[melferburque]

I've been having some issues with the last few batches I've brewed. I've been getting pretty consistent OG yields (70% or so) mashing for an hour with a ten gallon igloo cooler at 152-154 degrees. I bought a refractometer and have had OGs of 1.061, 1.066 and 1.071. using fresh dry yeast, I've pitched two packets of US-05 for each of these (sierra pale clone and two IPAs), let ferment in a 60-65 degree room for 2-3 weeks in primary, and the yeast has pooped out around 1.030 for all three.

I've been using beersmith, and I know I'm mashing a bit on the high side (I like some extra body) but I should still end up with FG in the 1.0teens. I'm missing my final mark by a full twenty points, which makes for some weak (though not unenjoyable) beer. I just can't figure out why. the yeast is fresh, only a couple months old and kept in the fridge, warmed to room temperature prior to hydrating. 350+ billion cells should be more than adequate for those OGs. I've left them primary a full week or two after airlock activity has ceased, and I've done plenty of aeration of the wort prior to pitching. I don't have much control over the fermenting room temp, but so far this year it hasn't gone over 70 degrees in there.

any ideas what I might be doing wrong?

I'm taking a couple of these to my club meeting tonight (north seattle homebrewers), and plan to bring them to NHC assuming they pass muster with my club.

[tygo]

Are you sure the thermometer you're measuring your mash temp with is accurate?

[melferburque]

Are you sure the thermometer you're measuring your mash temp with is accurate?

within a degree or so, yes. I have calibrated it and checked against my kettle thermometer. if it's off, it's not enough to cause that massive a swing.

[tygo]

Ah, wait...are you using the refractometer to measure your final gravity? 

[wonderbread23]

How are you testing your final gravity?  If you are using the refractometer, note that the alcohol will skew the readings and you'll need to use a formula to adjust the reading.  If you are using a hydrometer, make sure it is calibrated and accurate.

[tygo]

Yeah, exactly what wonderbread said.  You can use Sean's calculator to estimate the actual final gravity. 

http://seanterrill.com/2012/01/06/refractometer-calculator/

But a hydrometer is more accurate post-fermentation.

[melferburque]

Yeah, exactly what wonderbread said.  You can use Sean's calculator to estimate the actual final gravity. 

http://seanterrill.com/2012/01/06/refractometer-calculator/

But a hydrometer is more accurate post-fermentation.

I did not know that! huzzah! I'm not making oddly refreshing beer with undiscernable massive amounts of non-fermentables. that's guys!

[tygo]

Another thing to remember when using a refractometer is that even pre-fermentation with no alcohol present you need to make some adjustments.  The refractometer is designed to measure the refractive index of sucrose, not maltose, which is the primary sugar present in wort.  Also, if you have a refractometer with an SG scale it's probably not accurate.

For post fermentation, use the brix scale, reduce the reading 4%, and then convert it to SG.