Refractometer: brix to SG

I recently purchased a refractometer. I used 3-differnet sources to convert from brix to SG and get three different answers!

My brix reading is 12.5.

1) http://brew.stderr.net/refractometer.html

this site gives me 1.050

2) ProMash & http://www.primetab.com/formulas.html

this gives 1.051

3) BeerSmith gives 1.048

(I also took a hydrometer reading and it was 1.048 at 72 deg f.)

Which is correct (why a difference)?


Mike

Mike,
 BeerSmith is right of course!  

 I don't have all of your data, but here are some observations:

1) Appears to be a straight Brix-SG conversion which will not be correct because refractometers are not calibrated for wort but are calibrated for water with sugar.  This is why you need a calibrated refractometer calculator to perform the calculation.

2) I can't really tell which calculation they are using...so it is hard for me to comment.

3) This number should be correct assuming you have done a calibration for your refractometer.  You calibrate your refractometer by clicking the "calibrate refractometer settings" from within the refractometer tool.

Cheers!
Brad

Thanks for the response - I went home and re-checked my #'s and found my hydrometer reading was 1.048 @75 deg f. BeerSmith corrected this to1.050. I also looked closer at your brix calculation and entered this calibration data for my brix of 12.5. Seems like I should be set from now on.

Mike