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difficulty with the end of roast profile
Howdy all,

I'm always wondering where a thread belongs so please let me know if this place is appropriate.

HG BM, modified such that the exhaust exits out the side of my extended bowl. HG is on top with a tight fitting lid. There is a TC which used to be very low in the bowl (0.25" up from the bottom) entering through the side. I have since moved the TC up where it's about 1.75" up from the bottom. I did this because I was getting some low temp readings and wanted to get readings more similar to others who have their TC closer to the hot air. Since the move, the readings are more in line with others. with 8 oz charges the TC is perhaps 70% covered in time with greens as the paddle moves and coverage increases as volume increases.

I'm having a funny thing happen where at the end of the roast the TC reading stalls. I'm using a HF router controller where i marked the wheel for amp draw, with a maximum of 5 (Yes the HF HG doesn't put out its rated 1500W, shocker). Here is the last roast data: (temp in C)

time(min), TC (C), Heater(amps)
0, 100, 3 (chamber pre-heated)
3, 132, 3 (heat manipulated to keep the temp at 132)
4, 132, 4 (start of constant power)
6, 175, 4
8, 196, 4
9, 204, 4 (first discernible crack @9:40)
10, 211, 4
11, 216, 4
11+, temp stalls at 217 and i get freaked out

Things seem reasonable except at the end of the roast where the power remains the same, exotherm should be happening but the temp stalls. I'm describing a huge decrease in ROR at the most critical point. I have pushed past this in the past, waiting for temps of 230 or so but i may be over roasting at that point. Any thoughts to this "wall"

added data: Colombian, 8.00 oz start, lost 1.03 oz

I use a similar heat setup as you with the HF heatgun and router controller and have been able to hit 232C (450F). Not that I do that with any regularity mind you Grin

You do have a larger roast chamber and exhaust port though. Maybe try insulating the chamber or restricting your exhaust a bit.
I've got power to spare. The log I've listed shows 4 amps as the max but I can go up to 5. You might have a point though, as the temperature increases the delta vs ambient is greater and potentially could be what i'm seeing. I just expected to need to taper power over time not increase it. To your point, perhaps a more restrictive exhaust.
Mad Mac
Here in the tropics, with a covered bread maker with the TC near the cover, I hit City+ to Full City at 231C.

My brother at Portland OR taught me a neat trick. Rather then end of roast temp, he uses the temp at which the first 5 beans start first crack. From that temp, he noted the delta temp to hit the roast levels he liked.

I tried that delta technique and it worked. Pretty consistent. However, since I roast the same beans and ambient temp and humidity had been consistent, I just use end of roast temp. Perhaps when the weather turns colder, I'll go back to the delta method.

For my roasts, I hit City with an 11C delta. Full City with 15C delta. End of temp City 226C and Full City 231C.
I was able to overcome stalling problems by improving exhaust flow. Exhaust is routed to envelope the roasting pan. Any insulation on the outside of roasting pan will prevent this. Exhaust exits bottom through outer walls. But first, it goes around the outside of the roasting pan. This fixed my stalling problems. If your router controller is defective try toggling your power switch between full and variable at the crucial stages. (Until you can replace it) .
Bad coffee prevails when good coffee roasters stand by and do nothing.


AMAC wrote:

I've got power to spare. The log I've listed shows 4 amps as the max but I can go up to 5. You might have a point though, as the temperature increases the delta vs ambient is greater and potentially could be what i'm seeing. I just expected to need to taper power over time not increase it. To your point, perhaps a more restrictive exhaust.

Ok I think I see what your concern is more clearly now, it's not that you can't hit higher temperatures, but that your expectation is that your temperature should still be increasing at a given heat input or even less. I would say that is a reasonable expectation because that has been my experience.

In my setup with 3/4 lb roasts, I preheat to 380F and charge. The speed control is set around the 3 o'clock postion. I usually hit "yellow" somewhere between :04 and :06 minutes into the roast at which point I usually bump the controller to around 4 o'clock resulting in a 25F or so ROR which starts to decline as I approach FC. I usually hit first crack around 385F to 395F (:09 - :11 minutes) at which point I'll dial back the heat (sometimes a bit sooner) to about 2 o'clock and open the lid to the roaster to bleed off heat until I get about a 7F-10F ROR. Then I will tweak the controller/open the lid to maintain that ROR until end of roast, typically somewhere between 420F - 430F.

I typically experience what is being discussed in the roasting sub-forum as a "vapor flash" after the onset of FC, for me the ROR declines somewhat sharply and may even go negative. I also typically have reacted as allenb discussed and juiced the power to avoid baking the roast. The last few roasts I resolutley kept my hands off the power during this phase and let things recover naturally to my usual ROR, which they did although I don't recall the time interval. So for now I'm going to go with a "Don't fear the flash" mindset, I'll have a better idea this weekend once those particular roasts have had a proper rest. So perhaps this phenomena, although probaly not totally responsible for what you are seeing is at least compounding the issue.
Edited by MerlinWerks on 10/30/2014 6:24 PM
Great feedback everybody. I'm going to restrict the exhaust a bit and keep my hand on the dial if necessary.
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