Does backsweetening mess up refractometer readings?

[J2W2]

Hi,

I just brewed a hard root beer that required backsweetening. I initially had a 3-gallon batch that had a starting brix of 15.2 (1.060 corrected OG). Once fermentation was complete, the final brix was 7.7 (1.012 corrected FG) for an ABV of 6.38%. I backsweetened with a pound of sugar and a pound of honey, dissolved in six cups of boiled water. All told, I estimate I added another 64 ounces.

If my math is correct, I diluted the original batch by 16.67% (0.5 gallons added to 3 gallons), which tells me the final ABV should be around 5.32%.

I just took a brix reading on the final product (letting it go flat and return to room temperature first) and got 12.1. That would give me a corrected FG of 1.039 and an ABV of 2.82%. Obviously that can't be right, hence my question of if a refractometer reading is unreliable after backsweetening? I really don't want to waste half a bottle to test it with my hydrometer.

Thanks for your help!

 

[VikeMan]

I just took a brix reading on the final product (letting it go flat and return to room temperature first) and got 12.1. That would give me a corrected FG of 1.039 and an ABV of 2.82%. Obviously that can't be right, hence my question of if a refractometer reading is unreliable after backsweetening?

It's not unreliable. It's telling you what it should tell you, i.e. that you added sugar and thus raised the gravity. It is, however, useless for ABV calcs, unless you want to do the math to compute an effective weighted average OG.

 

[J2W2]

It's not unreliable. It's telling you what it should tell you, i.e. that you added sugar and thus raised the gravity. It is, however, useless for ABV calcs, unless you want to do the math to compute an effective weighted average OG.

Ah, that makes sense. I'd seen posts about calculating ABV by the dilution ratio (like I did), but I hadn't really seen anything on taking a new gravity reading.

Thanks!