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When brewing an IPA about a week ago, my starting gravity reading came out to be 1.12 - significantly higher than the listed target of 1.07 on the recipe. I had suspicions of error at the time which I believe was confirmed when I took a reading this weekend while transferring to secondary - the beer is now down to about 1.02, which seems like a large drop in just a week.

Here's what I believe happened - I noticed after taking the first sample that it contained some sediment that had failed to settle out. Being relatively new to this, I didn't think much of it at the time but now I imagine this contributed to an artificially high initial reading. Can anyone confirm this?

Also, is there any way to estimate what the starting gravity may have actually been besides relying on what the recipe called for (I doctored the recipe a decent amount so I'm not confident in the printed target)?

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It would be useful to know two things: 1. What is the recipe you used? 2. What temperature was the wort when you took the gravity reading? –  Chris Plaisier Mar 5 '13 at 20:53
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3 Answers

up vote 4 down vote accepted

EDIT: I'm not sure I realized it was 50 points we were talking about here, or just let my attention wander for a bit! Suspended solids can make a difference (see the comments), but you'd almost have to be measuring the SG of slurry for it to make that much of a difference!

If the original recipe called for 2 cans of extract, and you used 3 then that's just under a 50% increase (the recipe also included grains.). 1.070 plus an extra 50% is 1.105, which puts you in the ball park for your reading of 1.120. Depending upon how much suspended sediment was in the jar, an additional 10-15 points is not improbable.

PS: 1.12 is not the same as 1.120 - the additional 0 shows you have measured it to 1/1000 precision rather than 1/100 precision.

My original answer follows.

It's most likely the sediment that caused the high reading, if the hydrometer is otherwise calibrated and you have corrected for temperature. It's widely believed that the hydrometer is not affected by suspended sediment - but in fact it is:

Hydrometer analysis begins after thoroughly mixing the sediment and water, after which particles settle out of the water column according to Stokes’s law. The density of a sediment-water suspension depends on the concentration and specific gravity of the sediments present in the mixture. If the suspension is allowed to stand, particles will settle out of the suspension and the density of the sediment-water suspension will decrease.

Although this is discussing drilling, the same principles apply. While the solids are in suspension, the density of the liquid/solid mixture is the average of the liquid and solid densities. When the solids fall out of suspension, then it's just the liquid density that is measured.

It's not possible to determine what the original gravity was without knowing the average density of the sediment in the sample jar.

Just chalk this one up to experience - no real harm done. Next time leave the gravity sample to settle and also to settle to room temperature, so the hydrometer reading is accurate.

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Downvote with no comment? This is not helpful. –  Tobias Patton Mar 5 '13 at 19:01
    
That's a very interesting link. I'm not the one who voted this down, and I agree that it's not helpful for someone to have done so without comment. That said, I'm still skeptical that particle suspension is relevant to brewers, especially to the extent that the OP seems to be experiencing. I can't run the numbers on the equations you linked because I don't know all of the constants, but just playing around I'm struck by how quickly larger particles fall. If we were talking about a point or two, I'd believe it, but 50 points? I've just never seen anything like it, even in my muckiest beers. –  MalFet Mar 5 '13 at 22:04
    
Yes, there's no way 50 points difference comes from suspended solids. More likely an uneven mixture of extract. But suspended solids do make a difference to the reading and should be avoided as much as possible. I frequently hear people say that suspended solids don't affect the reading, which is a fallacy. What I do agree with is that the affect will typically be small. I should have put that in my answer. –  mdma Mar 5 '13 at 22:39
    
The hydrometer works by displacement - the weight of the hydrometer equals the weight of the volume displaced. If there are solids present in the liquid being displaced these are also part of the displacement, adding to the total weight of displacement. When we are talking 1 part in 1000 being significant (1 gravity point) then suspended solids should be avoided if they are of any appreciable weight - protein and yeast don't have any appreciable weight so their affect is not significant. –  mdma Mar 5 '13 at 22:41
    
Definitely. I understand how hydrometers work, and certainly suspended solids can have a measurable impact on displacement. They're often used, for example, to measure particle size in soil samples. However, I'm not convinced it matters at the level of precision we work with (I've edited my answer to make that more clear). For example, ajdelange used some of his fancy equipment to compare trubbed & yeasted beer to centrifuged beer. He found densities of 1.053140 and 1.053037 respectively. That's a mighty small difference. (thebrewingnetwork.com/forum/viewtopic.php?f=&t=20854) –  MalFet Mar 6 '13 at 0:32
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There is absolutely no way to guess a recipe's OG without knowing the particulars of the recipe.

Likewise, suspended solids won't affect the reading by a significant margin unless they are actually bound to the hydrometer itself.

There are several things that could effect a hydro reading. Did you add top-off water before you took the reading. If so, there's a good chance the water wasn't mixed thoroughly.

In any case, 1.120 -> 1.020 is not particularly unlikely. For your OG to be fifty points above target, however, is strange. How did you measure your volumes? If you ended up with a much smaller final batch size than the recipe planned for, this could also affect your OG.

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1.120 to 1.020 is ~83% apparent attenuation. This is possible with some yeast. But in one week? I've never seen that much of a gravity drop in such a short period of time. My best guess is that the initial gravity reading was mistaken. –  Tobias Patton Mar 5 '13 at 19:03
    
I don't have the recipe in front of me so I apologize in advance for the vague numbers. I had been anticipating a slightly higher OG, as I used more extract than the recipe called for (3 full cans and about a pound of steeping grains). We also pitched 2 packs of Wyeast, which was more than called for on the recipe. Of course, the yeast wouldn't account for the high OG, but could it have contributed to the quick attenuation? –  Hock Mar 5 '13 at 21:51
    
You added three cans of extract in total, or three extra cans? Like I said, I strongly suspect that your OG reading was wrong due to incomplete mixture of the concentrate and water, but I've certainly seen beers ferment from 1.120 -> 1.020 in a week (especially if kept warm). Truth be told, you may just never know with this one. –  MalFet Mar 5 '13 at 22:07
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You can enter your final recipe in an app like beersmith or hopville.com to get a general idea of your expected starting and final gravities.

Did you brew from extract with a partial boil? If so, your starting gravity reading will most likely be inaccurate since the wort and top off water won't be completely mixed.

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I did brew from extract, perhaps that was the main culprit. –  Hock Mar 5 '13 at 21:44
    
There is a logical gap here. Extract brewing is being falsely linked to partial boil. While many (most?) extract brewers do partial boils, that does not mean this brewer necessarily did. This answer needs to clarify partial boil, rather than incorrectly implying extract as causing an inaccurate gravity measurement. So, -1 for that, but +1 for the suggestion of using brewing software; net score 0. –  Galapagos Jim Mar 6 '13 at 0:15
    
This is the correct answer, no doubt. You see this exact problem ("My extract OG is too high/low") all over brewing forums, and is virtually always related to top off water not mixing properly. –  Graham Mar 7 '13 at 13:14
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