Temperature of the drum during roasting

by **dustin360** on Sun Nov 27, 2011 7:04 pm

One important variable in roasting than seems to be missing is drum temp. It makes since to me to know, ambient, bean, and drum. I think there would be alot to gain by knowing the drum temp. But the drum seems the hardest to take the temp of. I installed a naked thermocouple that drags along the drum on my quest. But it seems to give me a drum/Met instead of one or the other.(i haven't giving up on the location yet, more experimenting needs to be done) So my question is, whats the best way to get drum temp? What about insulating the side of the thermocouple thats exposed to the air? Any out there successfully taking drum temps during the roast?

by **another_jim** on Sun Nov 27, 2011 7:42 pm

Why do you think the drum's metal temperature is critical? I'd be more inclined to think the the temperature of the envelope of air that surrounds the drum is critical. The rotation and the vanes of the drum are designed to minimize drum to bean contact, and to maximize the flow of the beans through this envelope of air heated by the drum. This is what most environmental temperature sensors measure; and it souns you were getting this with the sensor you found unsatisfactory.

In general, there is a very simple way to tell whether a roasting sensor is useful or not. A useful sensor's readings will distinguish between a certain class of bad roasts and good roasts. A well place environmental sensor will tell you if you are exceeding a bean's upper limit (which is lower for low grown and DP coffees) after the beans dry and go into the first crack. How would the drum sensor improve on this?

by **dustin360** on Sun Nov 27, 2011 8:47 pm

Heres my thoughts on why drum temp would be helpful, one is the charge temp of the roast. Knowing the drum temp here seem to help with not overloading the drum with to much heat. Seems knowing the drum temp here would be a lot more helpfull then the MET or Bean. At this point both these probes are only telling me air temp. Only once these temps flat line for a while do i have a reasonable guess to what the drum temp is. On the first roast on the day the drum is going to have to catch up to air temp, and on successive roasts the drum is going to be hotter than the air temp(once ive blown some hot air out with the element off).

One of the best things about the quest(in my eyes) is the ability to hit the brakes on the roast, but not stall. I found this a lot harder on the hottop. So the second reason a drum temp would be helpful is at the end of the roast. At the onset of first crack I blow out a lot of hot air out of the roaster and turn the element down low. I then control the temp with the fan. The element is almost off, but the drum has alot of energy left in it. So at this point the drum is a big part of whats keeping things moving along. But the drum only has so much energy in it, and sometimes I exhaust it(stalling the roast), which leads to me cranking the element way up. If I knew the temp of the drum I could avoid this.

And just to be clear I use a bean temp and MET (maxim environmental temp) to control the roast now.

by **chang00** on Sun Nov 27, 2011 9:56 pm

If you really must know.......use an infrared thermometer. Generally though, I don't think it helps much.

by **dustin360** on Sun Nov 27, 2011 10:31 pm

An IR thermometer would only work if I had a direct line of sight to the drum, which I dont. Just curious chang00 have you measured the continuous temp of the drum during roasting before and found it not useful?

by **another_jim** on Mon Nov 28, 2011 12:02 am

dustin360 wrote:One of the best things about the quest(in my eyes) is the ability to hit the brakes on the roast, but not stall. I found this a lot harder on the hottop. So the second reason a drum temp would be helpful is at the end of the roast. At the onset of first crack I blow out a lot of hot air out of the roaster and turn the element down low. I then control the temp with the fan.

That's good technique. I use the air temperature just outside the drum to do the same thing. It runs up to about 525-550 as I ramp up the first crack. Then, I turn off the heat and keep the fan flat out -- this pushes the heat from the drum to the bean and powers it through the first crack, while the ET I measure drops down to 475-500. Then I drop the fan and raise the heat to finish the roast without the run away effect of having a lot of stored energy.

So I think we are on the same page when looking to get an estimate of the stored energy in the roaster, and discharging most of it to power the first crack, then having the roaster more responsive to direct heat changes afterwards. Only I think the stored energy in a roaster is so closely bound up with the ET as measured like I describe that a second measure is unnecessary.

But if you find you get a stored energy reading that isn't completely correlated with the ET, but adds independent information, then there is a possibility it can be used to improve roast quality.

by **farmroast** on Mon Nov 28, 2011 12:23 am

another_jim wrote:........the temperature of the envelope of air that surrounds the drum is critical.

+1 measure the MET at a spot relative to where the most heat transfer to the beans is occurring which is within the envelope close to the drum as Jim mentioned.

by **dustin360** on Mon Nov 28, 2011 2:08 am

Ya, thanks Jim and Farm. Just to be clear Im not trying to replace the MET thermocouple in my M3, just supplement it.

So Jim are you saying you that the MET and the drum temp are so closely intermingled that there would only be a negligible difference between the two? Obviously I dont know, thats why Im trying to take the temp of both.

by **another_jim** on Mon Nov 28, 2011 2:57 am

It's my guess; and if I were a bookie, I'd be willing to lay odds on a correlation of MEt to drum of over 90%.

But nobody knows for sure until its tried. For instance, if the air temperatures front to rear evens out a lot more quickly than the drum's, and the drum holds a lot of heat, the measure would add information.

by **farmroast** on Mon Nov 28, 2011 9:51 am

Dustin, It's well worth doing what your doing. I spent a lot of time moving TC's around to learn how heat was acting in my roaster.


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