Thanks for the spreadsheet- it definetely helps sort out a bunch of confusion I am having with this instrument. Can someone explain why a refractometer if only good for pre-fermentation readings. I did not see instructions for only using the refractometer pre fermentation, but now I do notice the "SG wort" label under the specific gravities. Does yeast in suspension bend light that much to throw off the FG readings?
Denny means the the alcohol has a different refractive index
than both water and dissolved sugar, which throws off a direct reading.
If plain water is set to zero (by calibrating the refractometer) you could read a mixture of water with either alcohol or sugar. But in a mixture of all three you can't tell how much refraction is from alcohol and how much is from sugar. If you have a base reading before fermentation then you can use an equation to determine alcohol content, but in general a hydrometer is easier and more accurate. Sean's spreadsheet is very good, most of the others are off significantly in practice.