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Old 03-04-2013, 08:13 PM   #1
Mar 2013
Posts: 1

Do you know why do we perform refractometry on the first day but not once the fermentation had been initiated?

Does anyone know about that?
I think it may related to the alcohol appear when fermentation start, but still not sure.


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Old 03-04-2013, 08:17 PM   #2
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yes, the presence of alcohol in the sample will throw off a refractometer reading, though there are charts that will allow you to factor that in for later readings. But I can't vouch for their accuracy. If I need to take a reading down the line, I use my hydromter for that.
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Old 03-13-2013, 08:59 PM   #3
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Sep 2009
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I still use my refractometer after fermentation has begun. I just don't use the figures as they can be skewed by the alcohol and I don't trust the online calculators. I use it only to see if fermentation is still occurring or if it has finally finished. Once the reading stays the same for 3-5 days, then I'll whip out the hydrometer to figure out what my actual final gravity is.

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Old 03-15-2013, 01:54 AM   #4
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A refractometer essentially measures how much refraction occurs as light travels into/through the sample. The amount of refraction is density and material dependent and brewers/vintners refractometers are calibrated to handle sugar solutions. Alcohol has different refraction properties than sugar so our refractometers are unable to give an accurate reading once fermentation has begun.
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Old 03-18-2013, 09:18 AM   #5
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Both the refractometer and the hydrometer are skewed by alcohol when measuring sugar content. However, convention is to use hydrometer measurements, so that is what people understand.

This post explains it:

I use a refractometer everyday when tracking fermentation metrics, and sometimes use a hydrometer on the first and last day.
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Old 03-18-2013, 12:32 PM   #6
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With the hydrometer Tabarie's principle (that the decline in SG of a beer/wort from adding a certain amount of alcohol to to it is the same as the SG decline in water to which the same amount of alcohol is added) is used. Problem is that I can't find any references to Tabarie's principle and I can't make it work either. Thus, as is the case with the refractometer, if you want to know the true extract and the alcohol content to any reasonable accuracy you must separate the alcohol from the beer and assay the two parts separately. Unless you own the now well known* Anton Paar alcolyzer (and even if you have it's brain, the DMA 5000M) this means hours in the lab.

*Well known since the Anheuser-Busch lawsuit. One of the news outlets took samples to White Labs and they used an Alcolyzer to find that AB beers were well within TTB reporting requirements but the Alcolyzer got quite a bit of play.

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