Real Dumb Question About Brix

Ok, so I got a screaming deal on a ATC refractometer. Only catch is it only measures in Brix. Fine, I get it, brix x 4 = SG.

Or does it?

Apparently in attenuating there is some relative computation that occurs that I just don't understand.

For example:

If I put in 15 brix by itself that is 1.060. If I put in 5 brix by itself that is 1.020. But if I put in 15 brix as my OG and 5 brix as my FG I then find that I have a gravity of <1. In other words as an OG 5 brix = 1.020 but as an FG in relation to an OG of 15brix 5 brix is then .995.

I'm so cornfused by this. Can someone explain or point me to a document that explains????

Thanks!

 

one degree brix = 4 gravity points

5 degrees brix = 1.02
15 degrees brix = 1.06
and so on

 

well that is true as an OG, but not as the FG according to the calculator on my BeerTools program as well as my HBCalc app.

Anybody who knows why care to explain?

 

When using the software to calculate the difference between OG and FG the software adjusts for alcohol content.

Read this

 

Alcohol will affect the reading, since it reads the sugar content of the source. It's iterated repeatedly in all texts to not go by the reading for a fermented beer, as there is a wide variance. The sw tries to compensate as best it can.

 

ok, interesting. thanks so much!!

 

here is your mistake. You OG is not 5 it is 15

 

shotgunner: no, i think you misunderstood. jetmac and suprchunk hit it. but thanks!

 

for last 5-6 batches i used refractometer and hydro to take my readings and i found that online calculators do just fine for alcohol correction when taking FG readings (refraction index for sugar+alcohol in water is very different than sugar alone)
I use this one because its the first one i found:
http://onebeer.net/refractometer.shtml