Pre Boil Gravity vs Post Boil Gravity

Homebrew Talk - Beer, Wine, Mead, & Cider Brewing Discussion Forum

Help Support Homebrew Talk - Beer, Wine, Mead, & Cider Brewing Discussion Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

mcurtis431

Member
Joined
May 23, 2011
Messages
22
Reaction score
0
Location
Cumming
I am a little confused. I am very new to AG brewing. My first batch was an American IPA with a target OG of 1.062. I used Beer Smith to help create the recipe. After I mashed and sparged I took a gravity reading of the pre-boil and it was around 1.04ish. Beer smith uses this number to caSculate the extraction efficiency. Using this number gave me a terrible efficiency result. However, when I boiled down to my target volume my gravity was much closer to the target 1.062, thus bumping my efficiency up.

My question is, which reading do you use to calculate your efficiency? It doesn't make sense to me to use the pre-boil reading since this is not the volume you will be fermenting at. When designing a recipe, which gravity reading should I use as my OG?

I have yet to read a clear answer.....
 
I take a pre-boil gravity to determine my efficiency. You have X gallons at Y gravity and compare it to the max theoretical extraction you could get from your grain bill. I would guess you didn't adjust your volume for your pre-boil gravity so you were calculating your efficiency on your final recipe volume coming in at 1.040 vs 1.062.

The OG is what you have post boil in your final volume of wort that goes into your fermenter.
 
Pre and post boil should yield the same efficiency, since there's no change in the amount of sugar in the wort, just in the amount of water.
 
A mathematical example might help here.

You had a 5 gallon batch with a OG of 1.062. If you multiply the 62 times the number of gallons, you have 310 points in your wort. For a preboil reading, you likely had 7 gallons of liquid, which probably had a gravity reading of 1.044 (310/7).

So, if you know what your preboil liquid level is, you can get a gravity reading and estimate what your gravity will be postboil the same way.

If you had gotten a 1.044 reading preboil on 7 gallons, you would have measured 308 points in your wort (7 times 44). By dividing the 308 by 5, you'll get your postboil gravity, 1.062.
 
Use the post boil with adjusted water volume to calculate efficiency and you starting OG. Why take two readings ( pre and post ) when all calcs can be done with 1 reading? I get lazy sometimes.

What was your OG supposed to be after the boil?
 
Since the original poster is new to AG brewing, taking a preboil reading is valuable to understand whether or not efficiency was close enough to what the recipe called for.
 
I had a similar case where the pre-boil gravity of my beer should have yielded a lower post-boil gravity based on the boil off. Somehow, instead of the calculated post-boil gravity, it ended up higher, giving me two different numbers for my overall efficiency. I'm thinking OP had a similar problem. I also used BeerSmith for my math, so it could be a flaw with that.....
 
also, a pre boil reading is great when you don't have your process dialed in yet and need to know how much boil off you really need to hit close to your desired OG. When I started AG I had 1 batch that had a much higher effeciency than I anticipated and didn't figure that out until I had it boiled all the way down - and it was a long boil. I could have shortened my boil and had a more reasonable OG if I had taken a pre boil OG. Was still great beer but not percicely what I had envisioned.
 
when you have a reference to OG that is the wort chilled down to room temperature in the fermenter having the exact batch size volume before pitching the yeast.

The preboil gravity is the SG (specific gravity) of the worts preboil volume collected before evaporation, boiler equipment loss, or shrinkage from chilling. This stage is where you make your adjustments to the gravity so once boiled, chilled, and transfered into the fermenter, you hit your recipes targeted volume and OG
 
Pre and post boil should yield the same efficiency, since there's no change in the amount of sugar in the wort, just in the amount of water.

I agree with this in theory, but in practice my pre-boil efficiency always comes out a point or two higher than my post-boil efficiency.
 
I agree with this in theory, but in practice my pre-boil efficiency always comes out a point or two higher than my post-boil efficiency.

