BG 5 pts under, but OG 20 pts under!

[PDevlin75]

Greetings, and happy new year to you all!

So I brewed up two 1 gallon BIAB batches over the past two days. The first one went well enough, but the second one, not so much...

The second batch was a pale ale intended to have an OG of 1.055. Beersmith predicted that the BG would be 1.028. I received a refractometer for Christmas, and that told me that the BG was about 5 points shy, at 1.023. The refractometer came calibrated, and it worked perfectly on the batch made the day before... I just figured the 5 points were no big deal, as I had mashed with an extra half of a gallon of water. Given the extra water, I also gave the boil an extra 10-15 minutes. I still wound up with an extra quart of water on top of the trub, but whatever, right?

After cooling the wort, I checked, and the OG was 1.035! That's 20 points off target! I didn't mind the discrepancy with the boil gravity, as it wasn't that far off, and I figured it might work itself out in the boil. Even if My OG were only 5 points off, I wouldn't mind as it would probably be an efficiency issue... But 20 points? This seems weird, right? I even double checked the refractometer with distilled water, and it was still correct.

Any thoughts on where this would have gone wrong? Or am I just missing something?

Thanks!
Pete

 

[billl]

An extra quart on a 1 gallon recipe of 1.055 would bring it down 11 point. If you are leaving that behind and all the trub behind, you've located your missing sugar. In a 1 gallon batch, you can't say stuff like "only an extra half gallon" and expect it to not make a difference. That's half the finished volume!

 

[nealm]

I don't believe any refractometer comes calibrated. You at least need to check that it is zeroed with some distilled water.