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Brix and specific gravity refractometer
I finally got a refractometer (one of the cheap Ebay models) and I'm a little confused. I realize that the two scales are not linear, that post fermentation readings need adjusted because of the alcohol, and that both scales diverge as the readings increase.
I have tried it on a few of my kegged beers after degassing them and it matches my hydrometer within a few points.
My question is if I brew a beer and it reads 1.090 and 23.2 brix on my refractometer (see scale above) which one do I use?
1.090 is 21.56 brix and 23.2 brix is 1.097.
I must be missing something.
It sounds like the refractometers are screwed up on the SG side and are going to be corrected in the future.
From Morebeer:
A Caveat about the accuracy of the Specific Gravity Scale
Traditionally homebrewers used refractometers that only featured a Brix scale and converted to Specific Gravity by multiplying the Brix reading by four. The is fine at lower gravities up to around 1.040 but the higher your starting gravity goes, the less accurate the 4x conversion is. This same inaccuracy is found on the Specific Gravity scale on this refractometer, and every other model with a Specific Gravity scale that is available on the market today (July 2010). To get a 100% accurate reading you can plug your Brix Scale Reading into our free online spreadsheet, Promash, or a similar software to convert from Brix to Specific Gravity. If you are interested in more beer geekdom you can read more about this at the very end of the product description.
With that said we would still choose this model over a traditional Brix only refractometer because the distortion is relatively small and it sure is nice to have the instant Specific Gravity readings during the brewing process.
Getting Geeky about Specific Gravity Vrs. Brix
The inaccuracy in the Specific Gravity(SG) scale on this model is rooted in the non-linear conversion of Brix to SG. Take a Brix reading of 10, multiply by 4, and you get a SG reading of 1.040 - the accurate SG reading is 1.041. Take a Brix reading of 20, multiply by 4, and you get 1.080 - the accurate conversion is 1.084. When multiplying by 4 the conversion is worse as sugar levels rise. When this (and other recent)refractometers were made they used the same linear, multiply by 4, approach to create the Specific Gravity scale. Can they fix this - absolutely. We have initiated this change and we expect models with accurate Specific Gravity scales to be available by 2011 and you can be assured MoreBeer! will be the first to carry them.
I completely get that Brix and SG are no linearly scaled with each other. No argument there.
I do however often wonder how accurate the refractometer is period. Keep in mind that in the example of 1.080 wort and 1.084 wort is only 5% change. Is the refractometer even accurate to 5% differences?
There is always the hardness of theoretical science and true measurements to be aware of. But no one ever really talks about the accuracy of the instruments you are using to measure these end points at home vs. in the lab where these things come from.
If the refractometer isn't better then 5% accurate, then the whole argument is moot and no one should be worried about replacing their refractometer if they bought one with this apparent "issue".
Keep in mind the error in the measurement is not only a function of the refractometer. Error also comes in from the temperature of the liquid, bubbles in the liquid, the homogeneity of the liquid (stratification), is the rinse water on the slide still prior to applying the wort. When you start including all these sources of potential error into the process, I am sure that the refractometer is performing at worse that 5% accuracy. And 1.080 wort is not something many of us are making all that often. So the distortion is only 5% when you are out there. The distortion is more reasonable at lower gravities.... So the distortion difference becomes even less than 5%. I really doubt anyone, myself included, are being more accurate than 5% with our hydrometer or refractometer, or anything.
Just something to keep in mind when we use our equipment and start to think about hard #s in our recipes. Just because you make a Pale ale today at 1.050 today doesn't mean that one you make tomorrow at 1.048 is actually 2 points different in gravity. Statistically speaking, the reality is they are both the same gravity.
All good points. Especially when it comes to thermometers.
My "easy fix" is going to be to ignore the SG side. I'm coming from an engineering background, so I have to be anal about measuring. The "illusion" of precision and accuracy is comforting, even if it isn't real.
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