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Netphilosopher
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Joined: 14 Jan 2011
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Location: USA
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Posted Thu May 10, 2012, 11:17am
Subject: .
 

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Netphilosopher
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Joined: 14 Jan 2011
Posts: 1,602
Location: USA
Expertise: Just starting

Posted Thu May 10, 2012, 12:46pm
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jpender
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jpender
Joined: 11 Jul 2011
Posts: 694
Location: California
Expertise: I like coffee

Grinder: OE LIDO
Vac Pot: S/S Moka Pot
Drip: Aeropress
Posted Thu May 10, 2012, 12:48pm
Subject: Re: Coffee Extraction Discussion, Questions for the membership:
 

Netphilosopher Said:

The coffee is dry when weighed in the Aeropress.  Not sure why the bouancy error matters when I add water - added mass is added mass, even with air entrained in it.

Posted May 10, 2012 link

When you weigh something on a scale you measure two things, the sample weight (mass * g) and its bouyancy in air (unless you have an evacuated chamber). If the densities of the sample and calibration weight are identical there is no correction. Steel has a density of about 7.9 g/cm^3 and coffee has a true density of....? I'm not sure, but I believe it's something a little greater than water.

For example, if you were to measure out 100.00g of olive oil (density of 0.9 g/ml) on your calibrated 0.01g scale the actual sample mass would be about 100.12g. Would this matter? It depends.


Netphilosopher Said:

I think I'm "good enough".  Class E1 and E2 weights at 500g are in the kilobuck range, the scale is $25 - I don't need an albatross weight if I have four 500g weights that are repeatable within the scale resolution.

Posted May 10, 2012 link

Could they all be off in the same direction? Maybe the statistics are on your side, but if you really wanted to you could buy a class F1 or F2 weight for about $20. It doesn't sound like that's necessary, especially if all you need is to measure coffee to something a little better than 0.1g.
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jpender
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jpender
Joined: 11 Jul 2011
Posts: 694
Location: California
Expertise: I like coffee

Grinder: OE LIDO
Vac Pot: S/S Moka Pot
Drip: Aeropress
Posted Thu May 10, 2012, 1:50pm
Subject: Re: Coffee Extraction Discussion, Questions for the membership:
 

Netphilosopher Said:

Strength = Extraction / ((1/Brew Ratio) - Absorption)

Posted May 10, 2012 link

Isn't TDS concentration typically defined as dissolved solids divided by solution volume?
The equation above is TDS / solvent.

For normal concentration coffee that doesn't matter much but for higher brew strength it becomes more significant.

The most convenient way to measure coffee concentration is mass of dissolved solids divided by mass of solution.
I'm pretty sure that's what a VST refractometer reports and I'll bet it's what you've been measuring with oven dehydration.

If you do this it changes the "standard" equation a little:

strength = extraction / (1/brew_ratio - absorption + extraction)

where the standard absorption factor is 2.3 instead of 2.086 or whatever.
So many significant digits is pretty funny for something that is so variable!
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andys
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andys
Joined: 10 May 2003
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Location: NY
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Espresso: Speedster, Londinium 1
Grinder: EK-43,Robur, HG One, M3
Vac Pot: Yama
Drip: various
Roaster: PIDed Popper
Posted Thu May 10, 2012, 7:02pm
Subject: Re: Coffee Extraction Discussion, Questions for the membership:
 

Netphilosopher Said:

I expected reduced extraction and was absolutely shocked to find that the normal-high(ish) brew parameters all tasted pretty much the same (when diluted to similar strength) - MAYBE a bit less extracted for the 18% brew, but not severely underextracted as the brew charts would predict.

Posted May 10, 2012 link

Not sure what you mean here. The brew charts don't predict, they simply input two of the three variables (brew ratio, %TDS, Extraction Yield) and calculate the third.

 
-AndyS
picture page:  http://flickr.com/photos/andy_s/sets/
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andys
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andys
Joined: 10 May 2003
Posts: 845
Location: NY
Expertise: Just starting

Espresso: Speedster, Londinium 1
Grinder: EK-43,Robur, HG One, M3
Vac Pot: Yama
Drip: various
Roaster: PIDed Popper
Posted Thu May 10, 2012, 7:07pm
Subject: Re: Coffee Extraction Discussion, Questions for the membership:
 

Netphilosopher Said:

If you use 55g/1 liter of hot water, and achieve 1.24% strength, your calculated extraction is 20% on the classic brew chart.
This is actually the same thing as using 55g/962.5g of hot water.

BUT if you use 55g / 1000g of hot water and achieve 1.24% strength, your calculated extraction is only 19.2%  (because you achieved the same strength but used a higher brew ratio relative to volume to get there).  On the chart, it's the same thing as using a brew ratio of 57g/1 liter and achieved 1.24% strength.

Posted May 10, 2012 link

Yes, this error was identified and incorporated into VST's Extractmojo and MoJoToGo software. Many people nowadays would say that 19%, not 20%, is the center of their "sweet spot" for extraction yield.

 
-AndyS
picture page:  http://flickr.com/photos/andy_s/sets/
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Netphilosopher
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Posted Fri May 11, 2012, 4:54am
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Netphilosopher
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Expertise: Just starting

Posted Fri May 11, 2012, 5:52am
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jpender
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jpender
Joined: 11 Jul 2011
Posts: 694
Location: California
Expertise: I like coffee

Grinder: OE LIDO
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Posted Fri May 11, 2012, 10:30am
Subject: Re: Coffee Extraction Discussion, Questions for the membership:
 

Netphilosopher Said:

a concentration, by definition, is the percent of the solute in the solution by mass.

Posted May 11, 2012 link

There are a number of different ways to represent concentration and mass/volume is certainly one of them.
Illy uses g/L to represent espresso strength, but I wasn't sure what the standard is for normal coffee.

The SCAA literature I could find only shows it as a percentage. The brewing control chart gives the brew ratio
in g/L so it didn't seem crazy to assume that strength was in similar units.


Netphilosopher Said:

Revising this to your form makes it impossible to find a constant for the absorption

Posted May 11, 2012 link

It's possible. I believe VST uses the same form of the equation (do you use mojo?).
What isn't possible to is EXACTLY match what the mass/volume formula for strength produces.

If you choose 2.3 for the absorption constant (2.068 + 0.2 extraction) the difference between the two charts is insignificant over the pictured range. It's only when you're well outside what is shown on the chart that it starts to make any noticible difference.

But if you're going to use mass/mass concentration your formula has to reflect that or else it's nonsense... assuming you believe that absorption really is constant for a given mass of grounds.

I don't do pourovers, but my Aeropress and moka pucks are always well below 2.0 absorption.
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jpender
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jpender
Joined: 11 Jul 2011
Posts: 694
Location: California
Expertise: I like coffee

Grinder: OE LIDO
Vac Pot: S/S Moka Pot
Drip: Aeropress
Posted Fri May 11, 2012, 10:33am
Subject: Re: Coffee Extraction Discussion, Questions for the membership:
 

By the way, I'm looking for an inexpensive scale. I read a comment on Amazon.com about the one you purchased. The person was claiming that it locks into 100g increments. I'm familiar with the annoying but common tactic of reporting zero when small masses (presumed drift) are added after the tare is used. But does your scale also do this at 100g increments as the reviewer claimed? I'd like to avoid that "feature" if possible.
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