What is this thermocouple reading?

Discuss roast levels and profiles for espresso, equipment for roasting coffee.
popeye
Posts: 340
Joined: 18 years ago

#1: Post by popeye »

Hi all, I just added some additional thermometry to my roaster and I've got a question that i wanted to throw out and get some 2nd opinions on. First, the picture:

A lot of stuff here, but the relevant lines are bright red (ET thermocouple) and pink (alternate ET thermocouple). My question is what is the pink line reading that it drops suddenly at charge and remains lower through the finish?

Some additional info:
Roaster is an diedrich IR-7 - solid drum of about 14 inch diameter, rotates clockwise.
Greens were 2lbs beans (about the minimum i run through the roaster).
The ET thermocouple (bright red) is an exposed thermocouple placed at the 11 oclock position halfway between rotational axis and drum surface.
The Alt ET thermocouple (bright pink) is an exposed thermocouple placed at 2 oclock position, about 2 inches from drum surface and 5 inches out from the axis.
An additional ET thermocouple (faded red) is of the grounded type, but physically seperated from the metal of the roaster. The tip of the probe is about .3 inches from the ET thermocouple. (This all started because I'm upgrading and getting ready to pull this thermocouple.)
All thermocouples are long enough that stem effects shouldn't be a problem. (1/8" diameter thermocouples, protrude at least 2 inches into the roaster.
Of course, blue/faded blue are BT, delta BT. Green is airflow, yellow is gas.

The roast was normal, except I had a little difficulty when I turned the PID on at about the start of first crack and airflow got a little shaky. Interestingly enough, this is reflected in the ET and additional ET thermocouple, but not the alternate ET thermocouple.

I have a guess, but i don't want to bias anyone. I will post my guess after a while, but it's really a guess at this point and i want to see what everyone else thinks. Thanks!
Spencer Weber

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farmroast
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#2: Post by farmroast »

quickly read, quick guess, basically indicated stored energy of the drum.
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TomC
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#3: Post by TomC »

Notice your sensitivity of your bright red ET, placed midstream of the highest area of airflow in the middle of the drum, it's transducing the blips you create by throttling/cutting your airflow (you did it twice, once just before 11:00, and the second time shortly after).Your Alt ET (pink) in question is clearly more dialed in to the surface temp of your drum, not showing the slightest blip from your airflow changes. Like Ed mentioned, I'd just trend that one for data about the energy your drum has at any given moment. I'd expect to see that caved out initial waveform on that graph since it's shedding so much heat to the cool bean mass initially.
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dogjamboree
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Joined: 12 years ago

#4: Post by dogjamboree »

I agree with the others, and I'm envious of the readings you're getting. I've been trying, without success, to get good differentiated exhaust and drum temperature readings. My issue so far is the thermocouple hitting the drum vanes. I suspect the larger size of your roaster makes this a bit easier than it is on my 1-pound baby machine.

frank

popeye (original poster)
Posts: 340
Joined: 18 years ago

#5: Post by popeye (original poster) »

Well thanks for the replies everyone! Until yesterday, my guess was the same as everyone else's - that i was (luckily) somehow reading drum temperature, or at least a useful component of it. I'm not so sure anymore, because i've realized that the pink probe may be influenced by the air intake on my diedrich. Why would the incoming air drop suddenly on charge? Because on my roaster, the incoming air is channeled around the burners, up and around both sides of the outside of the drum, then, at the very top of the drum, it curls around and enters at the front top of the drum. So when the drum temp drops on charge, the intake air temp also drops.

I will shortly take delivery on an infrared thermometer to hopefully confirm drum temp (unless emissivity is a problem) and i will also re-position the pink thermocouple more fully into the intake air to see if the characteristics of that line intensify or abate.
Spencer Weber