Coffee Roasting-Best Practices- Scott Rao [Book] - Page 8

June 14th, 2020, 7:13 pm

hankua wrote:I'm guessing this does not apply to the roasters here using smaller machines. Even the percentages are questionable if they're a simple calculate 2kPa/3kPa=%66 gas. Maybe someone can comment if Probat/Giesen heat dial are calibrated to energy output, where 60% is actually a representative number.

A 12kg on up roaster can absorb so much more heat than say a Huky; there's a lot of energy stored in the machine available to use. If one could enter 1c at 20% gas and still have 15%, 10%, and 5%; then maybe it's workable?

Trying to understand, when you say 20% gas you mean 20% of your charge kpa? Example charge at 2kpa and be at .4kpa at 1c?

I have ordered the book but has not arrived. I roasted my 22-23rd batch yesterday and it produced my best coffee yet, not good mind you but far better than previous. I charged at 360 degrees and 1.75kpa, by first crack I was at .99kpa. If I'm understanding I should be charging hotter and ending cooler? Which would make sense as my ROR was not steadily declining as I'm trying to attain. I'm at work so I don't have a graph to share with me.

Can't wait to get book, sounds awesome

#72: Post by **hankua** » June 15th, 2020, 9:51 am

Hi Vince!
Yes that's what I'm saying, and how my Artisan buttons are programmed. My question is more about Probat/Geisen or any machine calibrated to adjust heat by percentages. It was pointed out on the old Huky forum, the equations in gas laws do not line up with the simple arithmetic I'm using. If I turn the gas down from 3kPa to 2kPa it would be a mathematical reduction ending at .666 or 66%. I don't know how many btu's my burners put out at max 3kPa or 2kPa either for that matter. Me turning the gas knob down one notch might not be the same thing as an actual heat reduction from 100% to 66%.

#73: Post by **mbenedet** » June 15th, 2020, 8:51 pm

Commercial machines that use percentages are usually talking about valve position. Unfortunately, valves generally aren't linear, so 50% gas is not half pressure of 100% gas. Here's an example from a Loring I roasted on:

Scott's gas percentages, however, are meant to be in reference to the maximum for your machine.

#74: Post by **Almico** » June 15th, 2020, 10:51 pm

mbenedet wrote:Scott's gas percentages, however, are meant to be in reference to the maximum for your machine.

Yes. My machine has 12"wc available. I consider that to be 100%. At 5"wc I am at 42%, 4"wc = 33% etc.

The tricky part to master is getting a feeling for BTU percentage reduction at both ends of the dial. In other words, reducing pressure by 1"wc from 10" to 9" is a heat power reduction of 10%. Reducing pressure 1" from 2" to 1" results in a heat power reduction of 50%. That is why an accurate meter and a fine control needle valve is most helpful, because down low on pressure at the end of a roast is where the magic happens.

#75: Post by **hankua** » June 16th, 2020, 12:26 pm

Here's the quote from the old Huky forum, thanks to Eugenm. At the time there wasn't any discussion on Bernoulli's gas law, and I'm ignorant on the subject.

I would like to share a bit of knowledge gained in my line of profession that would help understand how gas flow varies with the total pressure of the system.

I guess at one point everybody asked the question how much should the pressure of the gas has to be lowered in order to lower the heat power by a certain amount. This can be easily calculated using Bernoulli's hydrodynamic law.

Let us assume that at the beginning of the roast, when we drop the green beans in the drum the initial gas pressure is P1 (kPa or mbar or psi). This pressure corresponds to a gas flow F1 (m3/h or l/s or cfm) that generates a total heat power which we will denote Q1 (kW or BTU/h). The total heat power is directly proportional with the gas flow meaning Q2/Q2=F2/F1, where F2 is a new value for the gas flow which generates a total heat power Q2.
Bernoulli will help us understand that the ratio between the a new value of gas pressure and the initial gas pressure value is equal with the square of the ratio between the new gas flow value and the initial gas flow value : P2/P1 = (F2/F1)^2

Let us analyze a simple scenario : we have an initial gas pressure at which we charge the drum with green coffee beans of P1=2kPa. This pressure will corresponds to a gas flow F1 that generates a heat power Q1. We want now to reduce the heat power by 50% and we want to learn what the total gas pressure P2 would have to be :
We want to reduce the heat power by half which means Q2/Q1=0.5 ; That means the ratio between the new gas flow and the initial gas flow is also 0.5 : F2/F1 = 0.5
At what total gas pressure we will have the F2 gas flow ?
P2/P1=(F2/F1)^2=0.5^2=0.25 which means P2=P1*0.25=2*0.25=0.5kPa

The conclusion is that in order to reduce by half the initial heat power we need to reduce by 4 times the initial gas pressure !!!

