Burr RPM affects grind particle size distribution significantly

Grinders are one of the keys to exceptional espresso. Discuss them here.
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TomC
Team HB

#1: Post by TomC » Mar 23, 2014, 10:04 am

The folks at Black Water Issue recently posted some graphs detailing the significant difference in particle size distribution that occur on the same grinder, same coffee, same setting, same burr, etc, at different RPM. Makes me want to maybe keep/use my HG-One after all. 8)

Image

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dsc

#2: Post by dsc » Mar 23, 2014, 11:03 am

What's Slow / Medium / High in RPM?

Guess it wasn't such a bad idea to have the ZR-71 grinding at 50RPM:)

Regards,
T.

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RapidCoffee
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#3: Post by RapidCoffee » Mar 23, 2014, 11:15 am

Speculation: the beans fragments bounce around more at higher rpm, which causes more breakage, smaller particles, and more fines.

Perhaps one day we'll see grinders that allow us to manipulate the particle distribution by altering the grind speed. :shock:
John

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TomC
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#4: Post by TomC » Mar 23, 2014, 11:15 am

You'll have to translate the page

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dsc

#5: Post by dsc » Mar 23, 2014, 12:10 pm

RapidCoffee wrote:Speculation: the beans fragments bounce around more at higher rpm, which causes more breakage, smaller particles, and more fines.
If this was hopper fed, nothing should bounce.

Regards,
T.

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RapidCoffee
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#6: Post by RapidCoffee » Mar 23, 2014, 12:28 pm

Maybe "bounce" is the wrong term. I'm talking about the passage of bean fragments through the burrs during the grinding process. At higher rpms, the fragments will accelerate to higher velocities, and find it harder to escape between the burrs. So they "bounce" around more, causing further fragmentation.

Pure speculation, natch. Have you a better theory?
John

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another_jim
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#7: Post by another_jim » Mar 23, 2014, 1:29 pm

I hope it's right; since it would be a very simple way of improving grind quality.

But I share John's scepticism. You need different grind adjustments if you use a hopper and no hopper; why should it be different with changed grind speeds? The thing that has been missing in all these wonderful smoking gin diagrams is grinds that actually work for espresso. Once that is done, the guns usually stop smoking.

In this diagram, if the fast grind works for espresso, the slow down wundergrind is probably a gusher. The wundergrind has a coarser, more tightly distributed large particle distribution and less fines. Therefore, it probably resists the flow less and will gush. Note that tighter distributions and less fines is also what happens when you adjust the grinder more coarsely and do not change the speed.

But I hope I'm wrong; but you need to compare low and high speed grinds that actually flow the same at the same dose. I very much doubt the illustrated grinds do that.
Jim Schulman

jedovaty

#8: Post by jedovaty » Mar 23, 2014, 1:51 pm

If your grinder's motor runs on AC, you might be able to get away with something like a router speed control to test the varying RPMs, or possibly a variac/autotransformer?. Those who know electronics will probably cringe at the thought of the RSC, it's probably not good to use long term (I don't know much about electronics - so, before you do anything with this, consult with someone who does!), and it is possible torque may be affected. If there are other electronics on board, they may fail. Can be cheaper with coupons at Hazard Fraught (for those in the US). Variacs tend to be expensive. Maybe I'll give it a shot with my grindmaster and variac.

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RapidCoffee
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#9: Post by RapidCoffee » Mar 23, 2014, 2:43 pm

another_jim wrote:In this diagram, if the fast grind works for espresso, the slow down wundergrind is probably a gusher. The wundergrind has a coarser, more tightly distributed large particle distribution and less fines. Therefore, it probably resists the flow less and will gush. Note that tighter distributions and less fines is also what happens when you adjust the grinder more coarsely and do not change the speed.
As Jim notes, finer grinds cause more bean breakage and generate more fines, leading to increasingly bimodal distributions. But it's possible that changing rpm is another parameter to be manipulated. For example, instead of coarsening the grind setting for non-espresso brewing, you might decrease the grinding speed. Either of these changes would coarsen the grind size and generate a more unimodal distribution.

Perhaps these factors (grind setting, bean load, grinder rpm) are really just one parameter, and by careful manipulation of grind setting you can get the same results as changing rpm. There's no way to tell from the plot.
John

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dsc

#10: Post by dsc » Mar 23, 2014, 2:46 pm

RapidCoffee wrote:Maybe "bounce" is the wrong term. I'm talking about the passage of bean fragments through the burrs during the grinding process. At higher rpms, the fragments will accelerate to higher velocities, and find it harder to escape between the burrs. So they "bounce" around more, causing further fragmentation.

Pure speculation, natch. Have you a better theory?
I haven't but I don't know anything about the method used for getting those distribution graphs. Was this done using sifting or microscopy photography or a mixture of both?

Also was this done a single grinder with a VFD, something custom built or on different grinders with different burrs sets? In this case a picture isn't necessarily worth a thousands words.

Regards,
T.