This likely due to temperature. If you use a chart to adjust your gravity reading, it will be off a little bit. If you chill your sample down to calibration temp for the hydrometer (usually 59F) you will get more acurate results.
 
Don't forget to adjust water volumes for temp differences.

Also make sure your volume measurements are accurate. Try adding +/- 1 pint on all your measurements and see how much your efficiency swings. My margin of error is quite large, and my sample size is far too small to determine anything meaningful from <5% swings in efficiency.
 
Don't forget to adjust water volumes for temp differences.

You're absolutely right. Lets say you brew a 15 gal batch and collect 18 gal of preboil wort. By the time you get all of the runnings mixed and measure volume/gravity the temp is at 135F. You actually have 17.71 gal of wort @ 59F (the reference temp of you gravity reading), and this is what you should use to figure out efficiency and estimated OG. I have my software thermally balanced so that it accounts for this automatically.
 
A mathematical example might help here.

You had a 5 gallon batch with a OG of 1.062. If you multiply the 62 times the number of gallons, you have 310 points in your wort. For a preboil reading, you likely had 7 gallons of liquid, which probably had a gravity reading of 1.044 (310/7).

So, if you know what your preboil liquid level is, you can get a gravity reading and estimate what your gravity will be postboil the same way.

If you had gotten a 1.044 reading preboil on 7 gallons, you would have measured 308 points in your wort (7 times 44). By dividing the 308 by 5, you'll get your postboil gravity, 1.062.

I realize this is an old thread but was wondering if I have this correct. If the recipe says I should get a 1.062 SG but my pre-boil says 1.032 (6.25 gal, mashed at 153 degrees 90 mins) then after I boil the wort cool it down and only end up with a 1.050 I'm actually getting an 80% efficiency? (50/62) I'm pretty new at this.
 
I realize this is an old thread but was wondering if I have this correct. If the recipe says I should get a 1.062 SG but my pre-boil says 1.032 (6.25 gal, mashed at 153 degrees 90 mins) then after I boil the wort cool it down and only end up with a 1.050 I'm actually getting an 80% efficiency? (50/62) I'm pretty new at this.

Math is hard, just use online calculators. Here's one for boil off: http://www.brewersfriend.com/dilution-and-boiloff-gravity-calculator/.

And here's a bunch more: http://www.brewersfriend.com/stats/.
 
I realize this is an old thread but was wondering if I have this correct. If the recipe says I should get a 1.062 SG but my pre-boil says 1.032 (6.25 gal, mashed at 153 degrees 90 mins) then after I boil the wort cool it down and only end up with a 1.050 I'm actually getting an 80% efficiency? (50/62) I'm pretty new at this.

Extraction efficiency for the mash is based on the maximum yield potential of the grain that you're using. It has nothing to do with your intended OG. For example, the base malt I use has a maximum gravity contribution of 38 points per lb per gallon. If I mash 10 lbs, then that gives me a max of 380 points. Let's say we get an 80% efficiency, that's 380*0.80 = 304 points. For a 5 gallon batch, that's 304/5 = 60.8 So roughly a gravity of 1.061. If the intended OG was 1.061 and we actually got 1.058 then our mash yield was 5 * 58 = 290 points 290/380 = 76% mash efficiency

Calculators and software are awesome, but I'm of the opinion that you should learn the math involved before you rely on them. That way you can spot problems and trouble shoot more easily and have a better understanding of what's actually going on.
 
Meh. I've never wanted or needed to calculate what my OG will be by hand, and I really don't think it would add anything to my understanding of what's going on in the mash. Just like doing long division on paper rather than a calculator doesn't add anything to my understanding of math. It's just a longer, more tedious, and more error prone way of doing something.

I feel like the only time I would need to do it by hand would be in a post-apocalyptic society with no computers or internet. But in that situation I think I could just estimate the amount of whatever grain I could get my hands on, and that would be good enough.

But I guess somebody needs to know the formulas and whatnot so that they can write the computer programs that make the calculators that we all use. So I can see your point.
 
Back
Top