How many points of gravity should i get when i boil 8.75 gallons down to 6 gallons?

I just made a Strong Brown Belgium and I use Beersmith. Everything was right on the mark and I started the 90 minute boil with a pre-boil gravity of 1.074. Exactly what Beersmith had estimated. But after the boil I had 6.25 gallons with an OG of 1.085. Beersmith estimated it should be 1.095. That sounds about where it should have been. All hydrometer readings were corrected for temperature and all were taken with the same hydrometer. And the FG came in at 1.012, exactly as expected. Why was the OG so low?

A 20/20 SG of 1.074 implies a wort of strength 17.97 °P. A 20/20 SG of 1.085 implies a wort strength of 20.45°P. A volume of 6.25 gallons of that (at 20 °C) would weigh 8.34*1.085*6.25 = 56.5556 pounds of which 8.34*1.085*6.25*0.2045 = 11.5656 pounds would be extract and 44.99 pounds would be water. During the boil only water is driven off. Calling the pre boil volume V we have its extract content as
V*8.34*1.074*.1797 = 11.5656 so V = 11.5656/(8.34*1.074*.1797) = 7.18538 gal. Thus if you boiled off 7.18 - 6.25 = .93 gallons or 13% in 90 minutes (not an unreasonable number) everything is as it should be. If a calculator gave you a different answer it assumed, or you gave it, a different boil off rate.

Not to dispute your math, you seem to know what you're talking about, but I just thought it was calculated more simply than that. Shouldn't you just be able to get a concentration factor by dividing the pre-boil volume by the post-boil volume and multiply by the original gravity points to get expected post-boil gravity? If this were so, then 8.75/6.25 = 1.4 and 1.4*74 = 103.6 which would indicate that something is, in fact, wrong. Like I said, I'm sure you know what you're talking about, but I just wanted to clear this up so if I run into this problem I know how to handle the math.

To begin with I scanned the OP a dozen times looking for the original volume and never thought to look for it in the title. That's why I answered the way I did. Knowing the original volume we calculate the amount of extract as
8.75*8.34*1.074*.1797 = 14.084 pounds. After the boil we still have 14.084 pounds but in 6.25 gallons. Thus we have 14.084/6.25 = 2.2534 pounds per gallon. That's (14.084/6.25)*(453.592/3.7854) = 270.02 grams per liter. So we need to know what strength of solution has 270.02 grams of extract per liter. That's determined by solving 998.203*SG(°P)*°P/100 = 270.02 for °P, where SG(°P) gives the specific gravity (20/20) for a solution of strength °P. That's a painful but doable job with the ASBC tables but trivial with a computer. The answer is 24.52 °P corresponding to a specific gravity of 1.1034

Thus:
1. Something is clearly wrong
2. Your 'rough' calculation using points is, as we say in Washington, plainly 'close enough for government work.'

Glad to know we got that cleared up. I like your way of doing it. I'm in the Navy and in an engineering rate, so I naturally like to be as precise as possible. I didn't even know this way of calculating it existed. Thanks for the explanation.

Thanks for the feedback. Everything you said makes sense, but for the life of me I can't figure out how I screwed up the hydrometer reading. For my brew log I am going with the calculated FG.

I'd say the most likely cause is in your volume measurements. Its harder to measure your liquid volumes than it is to read a hydro or refractometer.

pmcquillen said:

I just made a Strong Brown Belgium and I use Beersmith. Everything was right on the mark and I started the 90 minute boil with a pre-boil gravity of 1.074. Exactly what Beersmith had estimated. But after the boil I had 6.25 gallons with an OG of 1.085. Beersmith estimated it should be 1.095. That sounds about where it should have been. All hydrometer readings were corrected for temperature and all were taken with the same hydrometer. And the FG came in at 1.012, exactly as expected. Why was the OG so low?

Click to expand...

Did you measure the pre-boil volume & post-boil volume in the same vessel? Transferring losses could have an impact when trying to figure out what happened between Beersmith and reality. Is your boil really set to 90 min in Beersmith?

If you trust this set of numbers:
preboil OG = 1.074
postboil OG = 1.085
postboil volume = 6.25 gal

then you had 7.2 gal preboil and your actual boiloff rate is 0.62 gal/h

If you trust this set of numbers:
preboil OG = 1.074
preboil volume = 8.75 gal
Beersmith postboil OG = 1.095

then you have 6.82 gal postboil and 1.29 gal/h boiloff rate (about 15% of the preboil vol). What do you have entered in Beersmith for boiloff rate?

I don't think boiloff rate should be calculated as a function of boil volume, but as a volume/time. 0.62 to 0.75 gal/hr is accurate for my kettle.