Refractometer and Variability of Readings

I see your point, but I'm for as much accuracy as possible when it's relatively cheap and easy. Conversely, I'm perfectly willing to be ignorant about my actual IBUs (for example), as long as they are subjectively in the ballpark of what I calculated, because measuring them would be too expensive (on my own somewhat fuzzy sliding scale). I also have no issues with rules of thumb (like "multiply the brix by 4 to get sg"), as long as they are understood to be such.

 

sugar refracts light one way, alcohol another. In other words, once you start fermenting, the alcohol content will interfere with refractometer readings. Once you start fermenting you need to use hydrometers.

 

You don't necessarily 'need' to. You can adjust your refractometer readings using a calculator. Like this one...
http://seanterrill.com/2011/04/07/refractometer-fg-results/

 

Not to pick nits, but alcohol, being less dense than water, interferes with hydrometer readings. The error increases with the ABV. Thus, a 1.100 beer that finishes at 1.010 has more residual sugar than a 1.050 beer that finishes at 1.010. How much more? Who knows? Who cares? We're homebrewers! All we're interested in is hitting the target - screw the bulls-eye. I'm always amused by those who specify their homebrews' ABV to two decimal places.

 

Yeah, because if they ignore significant digits, they could easily specify to three decimal places.

 

Just a thought, you said you calibrated your refractometer. Did you do the same to your hydrometer?

 

Granted, but the degree to which alcohol interferes with sugar readings is much less in density than it is optical rotation.

 

If it's wrong, it's wrong. My point is, without adjusting for the presence of alcohol, both are incorrect for the purpose of measuring residual sugars. Homebrewers tend to view the hydrometer as the ultimate authority without regard to this inaccuracy. I suspect many are not even aware of it.

The formulae we use to adjust Brix post-fermentation use original Brix as a factor, so they have the potential to accurately adjust out the effect of the alcohol and, thus, provide a far more accurate measure of residual sugars than the raw SG we get from a hydrometer. Alas, these formulae typically return an SG number that replicates the inaccurate SG we would get from a hydrometer (we tend to confirm the accuracy of the refractometer and the formulae by comparing the result with the hydrometer. If they're different, we reflexively - and likely incorrectly - conclude the refractometer is incorrect). These SG numbers are loosely interchangeable and we're comfortable with them, so we use them as-is.

 

Once when I had access to a vibrating u densitometer (I think thats what it was called. Basically its the thing legit brewing scientists use to measure Plato) I checked its reading which was accurate to like 3 or 4 decimal places against my refractometer, with and without alcohol present. When alcohol was present I used an online adjustor like the one vikeman posted. The refractometer was accurate in all cases, just not with the precision of the densitometer.

So they are pretty accurate in my exp.

 

Yup. I wonder how many brewers know to use a different (and supposedly more accurate, especially at higher gravities) ABV calculation than the standard easy one, as the alcohol affects it. My RIS was ~12.5% using the "usual" calculation, but 15% when the calculator considered the density of alcohol in the beer...