A basic question about AG wort gravity

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Well-Known Member
Jul 30, 2007
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Lincoln, ND
I have a question that popped into my head today. Though I'm sure it's not common practice, what would be the result if you tried to take a gravity reading on unboiled wort collected after sparging? Would it be somewhat/unusually high or would it be low? I'm going to take a wild guess and assume that it would not be reliable as an OG reading, but instead, you'd have to wait until you had your boiled beer, hops and all, in order to take an original gravity reading. This is just a question for the sake of knowing, since I got a hydrometer for Xmas and I want to make a good practice of taking OG and FG readings.
Many people do this as a way to tell how long they need to boil. If you know your boil-off rate and your preboil gravity you can calculate the exact amount of time you need to boil to get to your desired OG. Also, this reading combined with your volume collected from the sparge should be able to tell you your efficiency. It should be lower than your expected OG since by boiling you concentrate your wort and increase the gravity. Hops should not effect your gravity reading.
Thanks for the info. So, let's say the reading was 1.010 or 1.020 or something in between. Would that be normal for unboiled wort after sparging? I'd like to have an idea of what numbers I'm looking for.
A lot of the brewing software will make these kind of claculations for you. But here is an example:

Expect OG of 1.065 in 6.5 gallons--->65*=6.5=422.5
Expect to start the boil with 7.5 gallons--->422.5/7.5=56.3
So I would expect my gravity of the wort after sparging, before boiling to be around 1.056.
you got to remember when mesuring pre boil that hydrometers are calibrated at 60F. When you mesure post sparging wort at 158F, your mesurment is going to be lower than it acctually is. So your mesurment of 1.020 is probably more like 1.040
Hm. If that's the case, my efficiency may be utter garbage. My sparging method isn't very good, in retrospect. I do a cross between batch sparging and fly sparging. If possible, I'll keep a constant flow of water (possibly not slow enough) and other times I'll let it sit momentarily and leach the sugars from the grain. Is it uncommon for there to be a bit of sweeter grain collected at the lowest side of the grain bed? Maybe it's my underlet or maybe it's my sparging technique, but something can't be right.
I would say stick with one style or another for sparging, IMHO. Bt post your recipie and we ma be able to give you a bit more input on whats going on
Sure, I'd appreciate the input.

Here's my ingredients list:

8 lbs. American Two-row Pale
2 lbs. Amber Dry Malt Extract
1 lbs. Belgian Biscuit Malt
1 lbs. Belgian Aromatic
1/2 lbs. Gambrinus Honey Malt
1/2 lbs. Dextrin (CaraPils) Malt
4 oz. Cane sugar.

Besides that I had 1 ounce each of: Willamette, Fuggle and Tettnang hops. I also used 4 oz. of hop suey for late hop addition. The yield was a little over 6 gallons.

Nothing too unusual. After I got everything in, I did about an hour and 10 minute long boil and cooled the beer in about 45 minutes or so. I filled my kettle up the about 9 or nearly 10 gallon mark to account for condensing the wort, evaporation and wort loss to hop trub.

Here's the thing - the wort that I took my hydrometer reading with was the final runnings off my grain...what was left over after I had gotten that initial 9-10 gallons I mentioned. I kept the extra final runnings in a pitcher since I wasn't sure what I wanted to do with it or not. The gravity of that stuff couldn't be terribly high anyway, and not reflective of the total gravity of the wort collected.

I think I just had an "aha" moment with that. Unless I took a hydrometer reading of all the combined wort, the stuff I did take a reading with is an inaccurate reflection of my true gravity.

Thanks for talking me through this guys...I hope I've got this one figured out! :fro:
Well, I don't really have an answer for that. That's why I asked my original question. I was in such a hurry to get the beer into the carboy, I forgot to take a hydrometer reading. : / So I tried to gauge the gravity by measuring the excess runnings, which didn't do a bit of good. Sometimes I get ahead of myself and forget basic stuff like that.
two things,

1. really warm temperatures do throw off your hydro reading. there are calculators out there for this, but if its REALLY warm, even the calc. is wrong. the further away from 60F the more skewed the reading gets. so sometimes these readings are just a ballpark value.

2. i agree with sticking to just fly sparging or just batch sparging. if you're doing a combination of both because your MLT can't hold all the sparge water in one go, I'd simply suggest two sparges of equal volume. vorlauf after each sparge.

you should get better efficiency doing that, than draining part of the sparge and then adding more water while continuously draining because you get a better rinse of the sugars.
During the boil, you will lose water, but not sugar.
The gravity will therefore increase as the water evaporates.
Assuming you had a gravity of 1.036 pre-boil and a pre-boil volume of 6.5g, and you boiled down to 5.25g you would end up with a gravity of 1.045
Take the pre-poil gravity and subtract 1. This leaves .036 which identifies the amount of sugar in the wort. Multiply by the pre-boil volume, and divide by the post-boil volume (.036 * 6.5 / 5.25 gives .045)
Then add the 1 back again, and you end up with 1.045.
I suspect however that when you took the pre-boil gravity, you didn't stir the wort. This would give you a very low gravity as the bottom of the wort would have much more sugar dissolved in it than the top.

Hope this helps