Agtron Scales

[rojo]

hey,

i recently heard people talking about different Agtron scales... meaning people using the Agtron getting different numbers for the exact same roast. Can anyone help me understand what the different scales are and how they relate to one another.... Like how some people can say their french roast is 40 while others a 25???

Anything helps...thanks.

[ckornman]

I believe the main difference is a 'gourmet scale' versus 'commercial scale.' Gourmet is geared towards lighter roasts, I believe. The Agtron is also fairly sensitive to how well it is calibrated: a small difference in the reference number used to program the machine with the discs can result in a large difference the further away the numbers get from the reference set. E.g., if your discs are say 35 (high) and 7 (low), but you program them in at 36 & 6, your lighter roasts will vary by much larger degrees from their accurate reading, as much as say 10 points. A while ago, Mr. Staub was kind enough to send us some freeze-dried grounds with predetermined gourmet & commercial agtron numbers, which we've been using to periodically check the calibration of our machine with ever since.

[rojo]

so gourmet scale numbers look higher in general than commercial numbers for the same roast?

[ckornman]

Typically, yes. For example, one of our reference sets is marked for Commercial Scale: 28.9 and for Gourmet: 37.4.

[c.hallien]

The Coffee lab I was employed with published a document several years ago entitled "Mysteries of The Agtron" to assist in the understanding of this piece of equipment. Agtron does have 2 scales; Commercial (0-75) and Gourmet (0-100). The explanation I've been provided is that the Gourmet scale allows for higher resolution to identify roast development than that of the the Commercial scale. "Commercial" roasting operations, having tighter tolerances within their specs, actually benefit from a compressed scale. As the title of the document indicated, the Agtron is misunderstood instrument and thus highly debated among some of the industry folk; I highly endorse the use of it when properly calibrated, maintained and applied but it can cause almost as confusion while actually trying to provide a standard for evaluating roast development. Agtron measurements are influenced by
1. Grind size
2. Time since roasting
3. General Sample preparation
in practice you need to standardize the sample preparation and grind (the grind particle size distribution is spec'd in the Agtron manual and approximates a "drip grind"). Agtron measurements should be performed within 20 minutes of the completion of the roast and only after the coffee is fully quenched (cool to the touch). I personally have documented the influence of sample preparation to be as great as 10 points. In addition there is a darkening factor that we have documented, that occurs over a period of time; meaning that a 70 (Gourmet) = 53.7 (Commercial) BUT in this number will change over a period of weeks to a lower number.
The message here is to standardize your sample prep and try to get the proper info on the Agtron before relying too much on it as a measurement. When applied properly it is a very good supplement to other QC practices but on its own and without thorough understanding can cause some issues and a false sense of security in production.