 cubic inches within the roasting drum. The variations between the graphs within the thermocouple readouts; plot form; fee of rise (RoR), which measures the temperature change over time; and completed temperature all occurred as a result of there have been three totally different thicknesses of thermocouple measuring the identical espresso concurrently (Determine Four).

Whereas a thermometer makes use of the recognized enlargement of a cloth (liquid or strong) to measure temperature based mostly on modifications in quantity or measurement, thermocouples perform in another way. Merely put, a thermocouple is a temperature measurement gadget that makes use of the distinction in voltage between two wires manufactured from several types of metallic. The temperature is measured on the junction of the 2 wires. These wires are sometimes sheathed in a metallic coating, which varies in thickness and materials. Alternately, there are resistance temperature detectors (RTDs), which base measurements on the distinction in temperature by passing a low voltage present by way of the system and measuring the voltage drop, which has a recognized correlation with temperature. Steadily in our business we inaccurately confer with any measuring system in a roaster as a thermocouple. On this article we’re speaking about precise thermocouples, particularly the thermocouple situated within the bean pile contained in the roaster.


As specialty espresso roasters, we’re steadily introduced with a big drawback: We’re successfully unable to speak to every different about our roasting course of, 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 on a regular basis about turnaround time and temperature, most/minimal RoR, finish temperature, and so on. Nevertheless, the drawback with these questions is the relative nature of bean probe thermocouples and the way they relate to temperature readings.

On this article, we’ll assessment some experiments utilizing three totally different roasters and ranging thicknesses of thermocouples. On the finish of the day, I hope to make you query your thermocouple, maybe expertise some existential disaster over the truth that we don’t actually know exactly what is occurring and, lastly, make you are feeling a way of calm as you understand our thermocouples nonetheless give us an distinctive means to regulate high quality and produce superb espresso.


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

What Are We Seeing?

I need to begin by asking a seemingly easy query to assist us body the dialog: Of what precisely is the thermocouple positioned within the bean pile reporting the temperature? The seemingly easy reply is, “The mass of beans rolling round within the roaster.” Nevertheless, this reply is wrong. The reality is, the thermocouple is reporting its personal temperature, particularly at and close to 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 by way of the bean mass, air velocity and burner changes. We should always remind ourselves that the actions of a skinny (1.6-mm [1/16-inch]Three.2-mm [1/8-inch] or 6.Four-mm [1/4-inch]) piece of metallic jammed inside a metallic machine — with scorching airflow filtering by way of the espresso beans and radiative warmth, and full of natural materials — usually are not going to report with 100 % accuracy the temperature of that natural materials’s floor, not to mention its internal core. We should always as an alternative be content material to debate the measurement of bean temperature by means of a thermocouple buried within the bean mass contained in the roaster as being a proxy for the theoretical bean mass temperature. Maybe it’s best to name the “bean” temperature thermocouple the “course of” temperature thermocouple as an alternative. (In mild of this, we’ll discuss with it as the method temperature thermocouple from right here ahead.)


Experiments in Thermocouple Readings

Now let’s start to delve into the causes behind temperature readouts on the method temperature thermocouple in relation to roasting espresso, and the way these may affect our understanding of the info we obtain through the espresso roasting course of.

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

As one other instance, Determine 5 exhibits three widespread thicknesses of thermocouple shifting from boiling water to ambient air temperature. As you possibly can see within the diagram, there are vital variations within the price of cooling between the three thermocouples. The 1.6-mm thermocouple cools the quickest, with the three.2-mm thermocouple shut on its tail. The thickest thermocouple (6.Four mm) is remarkably sluggish to return to room temperature, and after almost 10 minutes nonetheless registers 10 levels F larger than the opposite two.


Turnaround Time/Temperature as a Perform of Response Time

One of many first locations we see the response time inflicting a big distinction is within the turnaround time and temperature. Turnaround is how we describe the thermal equilibrium between the thermocouple and the bean mass firstly of the roast. In most roasting techniques, there’s a requirement to preheat the metallic and the air circulating by way of the machine to be able to construct a thermal capacitance or cost to assist the roast transfer by way of the earliest stage, the place the steepest RoR is required. Subsequently, the thermocouples are heated considerably because the roaster is preheated.

When the beans are dropped in at room temperature, this causes the thermocouples to chill shortly because the beans are being heated quickly because of the thermal gradient (the variations in temperature throughout the substances within the roasting drum). The place 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 check with as turnaround.

Turnaround, then, is just a perform of the thermal cost of the roaster, the thermal mass of the espresso beans and the responsiveness of the thermocouple. For the roast pictured in Figures 1–Four, we noticed three remarkably totally different turnarounds for a similar roast. (See Determine 6 for a better take a look at the turnaround factors.) The thinnest thermocouple (1.6 mm) skilled turnaround in 47 seconds, the three.2-mm in a single minute 43 seconds, and the 6.Four-mm in two minutes 46 seconds. Successfully, there was a one-minute distinction between every of the thermocouples. Observe that this doesn’t imply the espresso skilled something totally different, merely that the temperature studying system was totally different and thus gave totally different outcomes.