#76: Post by **drgary** » June 18th, 2020, 12:18 am

Related to earlier post about suggested charge BT: He's suggesting that charge temperature for machines of 500 gm to 1 Kg capacity. I generally charge at 400 to 450 but do a long soak, and my drum is perforated. My perforated drum may be harder to overheat than most so that reducing or eliminating the soak and setting gas at maximum for that roast at the start is an alternative strategy he suggests so that a lower charge temp would make sense. I'm not suggesting this is the right method for my roaster. Haven't tried it. Then there's the difference between my probe readings and his. Mine tend to read higher. But so far I'm very impressed with all of the aspects covered in the book with a minimum of excess verbiage and very helpful illustrations.

June 18th, 2020, 9:01 am

FWIW, the following is feedback, not right or wrong, good or bad, and not criticism of the book

I would have liked more precision in the book for learning purposes. The book assumes a 375F FC (confirmed in an email) but didn't state that so you have make adjustments to the tables, including the ones for charge on page 26 and gas on page 45. The page 48 soft and hard crashes are not well defined. The gas percentages on page 95 are without a reference point to max gas or as described above for pressure changes.

Further the book makes it sound like there is only one and one only gas dip method and calls it high risk. @drcraig has been experimenting with this on his North for quite a while and schooled me on this back in 2018 when I first got mine. From my experimentation, I don't think there is only one way and I also don't think raising gas back to 100% of where it was applies to all situations and all coffees. My experimentation has suggested that returning it to around 40-60% of the dip works better. Said another way, try correlating the gas dip with the suggested gas reductions on page 45. Map out the timing of these. Maybe the return to less than 100% is your next gas adjustment. I also don't believe 20 seconds is a magic number that works for all beans. I have ranged from 15-45 in the past. The book also calls it high risk that can ruin a batch. Sounds dramatic; but any worse than the crash you are trying to fix?

So I encourage everyone to dig deeper here and experiment as the implementation of a number of things is not as clear as they could be. If anyone has further observations or learning on these points, please comment.

Once again this post is feedback and not criticism.

#78: Post by **drgary** » June 18th, 2020, 3:36 pm

I don't know if I want more precision. I like that Scott Rao is identifying things to think about and to try. Precision would be hard to achieve with the great variety of roasters out there, and this includes the size of the roaster. His book is obviously aiming at commercial, production roasting, mostly on machines much larger than we are using at home. The kinds of takeaways I'm getting include thinking about what my gas settings actually mean. I haven't previously thought about the BTU rating of my burners or that when I'm adjusting heat between numerical settings the difference in heat applied is not linear. I also hadn't known I could identify the start of first crack by watching the probe in the exhaust path. Now I'm going to reconnect that to the Phidget. There's plenty more when I have time to go over the book and think about it. I very much agree with the following:

CarefreeBuzzBuzz wrote:Further the book makes it sound like there is only one and one only gas dip method and calls it high risk. @drcraig has been experimenting with this on his North for quite a while and schooled me on this back in 2018 when I first got mine. From my experimentation, I don't think there is only one way and I also don't think raising gas back to 100% of where it was applies to all situations and all coffees. My experimentation has suggested that returning it to around 40-60% of the dip works better. Said another way, try correlating the gas dip with the suggested gas reductions on page 45. Map out the timing of these. Maybe the return to less than 100% is your next gas adjustment. I also don't believe 20 seconds is a magic number that works for all beans. I have ranged from 15-45 in the past. The book also calls it high risk that can ruin a batch. Sounds dramatic; but any worse than the crash you are trying to fix?

So I encourage everyone to dig deeper here and experiment as the implementation of a number of things is not as clear as they could be. If anyone has further observations or learning on these points, please comment.

Finally, this book really is for people who have been roasting for awhile and are familiar with some or most of the terms and techniques we've been discussing, including how we're trying to understand roast profile readings presented by roasting software.

June 18th, 2020, 5:06 pm

@drgary,

maybe precision isn't a precise enough word

or bad choice of word. Maybe underlying assumptions is a better descriptor. I would hate to see people who have FCs at 390 use his temps as is, and it would help to understand the assumptions underlying hard vs soft crash.

I agree that he can't cover all the roasters and roast operators.

#80: Post by **drgary** » June 18th, 2020, 6:34 pm

Michael:

He also writes something to the effect that even using all of his techniques, we are not at the point where even a professional roaster can achieve identical batches of the same coffee.