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Poor TC placement- Drum (solid)
I'm almost finished making a solid (not perforated) drum roaster and due to the shape of the drum (see here for pictures) I cannot get the thermocouple (in a 18cm/7in sheath ) to actually touch the beans, instead it is just measuring the air temp inside the drum. Which leads me to my question, how will the temperature that I am measuring relate to the temp. of the beans? I haven't started roasting yet (variac problems), but at a constant 220V for 7+ minutes the temp in the drum gets up to 150C/300F. I put in a handful of beans and at this temperature I heard first crack at about 4.5 minutes. (I took them out a minute or two later, they were dark but hadn't reached second crack). Is there anybody who is measuring the temperature the same way and can give me some advice about this? Thanks
Edited by NetriX on 01/15/2019 9:44 PM
Welcome to the Forum! Welcome

I do not see any photos though.
With or without pictures, bringing into FC in 4-5 minutes with only 150C ET is weird science...
Could be many explanations, I would measure the drum surface temperature too, also let it preheat longer, at least 30 minutes, and also specify the rotation speed.
The most critical measurement on any roaster is bean temp. Seeing bean temp rate of rise is of utmost importance and without it, we're flying blind.

Most small drum roasters are a big challenge in finding a good bean temperature probe location due to all of the rotating obstacles. One method many of us use is to insert the thermocouple into as small as possible diameter sheath that's been bent to 90 degrees. The sheath stays in between the front plate and the rotating forward and reverse vanes with at least 3/8" between sheath and front plate and vanes to prevent lodging beans in between those parts. Locate the sheath tip so it resides in the bean mass about 45 degrees into the upward bean travel with bottom being 0 degrees. Its important to have at least 2" of sheath in the drum to prevent false readings from front plate to sheath conduction.
1/2 lb and 1 lb drum, Siemens Sirocco fluidbed, presspot, chemex, cajun biggin brewer from the backwoods of Louisiana
@allenb I am very interest in what you described but am having real trouble visualizing and understanding the positioning. Could you post photos, diagrams, links?

I have a small ss perforated drum.

Check this picture:

The BT probe should be inserted somewhere in the bottom part of the "Bean pile" text, as deep until is close to touch the vanes, and bent such way to be inserted in the bean mass exactly as the "Bean pile" text, at 45 degrees, with the tip of the probe in the direction of beans flow.

A possible right implementation is in the picture below, with probe mounted in the beans dump door
Look carefully how is probe bent and try to figure the position when door is closed.
Thanks but can't access 2nd picture - asks to login.

But re-reading seems the front plate mentioned is one of the extremities of the drum that does not rotate, right? Mine do rotate and are soldered to the drum...

I have a small video of the drum here:
Edited by dncpax on 05/01/2019 9:31 AM
Indeed, the link has a strange behaviour for me too... when posted here...
Please check the pictures in HB article page 3, post #30, link below:

Attached the picture as a file, if the link to HB still require login
Later edit: I seen the drum you are talking about... is open air, not cased inside any oven... so much more difficult to measure bean temp for such setup.
Even heat control is a lottery, imo.
As I see, the drum is mounted using two flanges with hollow shafts, the best you can do is to insert there a custom a TC probe inside a custom sheath made from bent brass pipe.
PS: your drum doesn't look solid Grin
renatoa attached the following image:

Edited by renatoa on 05/01/2019 10:06 AM
Thanks for taking a look.
I have bought of buying a metal tube from a hardware store and bend it . Didn't have a clue on which metal though. So brass is good.
I have a termopar that I already melted by having it loose in the drum. Do you have a suggestion for the kind of probe I could use in this scenario?

I know it's not really a drum but I am just starting toying with roasting....
We should figure a way how to mount the probe on the drum support...

Do you have any instructions ? What are they saying to do at the end of the roast? Is the drum designed to be removed for beans dump, or this operation is with drum in place, without removing?
Asking because this is important for probe mounting choice.

I guess your machine should look as this, when mounted and ready for use
renatoa attached the following image:
yes, that's exactly how my roaster looks like.

To dump the beans I stop the motor, open the "window", and turn the motor on again. The beans drop when the windows turns downwards. I don't remove the drum from the support at all.

One of the shafts (opposite the motor) is hallow, so that's where I inserted my thermopar probe. I clipped on a weight on the end (a tea "ball") so it tumbled along with the beans ;) I thought that was smart but the metal in the tea ball melted the probe plastic sheath. This is the probe I used type k:

A big difficulty I'm having is avoiding the rotation twisting the probe cable... unless I can put the measuring unit case "glued" to the top of the drum to make it solidary with the drum movement... I also looked for wireless probes imagining there would be something I could just stick inside the drum, but I guess that doesn't really exist...
I do not know how to link to my build thread but if you look you will see where i put my TC. On a drum you have two different veins. The one next to the drum pull the beans forward the one on top of it push the beans to the back. This serves two purpose it constantly mixes the beans so a bean does not sit next to the drum too long but also dumps the beans when you open the door. this also make a small pile of beans next end. You want your probe in that pile. The return veins start 1 inch or so away fron the front drum face. this gives you room to put the probe. Mine is between 8 oclock and 9 oclock on the drum face. I hope this helps.

Sorry I did not read all the post before I posted this, this will not work with your design. This is how a conventional drum roster works. I believe you could learn to roast without a probe into the beans but its better if you can get one.
hopefully you will be able to see and smell the beans. The smell of baking bread is a very good indicator of where you are in the drying faze. I know a commercial roaster that logs that on every roast he feels like its very important information.

Link to Build:

Edited by allenb on 05/03/2019 1:07 PM
All you need in life is ignorance and confidence, and then success is sure. Mark Twain
In this particular case I would be worried also about heat source consistency and control...
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