Installing ET and BT probes in a Hottop

Discuss roast levels and profiles for espresso, equipment for roasting coffee.
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Postby Dieter01 » Aug 30, 2010, 5:20 pm

After posing a few questions in a different thread I decided that I need to start measuring the temperature profiles while I roast. In order not to not stray off topic in the original thread I am starting this new one. Perhaps you guys could help explain some of the things that are a unclear and eventually help me make a shopping list !

I understand there are three terms: BT, ET and MET. Bean Temperature I assume is just a probe inserted somewhere in the bean mass. Environmental Temperature (if I got it right?) is the air temperature inside the chamber. This measurement is highly dependent on where you place the probe but I have seen installations both inside and outside the drum, high up and far down. MET has me a bit confused though... Is that the ET when its measured at the hottest place in the roasting chamber? There were so many threads on this but the closest ones I came across where this one and this one.

OK... Regarding the installation itself some people do like Randy Glass and installed the BT sensor through the bean loading chute. Its even possible to fit an ET sensor through there by doing like Max did. Another solution would be to drill a hole in the back wall and insert the ET probe there, like John did.

I'm leaning towards installing two sensors through the chute. Would that provide me with adequate data or would a different placement be more optimal?

Edit: This glossary was also useful, although it doesn't answer my question

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Postby cafeIKE » Aug 30, 2010, 6:15 pm

MET = Maximum Environmental Temperature

Each placement will yield different results, but as long as the probe positions are consistent from roast to roast, it should be fine.

You will have different numbers than another user with different probe placement, but you will also be roasting in a different location possibly with different voltage, ambient conditions and beans. As long as the numbers you get are meaningful to you and allow yours roasts to improve, then it should be fine.

A negative against using the chute is the leads sticking out the top of the machine making loading a bit more of a pain.

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Postby Dieter01 » Aug 30, 2010, 6:37 pm

From reading various threads it sounds like people are referring to MET as a continously changing value. What makes ET different from MET? If you had 5 ET probes in there, they can't all be measuring MET?

A second question... I am reading through the Scace Thermofilter Temperature Device thread. On page 5 Barry mentions that

barry wrote:i used to use a 1/4" probe in my roaster as a bean temp probe. i logged lots of data and looked at lots of curves. one thing which always bugged me was the turning point on my roaster (the shift from decreasing bean temp to increasing bean temp) was well over 2 minutes, when most experts i'd talked with said the turning point should occur around 90 seconds. hmmm... i was convinced my roaster was radically underpowered. a while back, i put in a couple of more probes, and decided all the probes in the roaster should be of the same type/size, so i replaced the 1/4" probe with a 1/8" probe. guess what? my turning point shifted to just over 90 seconds. so, did my roaster suddenly gain power? no, of course not. the thermal mass of the probe decreased, so the system lag was reduced. if i stuck a bare wire bead probe in there, the turning point might change yet again. so, the problem wasn't with the roaster, but with the measurement system.

Something to consider?

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Postby cafeIKE » Aug 30, 2010, 7:24 pm

The maximum temperature is the Maximum the beans are exposed to. Could be 700°F at the vent, but as long as ~500°F at the beans, it's cool 8)

I used a 1/8" probe in the beans for robustness. It has a 12x diameter to depth ratio, so response is tolerable. Bean temperature doesn't change that fast. Bean turn is about 1:10.

I used a bead probe for the ET so I could see what happens when it happens. ET turn is dependent on the roaster temperature at bean drop. The higher the ET, the faster the recovery.

Thermocouple response times in air @ Omega

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Postby Dieter01 » Aug 30, 2010, 8:27 pm

Sorry for being sloooow here... :oops:

Is it the ET reading in a hot area very close to the bean mass over time? Meaning that if an ET probe is positioned well it will measure MET over time?

Or is it a fixed value, say that for a certain roast the ET went from 200 to 280 and then back down to 230 -> the MET is 280?

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Postby cafeIKE » Aug 30, 2010, 9:24 pm

cafeIKE wrote:The maximum temperature is the Maximum the beans are exposed to.

Should have written "The MET is the Maximum the beans should be exposed to" :oops:

Carl Staub wrote:"Severe damage occurs to the cell walls of the matrix at distributed temperatures above 446 degrees F and bean surface temperatures over 536 degrees F The actual temperature values will change due to varying levels of other constituents. Second crack, associated with darker roasts, is the fracturing of this matrix, possibly associated with the volatilization of lignin and other aromatics. Under controlled roasting conditions, the bean environment temperature should never exceed 536 degrees F. A wider safety margin would be achieved by limiting the maximum environment temperature to 520 degrees F. These temperature limits minimize damage to the cell matrix and enhances cup complexity, roasting yield, and product shelf life. "
from Basic Chemical Reactions Occurring in the Roasting Process by Carl Staub

The ET is measured to ensure that it does not exceed MET, the maximum permissible temperature.

