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Old 03-15-2012, 04:22 PM   #1
Geordan
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Default Discrepency in gravity with BIAB, not enough experience to know why

Hey everyone!

I'm trying to pin down a variable in my BIAB brewing. I've done 4 batches so far, and had a similar issue with each: at mashout, with the proper amount of water, my gravity works out as such that I should hit my FG pretty much dead on, using the GU formulas given in Designing Great Beers. At the end of the boil, I'm left with exactly the amount of wort I expected, +/- a quart, but my gravity tends to be 10 points or so above what I was expecting. I had done the recipes in BeerTools using 75% efficiency the first few times, which I figured had caused the variance, as I am actually hitting closer to 81% brewhouse efficiency. However, changing my efficiency in BT even to 85% still doesn't account for a 10 point gain in FG. For specificity's sake, here's some specifics: last night I did an Irish Red with 3.5lbs MO, 2lbs Vienna, .2lbs C120L and .1lbs Roasted Barley. My final volume was 3.05 gallons, and my FG was 1.066. Mashed at 150f for an hour, mashed out at 170f for 15 minutes.

Would a wonky thermometer reading too low cause this -- that is, I know that mashing too high leaves you with too much unfermentable sugar; does that also bump the FG higher than expected, or does that variance not show itself until fermentation begins?

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Old 03-15-2012, 04:40 PM   #2
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Are you saying that in general your post-boil SG is 10 points higher than your pre-boil SG? If so, this is what happens when you concentrate the wort by boiling off some water. If that's your question, it's very easy to answer

Or... are you saying that your pre-boil SG is usually spot on with what the software predicts for a boil gravity but the post-boil SG ends up higher than what the software predicts for an original gravity?

The increase in SG due to boiloff is one of the most predictable things in homebrewing. This is basic physics, so if you know the exact volumes before and after the boil, you should be able to perfectly predict the gravity change.

If the software predictions are consistently off, I would say that you're measuring your volumes improperly, or you're measuring your SG improperly. Those are the only two possibilities (assuming the software is calculating it correctly).

So I would check three things:
1) Are you sure of your volume measurements?
2) Are you sure the wort was well-mixed when you measured the preboil SG?
3) Are you properly adjusting your hydro reading for the temperature?

It really can't be anything else! Good luck!

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Old 03-15-2012, 04:50 PM   #3
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Zach,

Thanks for the response -- see, this is where I'm confused. If my volume measurements are wrong, they're consistently wrong across all of my pieces, since it all matches up. I'm confident that the wort is well mixed, and I'm accurately adjusting for hydrometer temperature.

Does a too-high mash temp result in a higher FG, or does that issue not manifest itself until fermentation is complete?

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Old 03-15-2012, 05:07 PM   #4
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Quote:
Originally Posted by Geordan View Post
Zach,

Thanks for the response -- see, this is where I'm confused. If my volume measurements are wrong, they're consistently wrong across all of my pieces, since it all matches up. I'm confident that the wort is well mixed, and I'm accurately adjusting for hydrometer temperature.

Does a too-high mash temp result in a higher FG, or does that issue not manifest itself until fermentation is complete?
Hmm that's a puzzler. Have you checked your hydrometer in water? Are you measuring the final volume when it's hot or once it's cooled? That's only a small difference (4%) but it might explain something.

Other than that, I don't know. Like I said, the variables are preboil volume, preboil SG, postboil volume, postboil SG. Accurately knowing 3 out of the 4 is enough to calculate the one unknown! So again, assuming the software is right (and you're inputting your equipment profile and boiloff rate correctly) it has to be one of the measurements. Unless there's something silly I'm overlooking.

I'm sure someone else can confirm this, but I would say that you are correct, a too-high mash temp will still show the predicted OG, the differences will show up in your post-fermentation FG. The hydrometer measures density, if the sugar in the wort is fermentable or not it will still increase the density of the liquid.
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Old 03-15-2012, 07:34 PM   #5
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I'm reaching here, but I think it's an issue with BIAB in general.
It seems (in my experience anyway) that efficiency is inversely proportional to amount of grain.
I used to do 5 gallon batches, and with 7-8# grain bill, I would see 75-80% efficiency.
If I tried to do a wee heavy (grain bill more like 14#) , I'd see 55% efficiency.

I wonder if you're seeing insane efficiency because of your small batch/grain bill size.

God only knows how you accurately predict this. I gave up and went to 3 vessel.

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Old 03-15-2012, 07:37 PM   #6
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what temp are you taking the readings at? hydrometers aren't very trust-worthy over 90F or so.

mash temp doesnt affect your OG, it does effect your FG tho

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Old 03-15-2012, 09:18 PM   #7
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wolfman: that's a very interesting observation -- I might end going the same route.

dcp27: I measured at 65f and converted the results to 59f using the Brewer's Friend calculator. Glad to know that I wasn't crazy thinking a high mash wouldn't affect my OG.

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Old 03-15-2012, 09:57 PM   #8
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Quote:
Originally Posted by wolfman_48442 View Post
I'm reaching here, but I think it's an issue with BIAB in general.
It seems (in my experience anyway) that efficiency is inversely proportional to amount of grain.
I used to do 5 gallon batches, and with 7-8# grain bill, I would see 75-80% efficiency.
If I tried to do a wee heavy (grain bill more like 14#) , I'd see 55% efficiency.

I wonder if you're seeing insane efficiency because of your small batch/grain bill size.

God only knows how you accurately predict this. I gave up and went to 3 vessel.
In my (limited) experience, this is true, but I don't BIAB. I took a huge efficiency hit on a big beer I made a few weeks ago.

But the OP says that his preboil gravity matches his predictions, which means that he must be accurately predicting his efficiency (assuming all measurements are correct)

OP, what's your predicted/actual preboil gravity, preboil volume, postboil gravity and postboil volume? If you hit your preboil SG there's no need to bring efficiency into this.
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