Hydrometer Calibration - How to solve this riddle?

[Imburr]

I will attempt to keep this short and sweet. I am brewing a quad. Target OG was 1.098. due to volumes etc, I hit 1.068.

I fermented, and after 2 weeks I checked G and it was at 1.043, but the sample I used was chunky from the top.

On day 17 I got a good sample, and read it at 1.036. I also took a reading with my glass floating hydrometer and read 1.020.

I was like woah, and realized my ATC had not been calibrated in a long while. I out in distilled, and it was 1.006 off or so on the high side.

After calibrating, I checked it again... And it still read 1.036. I checked with glass hydrometer again, and it read 1.020.

???? How do I figure out what my reading should be? And was my original 1.068 @ 80 degrees accurate? If not how can I correct it... Go 1.006 G higher?

 

[ancientmariner52]

I'm assuming you are using a refractometer? Your refractometer with ATC is intended to measure sugar solutions, which wort is (mostly). As soon as fermentation starts, alcohol enters the mix and the refractometer is increasingly inaccurate as the percentage of alcohol increases. There are ways to calculate corrections, but why bother? That's what the floating hydrometer is for. So a difference between the two instruments is expected at this stage.

When you believe fermentation is complete, and it ought to be at day 17, you should take hydrometer readings 2 or 3 days apart. When you see no change fermentation is finished.

As far as your OG reading goes, who knows? You know it is wrong, because you had to recalibrate your refractometer. We could guess at it, but there's no point at this stage. Your going to have beer, just not the beer you wanted.

I've learned the hard way that you can't rely on even brand new instruments until you have tested them yourself.