A bit more on this...
Last night I got thinking about an experiment to see if my refractometer gives expected specific gravity results when using the onebeer/primetab equation for brix-to-FG in the presence of alcohol (via my BrixCalc app). I did the experiment this morning, however, I have a sneaky feeling that the might be some circular reasoning flaw to it, although I have so far convinced myself there isn't...please shout out if you spot logic issues here
- I mixed some hot & cold water together to get 20Â°C.
- I then added sugar to the water until it was about 1.016 with my glass hydrometer.
- Then I took 90 mls of this water and added 10mls of 37.5% ABV Gin (tesco value - used for air locks!!), giving a final alcohol content of 3.75%.
- I then measured the gravity again with the glass hydrometer: now 1.009
- I also quickly measured the Brix with my digital refractometer (3.9%).
- In order to end up with 3.75% alcohol at 1.009, one would need to start with a sugar solution of 1.0375 (back-calculated).
- Calculating the original brix from the 1.0375 gives 8.4% (all calculators seem to agree on OB-to-OG results).
- Putting all this data (starting brix of 8.4, final brix of 3.9) back into BrixCalc gives me:
A predicted Final Gravity of 1.009
(exactly as measured with the glass hydro 1.009)
A calculated ABV of 3.7%
(slightly below actual...but this is a different equation again)
experiment 2 (same as above, but more careful)
- I mixed some hot & cold water together to get exactly 200mls at 20Â°C.
- I added 8g of sugar to the water to get a gravity of 1.020 (measured with the glass hydro to check)
- I took 84mls of the water and added 16mls of the 37.5% Gin, to give a final alcohol content of 6%.
- I re-read the gravity of the solution to be 1.018
with the glass hydro.
- Quickly measured the brix with the refractometer (7.5%)
- To end up with 6% alcohol at 1.018, one would need to start with a sugar solution of 1.062
- Calculating an original brix from 1.062 gives 14.3%
- Putting all this data (OB = 14.3, FB = 7.5%) into BrixCalc gives:
A predicted Final Gravity of 1.016
(0.002 below the glass hydro of 1.018)
A calculated ABV of 6.0%
In short, this is an experiment to see what effect alcohol has on the refractometry measurement of brix, and the conversion of that to specific gravity using the equation at primetab. For me, the equation works acceptably (spot on in the 1st experiment, 0.002 out in the 2nd) with the correction I set in my BrixCalc app of +0.004 on the calculated SGs.
In simpler, more practical terms, imagine I brew a beer with a starting Brix of 14.3%. All calculators seem to agree that this converts to 1.062.
I let it ferment for a couple of weeks, until a good 4 or 5 days of no activity, and get ready to bottle it. I take my refractometer, and it tells me that the Final Brix is now 7.5%. Putting this into BrixCalc tells me that the Final Gravity is 1.016. The important point now is that if I were to dust off my glass hydrometer and measure the actual Final Gravity, it would say 1.018 ish... roughly the same as predicted.
I'm now happy to bottle without priming (I don't prime anymore anyway).
Putting this data in BeerEngine's calculator leaves me wondering why my beer is stuck at 1.025.
I think BeerEngine is great, I used it exclusively for my first 20 or so beers, and still use it in conjunction with another brewing program. However, for my gear, the refractometry tool doesn't match up to what my glass hydro says....even after setting the calibration factor. This is simply a mis-match of equipment and calculations, so I use a different combination of the two in order to get useable results.