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Old 08-22-2010, 04:16 AM   #1
NonServiam
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Default Implausible gravity reading on both hydrometer and refractometer

I just brewed a partial mash version of BierMuncher's Black Perl Porter. This was my second time brewing this beer; the first time went perfectly. This time, however, I'm getting some ridiculous gravity readings for the OG, and I just wanted to check with you guys that they couldn't possibly be right...

The gravity-contributing ingredients are (5 gal batch):
- 1.0 lb 2-row
- 2.0 lbs Munich
- 1.0 lb Caramel 20L
- 1.0 lb Chocolate
- 1.0 lb Flake oats
- 4.17 lbs Extra light DME (boiled 10 min)
- 8 oz Malto-Dextrine (boiled 20 min)
- 4 oz Lactose (boiled 15 min)

My target gravity was 1.074.

All of the grains were mashed using DeathBrewer's stove-top method. At the start of the boil I had 3.875 gallons of wort at 1.047 (~11.75% brix), which makes 182 gravity-gallons (a.k.a. "total gravity units" in Ray Daniels' Designing Beer book). My target of 5 gal at 1.074 was 370 gravity-gallons. So my deficit was 188 gravity-gallons, which at roughly 1.045 gravity per pound of DME per gallon of water implied I should add 4.17 lbs of DME during the boil in order to achieve my target gravity (my original recipe called for 4 lbs, so I was only slightly off).

I realize now, that when I did this calculation, I totally forgot to account for the gravity that would be contributed by the 12 combined ounces of malto-dextrine and lactose, which I hadn't yet added. So I should expect my OG to be higher than the 1.074 target, but not by a whole lot, right?

Well, after topping off to 5 gallons and aerating thoroughly, I drew my sample of wort and read its gravity first on the refractometer: 24.75% brix! This works out to about 1.110! I was like WTF, so I double checked with a hydrometer, and it confirmed, about 1.105 to 1.110 specific gravity.

Is it just me, or shouldn't it be physically impossible to achieve a gravity that high with the ingredients I added, no matter how great my efficiency was? There is not much hops in this recipe (1.5 oz), and in any case, I strained almost all of the hop trub out. What other kinds of factors/errors could lead to such crazy gravity readings? Is there a proper way to take a gravity reading that I'm possibly not following, which could lead to this?

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Old 08-22-2010, 04:54 AM   #2
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Yup, you're right, the ingredients you listed can't possibly add up to 1.110. Since you confirmed your refractometer reading with a hydrometer, my guess would be you mis-weighed one or more of the ingredients. Maybe a mix-up with the tare function?

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Old 08-22-2010, 10:00 AM   #3
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Could also be that the high gravity wort and top-off water weren't completely mixed, and the sample you took was mostly wort. It's much harder to thoroughly incorporate top-off water than it seems, and often takes several minutes of vigorous stirring.

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Old 08-22-2010, 11:10 AM   #4
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Either the stuff was extremely off in weight or the stuff didn't mix right.

Even at 100% efficiency the recipe can't be above 1.085 at 5 gallons. Now cut it down to 4 gallons (no top off water) and you could theoretically be close at 100% efficiency.

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Old 08-22-2010, 04:48 PM   #5
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Thanks for the replies, guys. It's pretty unlikely that I mis-measured the ingredients, just because of the way they were packaged -- even if I had screwed up the tare function, I couldn't have been off by much, because I know how many bags I used.

So the unmixed top-off water theory seems more likely, though that also surprises me. I would never have guessed it would be so hard to mix! This batch I happened to make a real point of aerating vigorously. I shook the carboy around until my arms couldn't take it anymore, and then my buddy shook it until his arms couldn't take it (kind of a contest ). Would you guys recommend actually stirring the post-top-off wort with a stick of some sort?

I've always just added my top-off water by pouring it through a funnel on top of the strong wort. Maybe I should try racking the water in through a tube that runs to the bottom of the wort, to ensure better mixing?

Edit: BTW, the airlock is bubbling quite furiously, and there's a nice krausen, so whatever happened the yeast (Safale S-04) don't seem to mind

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