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How to measure bean density

Postby King Seven on Tue Apr 28, 2009 8:00 pm

Is there a way to accurately measure the density of roasted coffee? I can't think of a good way but then I might just be a bit stupid....
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Postby cannonfodder on Tue Apr 28, 2009 8:15 pm

By weight but you would have to know the moisture content if you are comparing different roast batches or greens. The moisture content of the bean would make a difference in the weight but you would need a very accurate scale to measure just a handful of coffee.
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Postby cafeIKE on Tue Apr 28, 2009 8:16 pm

I'm assuming you want the density of the coffee if it were a solid mass without the inter-bean space:

You can fill a known volume with coffee beans and weigh it. Then fill the void area with a liquid the beans don't absorb quickly. A thin oil would probably work. Work back from the volume of oil added and you have the bean density.
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Postby AndyS on Tue Apr 28, 2009 9:59 pm

King Seven wrote:Is there a way to accurately measure the density of roasted coffee? I can't think of a good way but then I might just be a bit stupid....


Along the lines of what Ian suggested:

Fill a graduated cylinder half full with liquid and note volume.
Add a known weight of beans.
Note increase in volume as read on cylinder scale.
Divide bean weight by increase in volume reading.
Eureka, you've got it!

If the beans absorb liquid, spoiling the measurement, weigh them first and then spray on a very thin coat of some sort of lacquer sealant.
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Postby another_jim on Wed Apr 29, 2009 4:01 am

Whole beans contain a lot of airspace, and whole beans absorb water slowly, so it might be better to use Andy/Ian's method of a known weight of coffee in a water column, but using the finest grind possible, and letting it absorb all the water it can, and lose all the air, before recording the volume change.
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Postby AndyS on Wed Apr 29, 2009 5:19 am

another_jim wrote:Whole beans contain a lot of airspace, and whole beans absorb water slowly, so it might be better to use Andy/Ian's method of a known weight of coffee in a water column, but using the finest grind possible, and letting it absorb all the water it can, and lose all the air, before recording the volume change.


I assumed James wanted to measure the density of the whole beans as is, including the "air" space. Perhaps I'm wrong and your method would be more more applicable.

Meanwhile, Eric wrote me to say that beans float. Duh, I didn't think of that. The graduated cylinder method would also require a lid or disc on top of the beans to keep them submerged.
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Postby King Seven on Wed Apr 29, 2009 5:20 pm

To be honest I am not sure exactly what I want to measure. It is all part of my grand unified theory of espresso, acidity and extraction. I want to understand the relationship between a coffees natural density, its density after roasting and other coffees that may have had different starting densities but are roasted to match the density of another coffee. If that makes any sense at all?
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Postby dsc on Wed Apr 29, 2009 5:58 pm

Hi guys,

I think you can use a french press to easily measure it just like others said above. Pour some water into a big FP, put the plunger in and make a mark to show where the water level is. Remove the plunger, weigh a selected amount of beans and pour them into the water, put the plunger back to keep the beans at the bottom and make another mark of the new water level. Remove everything from the FP, pour fresh water in up to the first mark, weigh it, pour more water to reach the second mark, weigh it again, subtract and divide by water density to get volume. Divide the previously noted weight of beans by this volume and you get density.

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Postby barry on Thu Apr 30, 2009 1:07 am

King Seven wrote:To be honest I am not sure exactly what I want to measure. It is all part of my grand unified theory of espresso, acidity and extraction. I want to understand the relationship between a coffees natural density, its density after roasting and other coffees that may have had different starting densities but are roasted to match the density of another coffee. If that makes any sense at all?



Perhaps a relative density measurement would probably suffice for that. Take a non-porous container (eg stainless steel pail) of appropriate size (large enough to be meaningful but small enough not to be a hassle, perhaps 1 gallon?). Fill container to overflowing with beans, strikeoff excess with a straightedge, and weigh. All that really matters is that you follow the same method of handling each time. For example, don't shake the container some times but not others.
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Postby gscace on Fri May 01, 2009 11:22 am

King Seven wrote:To be honest I am not sure exactly what I want to measure. It is all part of my grand unified theory of espresso, acidity and extraction. I want to understand the relationship between a coffees natural density, its density after roasting and other coffees that may have had different starting densities but are roasted to match the density of another coffee. If that makes any sense at all?


OK, this method will work, but this method might lose some stuff you're intrested in as well. You'll need some toys including a vacuum pump, thermocouple vacuum gauge (Hastings Teledyne is sufficient), container ( 1L volume or so) that can be sealed at the top and evacuated, balancce with sensitivity around .01 gm. You'll need a thermometer and barometer as well. None of this stuff needs to be real accurate if you're looking for density accuracy in the percent level. You can prolly find most of this stuff on ebay for reasonable enuff I guess.

Mount the sensor for the hasting teledyne vacuum gauge on the container. Determine the internal volume of the container. You can get this from geometry, weighing with water, or by weighing with the top open, then with the top closed and evacuated. You should get the same answer to within a few percent. You'll prolly be surprised to learn that the air volume within a 1L container weighs a little over a gram when atmospheric conditions are 101325 Pa and T is near 0C. Anyhoo the air weight value would be useful as a check of the vac gauge, and you'll need it later.

Find the density of the coffee. Fill the container with coffee to an arbitrary height. Weigh the coffee in air. Close the conatiner and pump out the air until you get a modest vacuum that remains relatively constant so that you can get a damn weighing (might take a while) and weigh it again. The difference in weight is the weight of the air plus any volatile stuff you pumped off. Unfortunately that could be water, which might or might not be important to you (is it?).

It's sort of the same idea as filling with water, but doesn't have problems with air bubbles. It's got other potential problems, but has the benefit that you can buy toys. And vacuum pumps are highly underrated in my view. If you have a vacuum pump you can make the coolest carbon fibre (special spelling since you're in da UK) espresso machine bodywork etc. and you can vacuum pack your green coffee too.

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