The Unfold Throughout and on the Finish of the Roast

We additionally see this distinction in thermocouple response time through the roast and with the ultimate temperature. Some might presume the thermocouples ought to improve in alignment as time progresses, and ultimately ought to all report the identical temperature. This isn’t the case, nevertheless, as we’re always including warmth power into the system, and the thermocouples are continually studying barely in a different way because the precise temperature (and the purpose at which they might all ultimately attain equilibrium) is consistently climbing. Had we plateaued the warmth software solely, we might anticipate them ultimately to align, however the roasting course of is one during which we’re pretty continually making use of some warmth 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.

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What Does Response Time Imply for Roasters?

This leads us to the query: Which thermocouple’s response time is most consistent with that of the bean mass? I wouldn’t say it’s unattainable to know the precise response fee of the bean mass. Scientifically, we might determine it out (as some researchers have) by implanting a thermocouple inside a bean and holding one other thermocouple on the floor of the bean to watch the temperature gradient and the bean temperature. However this isn’t sensible for the manufacturing roaster.

The problem in determining a “most correct” response time is that the speed of warmth power absorption of the bean modifications, and the chemical and bodily properties of the bean change — together with the truth that beans of various sizes, moisture content material, water exercise, densities and cultivars possible will react barely in another way. Because the espresso dries out, because it loses density, because it goes via the glassy transition, all of those reactions trigger modifications to the speed of power absorption. Subsequently, it’s 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 with out them. Moreover, the method temperature thermocouple is a tremendous device for consistency. Not that it’ll essentially be constant from machine to machine, however inside the particular person machine it provides us the power to pretty intently replicate roasts. I might offer you instance after instance of a espresso having an identical roasted shade (whole-bean and floor, inside 1 to 2 factors on the ColorTrack and Agtron scales), the place the roast was ended utilizing solely the bean temperature thermocouple to find out the endpoint and drop time into the cooling tray.

One other necessary consideration is that thicker thermocouples are likely to naturally filter the info, or clean the road, as a result of they don’t seem to be capable of react as shortly. This will make it appear to be the curve being graphed (or the espresso being roasted) is progressing extra desirably, however it’s actually only 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 sufficient indication of what is occurring within the drum. Typically this will result in unintentional variations in roast which you could style on the cupping desk however not see on the info logger.


Loring’s new miniature demonstration roaster within the testing bay at Loring Sensible Roast in Santa Rosa, California. Right here you possibly can see the return air and bean probe thermocouples.

Bean Temperature and Price of Rise (RoR)

One other query is, how does this theoretical distinction between the response price of the bean mass and the response fee of the thermocouple have an effect on our understanding of RoR measurements?

If we’re not solely studying the bean temperature, and if the response time of the method temperature thermocouple is totally different from that of the bean mass, what can we perceive from RoR measurements and the way can these assist us?

Taking note of RoR is useful in trendy roasting, in that it acts as a number one indicator of the bean mass temperature and helps tremendously in our quest for consistency. It isn’t fallacious that roasters are taking a look at it with a brand new enthusiasm and curiosity. In reality, I want roasters would pay extra consideration to the temperature of the air getting into the roasting drum, then RoR, then course of temperature in relation to monitoring warmth switch for effectivity and consistency.

That being stated, I consider it is very important think about rigorously what RoR is telling us. As said beforehand, a thermocouple tells us its personal temperature, not essentially the temperature of the substance it’s immersed in. Likewise, RoR for the method temperature thermocouple isn’t displaying us the speed of change of the bean mass per se, however relatively the speed of change of the thermocouple caught inside the bean mass, as affected by its response time. Whether or not at any given level the beans are absorbing warmth extra 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 identical, merely that the method thermocouple’s RoR is a proxy for that of the particular bean mass.

Figures 7, eight and 9 present a number of thermocouples on three totally different machines throughout three totally different roasts. Particularly, let’s take a look at the top of the roast, the place I consider you’ll agree we see some fascinating outcomes.

I need to draw your consideration to the varied shapes the RoR curve can take throughout the identical roast. In all of the roasts, we see the 1.6-mm thermocouple react strongly to the burner adjustment round first crack. It even exhibits an fascinating phenomenon the place it dips after which comes again up when it comes to the RoR. In the meantime, the thicker thermocouples don’t expertise this in the identical method. They expertise a continuing decline (aside from the US Roaster Corp roast, during which the three.2-mm thermocouple rises zero.Four levels F from its bottom-out level to the top of the roast).

Theoretically, the temperature of the thinner thermocouple is probably going considerably greater than the precise bean temperature, and thus decreases with the removing of warmth software again towards the precise bean temperature, and begins to rise with it after the very fact. 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 quite a lot of barely different-sized items of metallic, how totally different is the precise bean mass in comparison with what we’re measuring?

I might additionally like us to think about this when it comes to defining or prescribing a most RoR at the start of the roast to keep away from defect, and a minimal RoR on the finish of the roast to keep away from what many discuss with as “stalling” or “baking.” As a result of thermocouples are so totally different, they demand us to have totally different expectations when it comes to each a most and minimal RoR, that are depending on machine, probe placement and probe thickness.


