Hydrometer inacuracy in mid scale?

Hi all. I have been brewing since the beginning of the year with 4 batches under me (last two in primary right now). I've been reading HBT almost daily for almost a year, and have read Palmer's book, How to Brew.

My first two beers finished stuck at 1.020 according to my hydrometer. Both started around 1.050 and used Nottingham yeast. I questioned the accuracy of my hydrometer at the time and even took a reading on tap water which read about 1.000 so I thought I was good.

I also have a refractometer (ATC) so the last 2 or 3 batches I took readings with both instruments. One was 1.054 / 13.1 Brix, the other 1.051 / 12.8 brix. I am using a correction factor of 0.9872 and I recalibrate it on every batch.

After these beers were in the primary for 5 and 3 days respectively I took readings with the hydrometer and refractometer again. The hydrometer readings were 1.022 and 1.023. The younger beer had competed fermentation to the point of no bubbles in the airlock (glass carboy) so I was pissed that my yeast quit on me again. The refractometer readings at this time were 7.4 and 7.5 Brix.

Again I question the accuracy of my hydrometer. I ran these numbers through the calculators from beer smith, morebeer, and brewzor - all with similar results. They all say my current gravity should be about 1.014 and 1.016 which is what I expect from the yeasts in these two batches (Nottingham and Saffbrew S-33).

Do you think my hydrometer is inaccurate in this range? After 4 batches with the same result, all in a pretty good temperature controlled room (65-70 degrees).

Thanks for your input!
-Rob

So the first thing you need to do is use Distilled water for the calibration.

Next, I recommend http://www.honeycreek.us/specfic_gravity_table.php.

Alcohol messes up refractometer readings, so that needs to be taken into account after fermentation has started. There are calculators and charts to adjust for this, but refractometers just aren't that accurate after fermentation has started. +1 on using distilled water to calibrate the hydrometer, although most tap water will get you pretty close.

My guess is that your hydrometer readings are pretty accurate. Were the beers extract or partial mash or all grain? How well did you aerate? How long did you primary the first two that finished high? Moving the beer off of the yeast too soon can cause a stuck fermentation, and 3-5 days is definately too soon IMO.

The first three were extract brews with about 50/50 LME/DME, the 4th a wort-in-a-bag (AG Festa brew). The first ones I did were in primary for 3 weeks (not much free time to rack, etc) and stopped fermentation (tried various methods or waking the yeast up).

I shook the crap out of my carboys for a long time (several minutes) on each batch to aerate.

I'm going to take more gravity readings tonight on these brews.

Extract beers tend to finish a little higher than all grain in my experience. The extract producers tend to favor a less fermentable wort for whatever reason. 1.020 is about what I would expect.

If this is not your problem, you might look at yeast pitch rates. It could be that you aren't pitching enough yeast, causing a stuck fermentation.

Also, as other poster have said, you can't trust your refractometer reading once alcohol is introduced. You have to adjust it based on some equation. The hydrometer is probably accurate though.