In the HotTop, the beans are heated by falling through the hot air and via mechanical contact with the drum.
I don't know the drum temperature as it gets to the bottom and the beans fall on it.

another_jim wrote:The environmental temperature is the hottest temperature the beans experience, their immediate heat source. The speed of the roast is determined by the difference between that temperature and the current bean temperature multiplied by a heat flow factor. The higher the airflow, the better the heat flow, and the lower the required ET-BT delta to maintain roast speed. If your environmental temperatures get very higher, they affect the outside of the bean, worst case charring them, best case deepening their degree of roast relative to the bean centers. In essence, the lower the required ET for a given profile, the more evenly the bean is roasted.

A fast roast, all other things being equal, will be better than a slow roast, since less of the aromatics are cooked off. Howver, things are rarely all-else-equal. In particular, a roaster has a speed limit for a given charge weight determined by the required environmental temperatures. In a drum, lower charge weights can give you faster roasts for the same ET, and it's your call how many extra roasts you want to do go with lower weights.
from Managing environmental temperature in the roaster

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Postby Sherman » Aug 30, 2010, 9:33 pm

MET is the upper limit of ET. BT is the measurement within the mass.

(e.g. You're cooking a roast in the oven. You set the oven to 175C. Due to the cyclical nature of ovens, the actual temp may fluctuate +/- 10C, giving you a reading of anywhere from 160-180C. MET is 180C. ET is the temp reading at any given point in time. BT is the temperature of the roast as it cooks. For medium-rare, say 55C.)

Your dog wants espresso.
LMWDP #288

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Postby Dieter01 » Aug 30, 2010, 9:56 pm

I appologize if this is turning into a MET discussion but I want to make sure I understand this...

In a different thread Jim said that
another_jim wrote:My goal was to hold the MET between 250C and 260C (490F to 510F) throughout. I started at the 375C drop in, 7.75 amp and 4.5 air setting recommended for 225 gram roasts. This turned out to hold the MET very nicely at the target without requiring any changes. After the second crack started, the MEYT started to climb, and I had to slowly run the airflow up to 7.5 to keep it steady. This produced a 5 minute roast finish, and an overall roast of 17 minutes. In any case, using air to hold the MET steady is a simple and effective profiling strategy.


Either he is mixing the terms (doubtful...) or I am (very likely heheh)... To me it doesn't make sense to speak of keeping MET between different numbers though. If I understand you correctly Sherman then once you reach 260 you can never go down and say your MET is now 250... Should Jim have said that he was trying to keep ET between 250-260 in order to keep MET below ~270?

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Randy G.

Postby Randy G. » Aug 31, 2010, 1:39 am

While doing some recent testing I installed two thermocouples into one of my Hottop roasters:
Here is a view (poor as it may be) into the roast chamber of the bean temperature probe. I made the lower bean probe using some thermocouple leads I had and an old stick thermometer. I bent it downward so that the beans would be caught between the probe and the drum, and to get a (theoretically) better reading than one mounted flush to the rear wall. Above the factory sensor is the ET sensor I added. It reads differently than the Hottop display. I think that because of the small area inside the drum, you can get ten different readings on ET by mounting the probe in 10 different locations. It doesn't matter. the ET probe will be most useful when learning how the roaster reacts to heating element power settings during the roast.. at least that has been my experience. That is particularly true later in the roast, between first and second and later when the beans have expanded and take up more room, "shading" the ET probe.

Here you see where the thermocouples came through into the electronics side.
Espresso! My Espresso!

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Postby Dieter01 » Aug 31, 2010, 3:32 pm

Randy, I think the Hottop sensor is correlated, ref this thread.

What did you mean by mounting the probe flush / directed downwards? Intuitively I would think that if possible the probe should be angled slightly upwards in order not to create a void in the bean mass below it? Ref the installation by JohnB, do you think yours is a better solution?

After giving this some more thought I think I will install the two sensors in the back wall instead of the bean chute. Now for the shopping list:

1 x HH506RA datalogger
2 x KTS-HH thermocouple probe (backup in case I decide to go with the chute installation)
2 x Omega XCIB-K163 for installation through the back of the Hottop
1 x OT-201-1/2 epoxy - is this what I am looking for?
1 x RS232 to USB converter. I understand this might not be the best converter available but if it fails I will try to find something locally, or even get me something like this.

I will also need fittings etc to make this work. I would like to order that at the same time but I am not sure exactly what I need. Since I live in Norway it would be of great help to just order everything from omega if they have what I need... I found these bushings, could something on that list be used or am I looking for something else?

I would also like to make one of these while I am at it:

I've been searching this jungle trying to find a thermocouple that looks like the one in the picture but I can't find anything that matches... Suggestions?