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snottywong 11-25-2012 11:42 PM

Mystery low FG
 
So, I'm a relative beginner, I've brewed maybe a dozen all-grain batches. Starting to get the hang of it. I recently purchased a refractometer to make gravity readings easier. Prior to that purchase, I had never measured the final gravity of my beer, mostly because of contamination concerns and because using a hydrometer wastes a lot of beer (relative to a 5 gallon batch).

Anyway, for the last 2 batches I've been able to measure the final gravity as I'm racking it to the secondary. And, both times it has been surprisingly high, in the 1.030 range. However, in neither case did the beer taste sweet, as one might expect in an incomplete/stuck fermentation. In fact, in both cases, the beer tasted quite good, close to what I was shooting for.

Both brews were all-grain batches. One was mashed around 150 degrees, the other around 148. I did an iodine test at the end of the mash on both batches to ensure that all starches were converted. I saw normal and vigorous yeast activity in the primary on both batches.

I can't understand why the final gravity is so high, 7+ days into the fermentation, well after yeast activity has plummeted. The beer tastes fine, but I wonder if something is wrong with it? Or maybe I'm just using the refractometer incorrectly? It seems to be calibrated correctly, since if I put water on it, it reads 1.000 exactly. And, when I'm testing the pre-boil gravity and original gravity during the brew, it seems to read reasonable/expected values.

Any ideas of what else I can check?

Newbeerguy 11-26-2012 12:00 AM

Quote:

Originally Posted by snottywong (Post 4620562)
So, I'm a relative beginner, I've brewed maybe a dozen all-grain batches. Starting to get the hang of it. I recently purchased a refractometer to make gravity readings easier. Prior to that purchase, I had never measured the final gravity of my beer, mostly because of contamination concerns and because using a hydrometer wastes a lot of beer (relative to a 5 gallon batch).

Anyway, for the last 2 batches I've been able to measure the final gravity as I'm racking it to the secondary. And, both times it has been surprisingly high, in the 1.030 range. However, in neither case did the beer taste sweet, as one might expect in an incomplete/stuck fermentation. In fact, in both cases, the beer tasted quite good, close to what I was shooting for.

Both brews were all-grain batches. One was mashed around 150 degrees, the other around 148. I did an iodine test at the end of the mash on both batches to ensure that all starches were converted. I saw normal and vigorous yeast activity in the primary on both batches.

I can't understand why the final gravity is so high, 7+ days into the fermentation, well after yeast activity has plummeted. The beer tastes fine, but I wonder if something is wrong with it? Or maybe I'm just using the refractometer incorrectly? It seems to be calibrated correctly, since if I put water on it, it reads 1.000 exactly. And, when I'm testing the pre-boil gravity and original gravity during the brew, it seems to read reasonable/expected values.

Any ideas of what else I can check?

When you use a refractometer to check the FG, you need to adjust for the alcohol that is now present. That is what is throwing your readings off.

Here is a good link to a calculator:
Refractometer Calculator

ajf 11-26-2012 12:04 AM

After fermentation has started, a refractometer gives inaccurate results due to the presence of alcohol. Beersmith and Promash provide calculators that help with this, but in my experience they are not very accurate. The only way I am aware of to get an accurate FG reading is to use an hydrometer.

-a.

Yooper 11-26-2012 12:07 AM

Quote:

Originally Posted by ajf (Post 4620633)
After fermentation has started, a refractometer gives inaccurate results due to the presence of alcohol. Beersmith and Promash provide calculators that help with this, but in my experience they are not very accurate. The only way I am aware of to get an accurate FG reading is to use an hydrometer.

-a.

Yes, that's been my experience also. You can tell when the fermentation is done, as it stops dropping, but it's not all that accurate even with the correction calculator. I use the refractometer pre-fermentation, and a hydrometer post-fermentation. The test jar uses like 3.5 ounces of beer, so it's not very much at all, and it gives accurate results.

snottywong 11-26-2012 03:18 AM

Seems I learn something new every brew. Thanks a lot for the good info.

pdxal 11-26-2012 07:07 AM

+1 Use the hydrometer for FG.
Also, a 1.030 beer would be really, really sweet. So, a dry beer with a measured 1.030 FG means the FG wasn't calculated correctly.

william_shakes_beer 11-26-2012 04:01 PM

If you are really concerned about loosing the contents of the test jar, try sanitizing the hydrometer and dropping it directly into the fermenter bucket.

WoodlandBrew 11-26-2012 04:08 PM

+1 to using a hydrometer for FG. Using a refractometer is stacking errors because it is dependant on OG.


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