Hydrometer/Refractometer Corrections

[jeeppilot]

I've been brewing for a couple years and broke ANOTHER hydrometer the other day. So instead of buying a new one, I tried instead with my refractometer (post fermentation) and an app that makes the corrections. This app lists the APPARENT FG and the ACTUAL FG with the former being what you'd read off the hydrometer and the the latter being the hydrometer reading corrected for alcohol effect. Well, I guess I didn't know you had to do that with a hydrometer!

So, my beer is a Belgian Dubbel, my target FG 1.019. My refractometer shows 9.0 Brix...the app calculates the APPARENT FG as 1.016 and the ACTUAL as 1.025. That seems like a big split! I always read the hydrometer and thought that was it. If I did that in this case, I slightly overshot my target. If I use the "hydrometer correction" value, I still have a ways to go. All visible fermentation has ceased and I'm 8 days post brew. So, am I at 16 or 25?

 

[billl]

You are at 1.016 by normal beer vocabulary. We always measure in apparent fg. Recipes are listed that way. Yeast attenuation is listed that way etc.

And the caveat - the "corrected" refractometer reading is less reliable than a hydrometer. The models are built on simple sugar and doesn't always match exactly to wort in the real world.

Also - your beer is "done" when the gravity is stable. Yeast stubbornly refuse to read recipes and never give in to peer pressure from Beersmith about where they should finish.

 

[day_trippr]

[...]Yeast stubbornly refuse to read recipes and never give in to peer pressure from Beersmith about where they should finish.

 

[jeeppilot]

You are at 1.016 by normal beer vocabulary. We always measure in apparent fg. Recipes are listed that way. Yeast attenuation is listed that way etc.

And the caveat - the "corrected" refractometer reading is less reliable than a hydrometer. The models are built on simple sugar and doesn't always match exactly to wort in the real world.

Also - your beer is "done" when the gravity is stable. Yeast stubbornly refuse to read recipes and never give in to peer pressure from Beersmith about where they should finish.

Whew, good answer. I checked Brewers Friend and their calculator says the same thing. I'm no expert, but I'm happy to know I haven't been doing something wrong all this time without knowing it.

 

[doug293cz]

There is no such thing as "Apparent Specific Gravity" when measuring with a hydrometer. A hydrometer directly responds to the density of the liquid it is measuring. And, at it's calibration temperature the scale is marked to read the value of the test liquid's density divided by the density of water at the calibration temperature, which is exactly the specific gravity. The specific gravity of a liquid is just the ratio of its density to the density of water. There are no errors created because of the composition of the liquid.

A refractometer is a totally different animal. A refractometer does not respond to density, but rather index of refraction. The scale is based on a correlation of the index of refraction of a pure water and pure sucrose solution, either in Brix and/or specific gravity. You need to use correction factors when using a refractometer, both with wort and fermented beer. With wort the correction is required because the solute is not pure sucrose. Some refractometers, designed for use with wort, may have the wort correction factor baked into their scales.

When it comes to fermented beer, not only is the solute not pure sucrose, but the solvent is no longer pure water. The alcohol in the beer also affects the index of refraction, so a much larger error occurs when measuring beer than when measuring wort.

So, with a refractometer, the raw scale reading is often an apparent Brix or SG reading.

Brew on

 

[jeeppilot]

There is no such thing as "Apparent Specific Gravity" when measuring with a hydrometer. A hydrometer directly responds to the density of the liquid it is measuring. And, at it's calibration temperature the scale is marked to read the value of the test liquid's density divided by the density of water at the calibration temperature, which is exactly the specific gravity. The specific gravity of a liquid is just the ratio of its density to the density of water. There are no errors created because of the composition of the liquid.



A refractometer is a totally different animal. A refractometer does not respond to density, but rather index of refraction. The scale is based on a correlation of the index of refraction of a pure water and pure sucrose solution, either in Brix and/or specific gravity. You need to use correction factors when using a refractometer, both with wort and fermented beer. With wort the correction is required because the solute is not pure sucrose. Some refractometers, designed for use with wort, may have the wort correction factor baked into their scales.



When it comes to fermented beer, not only is the solute not pure sucrose, but the solvent is no longer pure water. The alcohol in the beer also affects the index of refraction, so a much larger error occurs when measuring beer than when measuring wort.



So, with a refractometer, the raw scale reading is often an apparent Brix or SG reading.



Brew on

Doug, you say that with a hydrometer, the SG is a ratio of a liquids density to the density of water. Yet when we measure specific gravity in brewing, the density we seek is really the sugar content, right? We derive ABV and attenuation numbers by the difference of original sugar density to final sugar density. Alcohol content is not part of that equation yet in our measurements, even with a hydrometer, it is. So the ethanol being lower in density than water, makes the FG a bit "skewed" to low side. While I'd agree the hydrometer is simply reading the density of the solution, and is not in error, it seems it's no longer an apples to apples comparison. I never considered the effect of alcohol lowering the total solution density and I suddenly feel like I have been doing it all wrong.

 

[doug293cz]

Doug, you say that with a hydrometer, the SG is a ratio of a liquids density to the density of water. Yet when we measure specific gravity in brewing, the density we seek is really the sugar content, right? We derive ABV and attenuation numbers by the difference of original sugar density to final sugar density. Alcohol content is not part of that equation yet in our measurements, even with a hydrometer, it is. So the ethanol being lower in density than water, makes the FG a bit "skewed" to low side. While I'd agree the hydrometer is simply reading the density of the solution, and is not in error, it seems it's no longer an apples to apples comparison. I never considered the effect of alcohol lowering the total solution density and I suddenly feel like I have been doing it all wrong.

Yes we are interested in sugar content (actually total dissolved extract content, most of which is sugar), and SG is an indirect measurement of sugar content. As the sugar content goes up, the SG goes up (almost linearly) as well. Zero sugar equals an SG of 1, since 0 sugar means you have water, and the density of water divided by the density of water is 1.

SG ~= 1 + 4 * wt%_Sugar / 1000​

Thus a wort that is 10% by weight of sugar has an SG of about 1.040. Weight percent sugar is degrees Plato (°P) by definition, and degrees Plato is a common measure of sugar content in brewing (especially among professional brewers.) Also turns out that °Plato is equal to °Brix (to about the fourth decimal place.)

Directly measuring actual sugar content is difficult, but measuring SG is easy, so we use SG as a proxy for sugar content.

A more accurate equation for converting SG to °P is:

°P = -616.868 + 1111.14 * SG -630.272 * SG^2 + 135.9975 * SG^3
and
SG = 1 + (°P / (258.6 - ((°P / 258.2) * 227.1)))
​

No, FG is not skewed by the presence of alcohol. Since FG is the final specific gravity. Calculated attenuation is skewed since the residual sugar content is not accurately reflected by the SG anymore. That's why we call the attenuation calculated from OG and FG the "Apparent Attenuation" and real attenuation is actually lower, since you have more residual sugar than the FG indicates. It's possible to correct for the attenuation error if you know both the OG and the FG.

Brew on