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Hit my SG but low attenuation


Nov 27, 2019
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My last batch on my HERMS, I hit my OG and measured with a refractometer of 1.065(1 degree higher than expected).  My mash efficiency was 73%(first time BIAB).  I felt great that I hit my OG using the method of BIAB for the first time.

I used OLYMPIC Irish Ale Yeast for my Oatmeal Stout.  I cooled down (and maybe not cool enough and this is where I went wrong) to 82F and pitched the yeast(No starter since it was a 5gal batch and the yeast had manufacture date of about 3 weeks prior).  I put it in my fermentation chamber and set it to 67F.  The fermentation took off in under 12 hours and things were going good.  After about  days it really slowed and I checked the gravity and got 1.038 with the refractometer.  What?  I am expecting 1.020.

I roused the yeast and set the temp to 69F.  Btw, I am using a bung with the thermometer tube and an Ink Bird.

2 days later, I am still around 1.037. 

Did I stress the yeast out too much from my initial pitch?  Should I have made a viatility starter even though the yeast was only 3 weeks old?  These manufactures say all you need is one bag for 5 gal...I guess we can't trust that?

Thoughts, ideas on what to do now or future?

The presence of alcohol makes a refractometer reading inaccurate. I use my refractometer only pre and post boil. For FG I go to my hydrometer. There are correction calculators available and I ran your OG and measured FG through the one linked below to get a corrected FG of near 1.016 giving you an ABV of 5.6%.

To use this you will need to Google [your gravity reading] to brix.

PS. Your pitching temp was high but not enough to cause serious damage. As for how much to pitch I would go by OG rather than batch volume. One pack per five gallons of 1.050 wort will react much differently that when pitched in five gallons of 1.080. Anything over 1.060 and I start thinking of either using a starter or multiple packs of yeast.
You are right about the refractometer!  I ran it through the adjustments and I HAD hit my FGs!  Thank the beer gods!  All is well.