For those that were interested in this thread another approach has been developing at homeroaster.org
rate of rise

Ed- thanks for the link. What are your thoughts on that design vs yours?

The hardware seems very good with many more capabilities. I'm going to be speaking with Bill today on capabilities and the software/computer design part. So far with my experiences with live time RoR (rate of rise) gives me numbers to follow that I find easier to wrap my brain around rather than a live BT/Time graph line. I'm starting to see actual bumps and hollows in the beans heat demands that I'm assuming relate with various reactions happening during the roast.

Attached below is a screen captured (just before ejecting the beans) using the new roast logging system described on the Homeroasters.org thread that Ed referenced previously.

The yellow trace (channel 2) is ET. The red trace (channel 1) is BT. Scale is 0 to 500F for both ET and BT.

The green trace is BT-RoR (bean temp rate of rise). The scale for RoR goes from 0.0 to 50.0 degF per minute.

There was an interesting event that I observed during this roast on my Hottop. Even though I've been watching RoR on my analog meter for several months, I had never really picked up on this before.

There is a very sudden and large drop in the BT-RoR at first crack. Much more than I can explain by the fact that the gap between ET and BT is constantly getting smaller.

On the graph, you can see how the trend in the green line drops dramatically at a point just after 10 minutes into the roast. This drop followed the first snaps of FC by about 30 seconds.

While the RoR was dropping a like a rock, FC was really rolling along. Once FC started to taper off some, the RoR trace returned to its previous trendline.

I always thought that FC was an exothermic process. This graph clearly suggests that this not the case. What's going on here? Ed? Anyone?

Jim

(EDIT: Doh! I've answered my own question. The flat spot in the BT trace, and drop in RoR, must be due to the latent heat of vaporization of the water as it turns to steam and cracks the beans. Right?)

Jim,

I bumped into this post by Ken Fox (second post) which doesn't explain the why's of the phenomenon but is interesting. I've never found anything written on why, if beginning to end of 1C is supposed to be exothermic, it requires more energy input at its start.

Timing first crack heat reduction

Allen

The way I've looked at first crack is that at the start of first the beans surface temp. (what we measure in the bean mass) is higher than the temp. of the moisture being released. Thus we see a drop in RoR(rate of BT rise). Enough added or in some roasters stored heat is needed to overcome this and keep a positive momentum to the roast. As first nears end the beans no longer releasing the mass of moisture and again taking on heat more easily and is the cause of runaway roasts if heat input is not controlled at that point.

Then the claim that exothermic reactions begin at the same time as the beginning of 1C is most likely correct.

It makes sense how the release of vapor would cancel out the additional internally generated heat.

Have you found anything written about this (slowdown at 1C) other than in forums?

Allen

Gday all !

Just came across this excellent thread which prompted me to join your forums.

I've been home roasting for a few years now with a "Corretto" setup (heat gun on tripod, into converted breadmaker, temp monitored by DMM) and have been looking for a method of monitoring rate of change in a standalone tool like a DMM. To date have been manually recording temps at 30 second increments on a homemade spreadsheet & mentally calculating rate of change.

Looking forward to seeing further progress!


Regards & cheers.................Sean