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Monitoring bean temperature on Gene Cafe CBR-101
I've had a look at the various efforts at monitoring the bean mass temperature on the 101 (and also controlling the roaster) and although this is something that I would like to do, I really don't like the idea of putting a battery and circuitry on the drum, and I don't much like the idea of drilling through the drum and placing a thermocouple inside it.

Also, because of the off-axis design of the Gene, it's not actually possible to measure the bean mass temperature because the beans are constantly moved from the top to the bottom of the drum, and from one side to the other. So the positioning of the thermocouple would be critical (and even positioned with thought and care there is really no way of guaranteeing that it will give anything more than an approximate bean temperature).

So I wonder if anyone has tried to monitor just the exhaust, or better still, both the inlet and exhaust temperatures? It would seem to me that by monitoring both that a fairly good estimate of the bean temperature should be possible (a weighted average of the two with a lag to allow for the mass of the bean, perhaps).

It would also be very nice to monitor the set temperature, to allow for a correlation between the set temperature and the monitored temperatures.

At any rate, the calculated bean temperature plus a simple running ROR figure would be enough (without any graphical output ... nice but not really necessary) to give a very good manual control system. (The running figures could be kept for later analysis.)

Any thoughts or suggestions? Has anyone already done this?


Did you see this:


KKTO Roaster.
Thanks for the link Jack ... very interesting.

It made me think that it might be possible to use the 1st crack and 2nd crack theoretical temperatures to calibrate a dual-probe system (inlet and exhaust).

I still think that some form of automatic temperature recording and computation of ROR might be useful ... if only to provide the information during the roast (freeing one to focus on listening/looking/smelling) and also for post-roast analysis.

Or is this sort of data only really useful for the sort of repeatability that professional roasters must be able to achieve? And which home roasters will never achieve because we are always roasting different beans in different conditions with equipment that is not accurately controllable.
You can approach an ET based roast, with no more care about BT, with a little experimentation and tweaking the results are coming. This is how I roast now.
I don't need more than 4 fixed pwm value steps to perform any kind of coffee roasting.
So, a pid or power variator (% pwm) on the hot air path, placing the thermocouple at the drum input could do the trick.

Also, in the case of Gene, I think that on the final approach the exhaust air temp replicate very well BT, at least the differential (rate), which is more important than absolute value.
Roasters: DIY: TO based IR 200-640g
Moded commercial: Skywalker, Dieckmann RoestMeister, Nesco, popper(s).
Electrics: TC4ESP, MS6514, USB/Artisan/Apps, PID controllers
Grinders: Arco, Xeoleo ghost burrs, Krinder, vintage PeDe Dienes, MBK Feldgrind
Brew/presso: ALM pour over, Arin lever, Staresso Mirage, Hario Cafeor dripper, Flair, Kompresso, Rota Aeropress, Gaggia Mini
OK ... so you are basing your roast purely on the ET, plus experience? But you also seem to be controlling the heater (or is it the fan?).

I've put a probe in the outlet and compared it to the GC readings, and the two are pretty much identical, as one would expect.

With the GC's fast airflow it wouldn't be unreasonable to expect that the temperature inside the drum would correlate quite closely with the exhaust temperature. So at my next roast I will record and plot the exhaust temperature and see where that gets me.

I see that you use an MS6514 ... do you find it to be good?
Yes, nothing to complain about MS6514, especially the price/usability/precision ratio, I don't know other meter so far equals this device.

Is not only experience but also study of roasting thermodynamics.
As already wrote in the past, a given mass of "something" has a natural curve exponential temperature increase, if placed in a hot environment at a fixed temperature.

I am controlling heat only, airflow is constant by machine design, and is not so important to change it during the roast, as in a massive conduction based approach, i.e. solid drum roaster.

Someone measured the temperature gradient inside Gene, it seems the temperature drops along the roast chamber, but not dramatic, see below:
Many thanks ... yes, the thermodynamics will definitely cause a lag in the bean temperature compared to the air temperature. Have you figured out some way of estimating this? Possibly based on 1st & 2nd crack?

Also, have you documented your control system anywhere? It would be very useful ... I would prefer to use an MS6514 than go the Arduino route.


Nothing fancy, MS6514 connected to the laptop using Artisan for monitoring, and a variator like below for manual control.

