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jpender
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Posted Tue Feb 12, 2013, 11:59am
Subject: Density of coffee
 

Looking at the SCAA Gold Cup application recently I noticed that one listed option for measuring soluble concentration is the hydrometer. That surprised me a little bit. Does anyone still use a hydrometer for this purpose? I would imagine it would not be very accurate, require a fair volume of coffee, and probably be quite sensitive to temperature. Where can one find tables relating density, temperature, and coffee soluble concentration?
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yakster
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Posted Tue Feb 12, 2013, 2:52pm
Subject: Re: Density of coffee
 

You can do it, but it's a PITA.  I used a triple-scale brewing hydrometer, available at your homebrew shop, and just read off the readings in Brix.  It does take a lot of coffee and you do need to cool it down or compensate for the change in temperature.  There are also relatively inexpensive optical hydrometers available, but the SCAA has moved away from hydrometer readings towards using the ExtractMojo conductivity meters these days.  See this thread for more info.

CoffeeGeek - Coffee: Machines and Brewing Methods, Measuring Coffee Strength With A Brix Meter

 
-Chris

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jpender
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Posted Tue Feb 12, 2013, 4:39pm
Subject: Re: Density of coffee
 

Thanks, yakster. Did you ever go out to lunch with Alan Adler?

It isn't clear to me that the Brix to SG conversion on those triple scale wine and beer hydrometers is applicable to coffee solubles. Is it? And if so how do you know this?
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jpender
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Posted Thu Feb 14, 2013, 7:04pm
Subject: Re: Density of coffee
 

Yakster, I see in the first post of the thread you mentioned that Randy Pope of Bunn Technology is credited with correlating Brix (via refractometer?) and TDS via dehydration to obtain a factor of 0.85 %TDS/°Brix. But no mention of the temperature where this applies or the accuracy of the measurement. What is to made of this vague claim?

I know that coffee hydrometers have been produced and sold, so clearly at least an approximate relationship between density and concentration of coffee solubles has been worked out for at least some defined temperature. Where might someone find these data? Granted, it isn't easy to get high precision from a hydrometer. The bigger the better. The larger ones require something over 250ml of coffee -- basically a whole mug -- and you need to keep the whole ball of wax within a small fraction of a degree to maximize the accuracy. No wonder it isn't commonly done.

There are ways of determining density besides using a hydrometer. None of them are easy and inexpensive. And all of them include suspended, undissolved solids.

I used to use a hydrometer to estimate OG and FG in my home brews but I wasn't that worried about it. I just wanted a rough idea. With a good (large) hydrometer I believe you can obtain a precision of about 0.1° Brix. I think something similar is possible with optical refractometers. This translates roughly into 0.1% TDS. With digital coffee refractometers offering nearly an order of magnitude more precision one would wonder why anybody would bother with a hydrometer. I would venture to guess that nobody does.

So why is the hydrometer still on the SCAA Gold Cup application?
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yakster
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Posted Wed Feb 27, 2013, 2:24pm
Subject: Re: Density of coffee
 

Unfortunately, it's been a while since I was seriously working on measuring the TDS of coffee with hydrometers, BRIX meters, etc.  I had worked on it trying to understand it, and I had seen references to hydrometers in SCAA literature, but never any good instructions.  I'd even borrowed an optical hydrometer for a while.  I still have an HM Digital COM-100 conductivity meter that measures TDS and temperature but had problems relating brewed coffee conductivity to TDS measurements (since the meter measures the dissolved solids in the coffee that conduct electricity, not all the dissolved solids) and even had trouble with this meter for measuring water TDS when softening it with a CATION softener filter (since I'm exchaning conductive ions to reduce scale).

I did have lunch with Alan (we had a good conversation and swapped some home-roast) and I've run into him a few times since at coffee gatherings.  As for why hydrometers are still listed in the SCAA Gold Cup application, I can't say.

 
-Chris

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