Calculation of ABV

[bobtheUKbrewer2]

I looked on the Brewers Friend website and their calculator using hydrometer readings of OG 1.046 and FG 1.015 gave 4.07 ABV

However switching to Plato units of 11.5 and 6.2, ABV was 2.86

Why is there such a difference ?

 

[Minky]

The Promash calculator shows a gravity of 1.015 = 3.83 Plato.

 

[william_shakes_beer]

I use (og-fg) x1.33

The Palmer book has a lookup table in the back. I did a random sampling of many of these numbers and they all came pretty close.

 

[bobtheUKbrewer2]

but why is the plato formula so far out?

 

[kombat]

Because, as Minky pointed out in the second post, 1.015 = 3.83 Plato, not 6.2 Plato as you erroneously asserted in your original post.

 

[bobtheUKbrewer2]

Kombat = the 6.2 was measured with my refractometer, so my question is using 2 measured by refractometer figures, how is ABV calculated.

For clarification I inputted the 2 BRIX readings into the brewers friend calculator.

 

[jddevinn]

Bob,

Refractometers measure the refractive index of the liquid. The refractive index is dependent on the sugar content so it can be used to measure the OG. However the refractometer will not measure the density change that is caused by alcohol post fermentation. Therefore the 6.2° you measured is not the final gravity as it does not take into account the alcohol content.

I have a digital refractometer and use it for prefermentation measurements and to check on the fermentation progress but always take a final reading with a hydrometer before racking.

A formula for estimating an "actual" gravity post fermentation from brix measurements is on this website. http://www.musther.net/vinocalc.html#monitorferment

 

[evanmars]

Brewer's Friend also has this estimation calculator: http://www.brewersfriend.com/refractometer-calculator/
That gives a final of 3.29 Plato, with an ABV of 4.45%, the VinoCalc referenced above gives 4.3%, both very close estimates.
If your hydro reading was just 1 degree off, with 1.047 OG and 1.014 FG, Brewer's Friend gives you 4.33%