For new coffees or out of usual quantities I am using Artisan trending lines to find % of power for drying and then for development, For known coffees I roast without computer.
Typically I am preheating with 40%, charge, than keeping same level until TP, about 80C@1min, then ramp to first % for drying, about 60-70%, target is 200C ET at minute 4, then adjust a second %, about 5-10% lower, for development, target being ET 245C at minute 8.
Keeping ET there until FC, when comes fine tuning within 5-10 degrees.

I'm not sure that I fully understand what you are doing, but if I translate that to the Gene (with manual control) it would seem to be something like this:
- Preheat to 100C
- Set to +/-150C until ET (at exhaust) reaches 80C (target 1min)
- Set to +/-250C until ET reaches 200C (target 4mins)
- Set to 245C until 1st crack (target ET 245C at 8mins)
- Fine tune

Something like that? The Gene won't get these temperatures in those times, but I'm just trying to understand what you are doing, in principle.

I take it the MS6514 interfaces to Artisan without any problems?

The only problem I can imagine now is running an OS with issues regarding CP21xx serial usb drivers.
If this is checked, then all you have to do is select MS6514 in Artisan Config/Device... menu, then select serial port next dialog, press a button on MS6514 to start data streaming and... roast.

Assuming you mount one of the MS6514 probes in the hot air path, (else why you get a MS6514 ? ) and a power variator to replace Gene thermostat, my approach would be as follows:
- would dial the heater % to preheat at 200 C, is not that much, when opening the cylinder to load beans is a lot of temperature loss.
- keep the same % found when preheating, for 4 minutes, and check exhaust temperature evolution, this will give an idea about machine capabilities
- having the BT probe mounted in the exhaust path, dial starting from minute 2-3 more % power to see the Artisan trending lines showing BT reaching 150C at least at minute 6.
- after drying, raise power % to have 245C ET, desirably by minute 8, and keep this temperature.

Using Gene exhaust temperature as BT we can find the coefficients of the two curves, (drying and yellowing) and have a better idea about the machine capabilities to create other profiles.

Without a variable power mod of the heater I have no idea how to control something, to draw any conclusion... because Gene is using a high hysteresis
thermostat, that produce temperature jumps as big as 50 C... maybe the parallel diode mod, to lower the hysteresis to more acceptable values.
To be clear, are you suggesting having the BT probe at the exhaust and the ET probe at the hot air inlet to the drum?
Yes, as an immediate solution, without major changes, a true BT solution is much more complicate to implement.

But without an effective way of hot air temperature control, I don't see much gain adding these probes, because actual Gene system is too much on-off, for my taste.
This article http://coffeetime...dification shows a simple low-cost mod to the Gene to control the heater. Might be better to replace the dimmer with an SCR voltage regulator though.

As you mentioned, the on/off control on the Gene seems to be a problem (I didn't know that is how the heater is controlled ... time to get a power monitor!).

I realize this is an ancient thread, but it never hurts to mention new technology. Probably many of you are familiar with coffee cups that change color when you put something hot or cold in it. There is a family of pigments, which can be used with ceramics which change color with temperature. They are known as "thermochromic pigments". There are ALSO liquids, which would have to be put in a clear capsule which is rugged enough to bounce around in a drum. The liquid must be allowed to dry before sealing the ends. The Tempilaq product listed below "goes from a solid, to a liquid at it's target temperature". You paint it on the inside of a glass tube (and let dry), and then seal the ends of the glass. The Ceramic pigments are different, they change color at the target temperature, but fewer temperature ranges are available for it.

I have not had luck finding one with the right temperature range, but If I do, some small "beanlike lumps of clay" will be made and fired with the thermochromic pigments. Target temps I am looking for are dry, first crack, and second crack.

This may be an avenue for some enterprising soul can create a side business for the roasting community. I know me, I would rather give away a good idea (and see it done), than put in all the hours to bring it to fruition.

Below is a link to examples of the liquid based tempilaq (they are not meant to be fired into ceramics, they simply change to liquid at the target temperature).

Edited by addertooth on 08/26/2022 9:41 PM
You forget a little detail, for coffee roasting the temperatures values themselves are of little interest, what is most important is the variation.
190C could read 205 C on others machine, and both be right... but for both is more important to know at that point if the temperature rise firmly, and which rate.
8 vs 4 C per minute could be the difference between a well roasted and a baked coffee.
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