Deltas

[Slayton]

I've read a very little bit about Delta BT's, ET's, and MET's but not nearly enough to fully understand. Can someone please explain how they relate to what is happening temperature wise during a roast and how those measurements affect what you might change on the fly during a roast or knowing their measurements from a previous roast how you might change further roasts?

[SusanJoM]

There is a lot of talk in roasting theory about the ROR (Rate of Rise), or how fast the temperature is rising.  The Delta BT is the same as the ROR.  If you look at it it will tell you the RATE at which your temperature is rising (or not).

Current theory seems to be that the ROR should decrease steadily after the temperature peaks.  That means the temperature should keep climbing but at a decreasing rate.

Watching the Delta BT (either in an LCD or as a line on your graph) will let you know if that is happening.

And that's the sum of my scant knowledge on the subject ....I'm sure there are roasters here with more to add to that.

[Gregr]

Right- it breaks down to 2 things- how does it help you roast and how does it make the coffee taste better. First one is pretty simple, second... that one takes time and there's no beating old fashioned hands on experience. Solid info on the internet is kind of hard to come by but here and there experienced roasters have shared their wisdom; this recent article by Matt Perger is one of those pearls of wisdom:
http://baristahustle.com/lets-talk-about-roasting/
As far as helping during the roast- if you're good and quick at math you can watch the delta BT and calculate how far off your roasting milestones (dry point, first crack, etc.) are. For example, if the BT is 325F and the BT delta/RoR 25F/min, and if no changes were made the the fan or stove, first crack will come in a little under 3 minutes. Rough numbers, things don't always go in a smooth gradient but as the roast is progressing that quick calculation can tell you how on or off track you are from your pre-planned milestones.
The ET and MET deltas can also give you feedback, but I'm a simple roaster and really only use the BT- those other two will be studied at some point in the future. For now though, I get what I need out of BT feedback.

[Slayton]

Thanks for the info!!  That was exactly what I was looking for.

[jyalpert]

To add on to what GregR said, you can plan your entire roast with target RoRs.

If you want to reach 1C at 8:00, for instance, and you want a declining RoR, you could say:

- I will charge at 400F, which will give me a TP at 1:35ish of ~250Fish
- I will target a RoR decreasing through 25-30deg/min from TP to 300F, which should occur at around 3:30-4:00
- I will target a RoR decreasing through 20-25deg/min from 300F-375F, which should occur at ~7:00-7:30
- I will target a RoR decreasing through 15-20deg/min from 375F to FCs, which should occur at ~7:30-8:00
- I will target a RoR of around 8-15deg/min through FC, which should end at 410F and take around 1:30-2:00

Then you have boundaries and targets to watch throughout the roast, and you can adjust as you go.

[coffee_cat]

Rough numbers, things don't always go in a smooth gradient but as the roast is progressing that quick calculation can tell you how on or off track you are from your pre-planned milestones.
The ET and MET deltas can also give you feedback, but I'm a simple roaster and really only use the BT- those other two will be studied at some point in the future. For now though, I get what I need out of BT feedback.

Please note that a constant smooth rise in temperature will create a flat RoR rather than a descending one.

Cheers,

james

[hankua]

The delta BT/ET or the spread between the two, determines how fast BT is going to increase (ROR) BT is always trying to catch up with ET. The Huky ET probe is unique as its inside the drum rather than in the exhaust like most others.

By manipulating the ET where it's slightly higher than BT during 1C; it's much easier to control ROR. The Giesen temp. controller adjusts ET, not BT; and that Co. has outstanding engineering.

[SusanJoM]

So, if you were trying to get that value, would it be Delta (ET-BT) or Delta ET - Delta BT?

[SusanJoM]

I might be wrong, but I think this entry by Marko explains how to plot what Hank is referring to above

http://artisan-roasterscope.blogspot.com/2014/04/virtual-devices-and-symbolic-assignments.html