Posted Sat Oct 3, 2009, 8:27pm Subject: Re: Interesting thermocouple readings in modified Poppery
OK - I just went in with a probe to see what the differences around the various vanes were. No bean load, heater running full bore.
WOW! There is a great deal of variation. Even going just a few vane openings one way or another results in a 10-20° drop in the temperature of the air entering the chamber. I found the hottest vane entrances and will place the ET probe there.
Thanks Gary.
Dana Leighton - Espresso hack and CoffeeGeek moderator
wideasleep1 Senior Member Joined: 19 Feb 2005 Posts: 1,399 Location: San Francisco,Ca Expertise: I live coffee
Espresso: VBMDoubleDomo Grinder: Mahlkoenig K30 Vario Vac Pot: nope Drip: Bodum Press Roaster: IR1 and 2,SC/TO,Behmor
Posted Sat Oct 3, 2009, 8:29pm Subject: Re: Interesting thermocouple readings in modified Poppery
Related reading where Terry mentions 'kinetic' energy (my 'mass intertia') towards first crack and end of roast. http://www.ambexroasters.com/pages/articles/articles.html I like his concept of Total Energy (from roomtemp green to roomtemp-cooled roasted) for a given profile, and where roasters can control these parameters. :)
Posted Sat Oct 3, 2009, 8:39pm Subject: Re: Interesting thermocouple readings in modified Poppery
I believe I have observed some 'reactive' effects of the beans during roasting, but they are much more subtle and I think the high air flow of the popper swamps these subtle effects (tend to see it more at lower air flows)
One is during drying, when the 'vapor phase' of evaporating free moisture; there is a dip in bean rate of rise for the same ET-BT margin. (When BT is from 260-290F)
The other is the 'exothermic' response during first crack. I generally hold a constant ET through first crack (usually between 450-460F ET) As first crack is finishing up, there will be a sudden rise in BT of around 5F. This will tend to be more pronounced if the first crack is aggressive than if it is slow and drawn out in duration.
wideasleep1 Senior Member Joined: 19 Feb 2005 Posts: 1,399 Location: San Francisco,Ca Expertise: I live coffee
Espresso: VBMDoubleDomo Grinder: Mahlkoenig K30 Vario Vac Pot: nope Drip: Bodum Press Roaster: IR1 and 2,SC/TO,Behmor
Posted Sat Oct 3, 2009, 8:49pm Subject: Re: Interesting thermocouple readings in modified Poppery
I was just reading the related PID article, and happened to notice Kathi's Graph 4 (PID fine-tuned, to where she achieved a near-perfect profile for a 1# batch in a 15 kilo roaster!) also shows this crossover point: Click Here (www.ambexroasters.com)
Posted Sat Oct 3, 2009, 8:55pm Subject: Re: Interesting thermocouple readings in modified Poppery
Nice article by Terry.
For posterity and FYI, on my roaster (not sure if it will vary) the hottest vane entrances were at about the 7:30 position (if when looking down on the roaster the cord entrance is at 12:00).
Dana Leighton - Espresso hack and CoffeeGeek moderator
Posted Sun Oct 11, 2009, 12:46pm Subject: Re: Interesting thermocouple readings in modified Poppery
Update:
I installed the new probe at the hottest vanes, and it made a big difference. The environment temperature thermocouple now reliably exceeds the bean mass thermocouple by 90-120 degrees. It seems to really measure maximum environmental temperature. It starts out about 110° above, then reduces to about 90° just prior to first crack, and then post-first crack it exceeds by about 120°.
If I heeded Gary Frost's advice way back when - I could have avoided the confusion!
The difference between BT and ET seems pretty extreme, but it is predictable and seems much more realistic than the weird crossover from the cooler temp probe.
If I were to do it again, I might try finding a vane that is about an average of the maximum and minimum temperatures around the chamber. And, I just might! :) :) I considered Gary's solution of a steel mesh bottom "air mixing chamber" where the TC would be mounted, but I do not like the idea of a disruption of the rotation of the beans he mentioned.
Dana Leighton - Espresso hack and CoffeeGeek moderator
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