Thermocouple Placement

One other explanation for measurement variations is the location of the thermocouple, which impacts the quantity of publicity to airflow and radiant power, amongst different issues — and these variations could be amplified relying on batch measurement. Desk 1 illustrates the impact of thermocouple placement inside the roaster.

Observe that the 6.Four-mm thermocouple positioned in a special location provides us a good much less responsive studying relating to modifications within the bean mass. As we close to the top 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 may be, let’s take a look at the widest potential vary aspect by aspect. Determine 10 exhibits a roast of a Kenyan espresso with a 1.6-mm probe situated within the drop door and a 6.Four-mm probe situated to the highest left of the drop door.

Equally, Determine 11 exhibits a roast profile match accomplished on two totally different roasters, a Probatino and a Loring, by which the identical primary profile was accomplished to the identical whole-bean and floor finish colours (Probatino 60 WB, 55 GR; Loring 61 WB, 54.54 GR in accordance to the ColorTrack bench mannequin | Probatino 68 WB, 93 GR; Loring 68 WB, 92 GR, based on Javalytics utilizing the Agtron Gourmand Scale).

For each roasts, the espresso entered yellow, brown and first crack on the similar time. The Probatino’s thermocouple was positioned within the faceplate above the door and was thicker than the thermocouple positioned contained in the Loring’s drop door. In each of those situations, we see the potential for the vast 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 best way the roaster applies warmth to the espresso.


The Take Away

After finishing the checks detailed on this article, I consider we will make the next conclusions about thermocouples and the best way to use them most successfully:

Think about shifting to a thinner thermocouple positioned decrease within the drum (within the drop door). The Loring we use at Nossa Familia Espresso was the sixth machine constructed by Loring. When we began roasting on it, the method temperature thermocouple was close to the highest left of the drop door and was 6.Four mm thick. We roasted on that machine in that configuration for fairly a while. We observed that, sometimes, the profile can be bang on however the espresso would style a bit of baked. We chalked it as much as a type of issues in espresso roasting that is mindless, and we simply stored up on our manufacturing cupping to do cross/fail on the beans earlier than sending them out.

Once we upgraded to Loring’s present configuration (the 1.6-mm thermocouple within the drop door), we ran each for some time. Although we have been nonetheless utilizing the 6.Four-mm as our management, we have been watching the 1.6-mm thermocouple to study the way it was totally different and turn into acclimated to it earlier than absolutely switching over. What we started to see helped us make sense of what was occurring with these “off” batches. Although the thick thermocouple was smack on profile, the thinner thermocouple truly confirmed us going into damaging RoR. We had been dropping power publish first crack and “baking” the espresso with out realizing it. For us, shifting to that thinner thermocouple within the drop door made an enormous distinction in high quality management.

Please word that making this modification will make it crucial to transform your profiles both barely or completely.


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

Keep in mind that thermocouple measurements are relative and solely a proxy for the precise bean temperature. There are not often straightforward solutions 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 or not a espresso is sweet or dangerous, whether or not it passes or fails. You will get lots of info from the profile, and whether it is annotated with colour modifications, occasions, management modifications, weight reduction and finish colour (whole-bean and floor), you could even come near understanding what occurred in the course of the roast, however it isn’t good. For instance, it’s potential to have an RoR that’s destructive whereas the roast continues to be progressively getting darker and persevering with to crack vigorously. (This occurs by pushing the roast arduous till proper earlier than first crack, then dropping off the burner. The RoR plummets, however the beans proceed darkening and cracking vigorously. Then you’ll be able to finish the roast earlier than the beans truly finish their ahead momentum.) This isn’t widespread follow, nor am I saying roasters ought to goal to do that — 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 wish to purpose for a sure form of profile or a sure objective together with your RoR, that’s superior; nevertheless, I might encourage you to not obsess an excessive amount of over particulars which are associated 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 flavour of the espresso by means of roast manipulation into one thing you and your clients like, after which maintain delivering that very same nice espresso. Use colour measurement info, weight reduction, solubility measurements and manufacturing cuppings to double-check your self and keep constant.


Know that everybody’s roaster, fashion, buyer base and thermocouple is a bit totally different. If one thing sounds off about the best way somebody is roasting, it doubtless has to do with these elementary variations in how we view our roasting world. For those who’re simply beginning, speak to the roaster producer about tips on how to use its machine and speak to different individuals who use that very same roaster, however keep away from obsessively evaluating your self to others. Typically one thing so simple as a special air flow setup may cause your machine to roast in a different way and your thermocouples to skew barely due to variations in airflow by means of the machine.

Lastly, take pleasure in your self, take possibilities, innovate, and relish the fantastic depth and complexity that exist inside our business. Deal with espresso like jazz music. Study the principles — or tips, as it might be — then don’t be afraid to bend and break them.

It’s clever for roasters to proceed to attempt for consistency, and to do our greatest to know what is occurring contained in the espresso roaster, however we should achieve this with each eyes open. On the finish of the day, course of temperature thermocouples are extraordinarily useful for consistency, however the knowledge they report is just not as correct as many in our business may consider.


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