Hydrometer and Refractometer Disagree

[bearymore]

I'm at a loss. I have a hydrometer and a refractometer. I calibrated both using distilled water and both read 0 (1.000 and 0 Brix respectively), so I have to assume they are working correctly. When I measured OG of my last brew, the hydrometer read 1.071 (1.072 after temp. correction) while the refractometer showed 16 Brix implying an SG of 1.062. Unfortunately, this was my second all-grain, so I can't say which one was correct. My FG was 1.014 according to the hydrometer and 1.012 according to the refractometer assuming that 1.062 was the correct initial reading or 1.007 assuming 1.071 was correct. It was a Saison, so I was aiming at 1.012 or lower.

My last few batches had high FG's according to the hydrometer - 1.019 to 1.020. I didn't have a refractometer, but based on my experience above, I'd guess it would measure out around 3 or 4 points lower. That would be consistent with what Beersmith showed I should get. These have been in the bottle for up to 3 months - no bottle bombs and, according to my uneducated palate, not overly sweet.

What confuses me here is the fact that the hydrometer reads correctly in distilled water. With such a simple instrument and a linear SG scale, wouldn't it have to be correct at all SGs if it is correct at 1.000? Yet the evidence above suggests to me that the refractometer must be correct. What should I believe?

 

[Donasay]

refractometers only work for OG not for FG, they measure how much (stuff) is in the water solution. Once the yeast make alcohol it throws off the reading because it is measuring the amount of stuff floating in the yeast alcohol mixture.

Additionally unlike the hydrometer, the refractometer will measure yeast in suspension, so if you pitched first and they are in suspension it might give you a high reading.

 

[Bytor1100]

All of my readings between the hydrometer and the refractometer have been very close. Do you have an ATC (automatic temp. correcting) refractometer?

 

[Loup]

refractometers only work for OG not for FG.

you can calculate gravity with a refractometer once fermentation has started using the following formula

SG = 1.001843 - 0.002318474*OB - 0.000007775*OB*OB - 0.000000034*OB*OB*OB + 0.00574*FB + 0.00003344*FB*FB + 0.000000086*FB*FB*FB

          where:

                   SG = estimated specific gravity of the sample
                   OB = Original Brix
                   FB = Final Brix

 

[bearymore]

I tested before pitching. I used both Beersmith and the Morebeer spreadsheet to calculate FG (I know Morebeer uses the formula Loup posted), and in any case, the OG's differed more than the FG's. The refractometer has ATC.