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Old 11-16-2009, 07:51 PM   #1
Rugg1132
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Default Big Jump in Gravity Pre-Boil to Post-Boil

Hey, I just did my second all-grain batch and wanted to evaluate my efficiency. I used 12.5 lbs of grain, mashed in an igloo beverage cooler with cpvc manifold at around 152 for an hour, held the temperature very well and opened to stir every 20 min. Fly sparged with another cpvc manifold with 170degree water for about 45 min an collected right around 6 gallons. I stirred it and got a sample for testing. I checked the gravity right away and got 1.034. This seems really low, hinting at an efficiency of maybe only 44% as 100% should have yielded around 1.077.
I continued to the boil, added the 1 lb. of candi sugar as planned and after the hour boil and cooling gravity was at 1.082, which is right where it should be according to BeerSmith. I had probably boiled down to about 5 gallons.
Is it normal to see this big of a jump from pre-boil to post-boil? When the pre-boil came in so low I was expecting to be shy of the target OG but was surprised when I hit it pretty close.
Do you think my efficiency was really that poor or could I have tested a light sample? The coloring seemed to get a lot lighter after I collected about 3 or 4 gallons. I know hydrometers can be inaccurate at various temperatures so I checked around and found at 140-150 degrees I could add about 0.015 to my reading. That still comes out to an efficiency of only 63% or so.
I appreciate any input/help. Thanks a lot guys.

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Old 11-16-2009, 08:18 PM   #2
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I am going to take a guess on a few things, in all honesty I defy the laws of nature like this from time to time.

First, you may have gotten a lighter sample. I understand that the wort can stratify with the thinner wort sitting on top of the denser wort. You say you stirred it so that is really about all you can do, but maybe it didn't mix as well.

Second, you are right in that hydrometers are calibrated to a certain temperature and if the wort is a different temp you have to adjust. I know there are tables out there but I would guess at 140-150 there is a decent error factor. Not sure if this is enought to get you that out of whack, but you really should let your hydrometer sample cool some to get it closer to the calibration temp. I usually set it in a glass in ice for a little while.

Also, don't forget to take into account the candi sugar. That will make your gravity higher since you added it into the boil.

By my quick calculations, you had 410 "gravity units" in your final beer...5 gal x 82 = 410. 46 of this was due to the candi sugar, which put your wort at 364 w/o the sugar added. As such the theoretical "correct" reading at a preboil of 6 gallons should have been in the 1.060 range --> 364/6 gallons = 60. That would have put you at an efficiency of 78% ---> 364 preboil g.u. / 462 100% eff g.u. (got this by 1.077x6 gal = 462 possible gu)

I hope all that math works, I'm doing this on the fly! Good luck!

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Old 11-16-2009, 08:23 PM   #3
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I had this happen a month ago, and I think the cause was a massive amount of floculating proteins and other trub that was still in solution. I kept getting 1.040 on repeating readings but after the hot break I let a sample cool down and watched all the sediment settle out like snow, after which it read about 20 points higher. I'm not sure of the physics and chemistry behind it, though.

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Old 11-16-2009, 08:26 PM   #4
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1.034 at 160 degrees = somewhere around 1.068 at 68 degrees.

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Old 11-16-2009, 08:54 PM   #5
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Thanks a lot for the replies. Everything makes sense and it's encouraging that I maybe didn't build a horrible all grain system! I'll take all the advice on the next brew and hopefully get a more accurate reading and reliable gauge on my efficiency.

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Old 11-16-2009, 08:54 PM   #6
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Did you cool your sample to 60 degrees? Even with a "correction", SG readings are notoriously inaccurate over 100 degrees. I cool mine to 90 degrees, then take the reading and adjust it for the temperature. It's not perfect but it's far more accurate that way. (But I just bought a refractometer- and it went much better today!)

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Old 11-16-2009, 09:10 PM   #7
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Hi Rugg, this will not answer your question directly but here is a thread of Pol and Kaiser hashing out efficiency and it helped me understand how to break up and measure each step of the mash and sparge in order to quantify efficiency. Hope this helps:
http://www.homebrewtalk.com/f36/has-more-effect-efficiency-crush-sparge-143646/

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Old 11-17-2009, 04:17 PM   #8
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If you measure the specific gravity correctly (by stirring the wort first and cooling the wort sample down to get an accurate reading) you will know before you even start your boil exactly what your finishing gravity will be.

This is because total gravity (beg. of boil) = Total gravity (end of boil).

For example:
You have 8 gallons of runoff from the mash. The specific gravity of this wort is 1.038. You plan to boil long enough to yield 5.5 gallons of finished beer. Because the total gravity will not change during the boil, you can calculate the finishing gravity of the boil:

(38 GU x 8 gallons) divided by 5.5 gallons = 55.3 GU.

This calculation tells you that this wort will have a gravity of 1.055 when boiled down to 5.5 gallons.

If you look at the equation above, the GU (gravity units) are the #'s after the "0" when reading your gravity. 38 is 1.038 and 55.3 would be 1.0553 (rounded down to 1.055). Do the math from the equation and you will see that 38 x 8 = 304 total gravity units and 55.3 x 5.5 = 304 gravity units.

The only difference for you is that you are adding some fermentables to the boil, but you could figure how much those will add too. I didn't do the math on your situation, but if it doesn't equate, I'm guessing that your original hydrometer reading was off due to the temperature of the wort. I had a bad hydrometer for years that was screwing me up. Lastly, when figuring total gravity, don't forget that hot wort will show something like 4% more volume than when it's cooled.


Quote:
Originally Posted by Rugg1132 View Post
Hey, I just did my second all-grain batch and wanted to evaluate my efficiency. I used 12.5 lbs of grain, mashed in an igloo beverage cooler with cpvc manifold at around 152 for an hour, held the temperature very well and opened to stir every 20 min. Fly sparged with another cpvc manifold with 170degree water for about 45 min an collected right around 6 gallons. I stirred it and got a sample for testing. I checked the gravity right away and got 1.034. This seems really low, hinting at an efficiency of maybe only 44% as 100% should have yielded around 1.077.
I continued to the boil, added the 1 lb. of candi sugar as planned and after the hour boil and cooling gravity was at 1.082, which is right where it should be according to BeerSmith. I had probably boiled down to about 5 gallons.
Is it normal to see this big of a jump from pre-boil to post-boil? When the pre-boil came in so low I was expecting to be shy of the target OG but was surprised when I hit it pretty close.
Do you think my efficiency was really that poor or could I have tested a light sample? The coloring seemed to get a lot lighter after I collected about 3 or 4 gallons. I know hydrometers can be inaccurate at various temperatures so I checked around and found at 140-150 degrees I could add about 0.015 to my reading. That still comes out to an efficiency of only 63% or so.
I appreciate any input/help. Thanks a lot guys.
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Old 11-17-2009, 04:25 PM   #9
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I forgot to give credit in my previous post to the book Designing Great Beers by Ray Daniels I have a lot of brewing books that I like for different reasons, but this book is by far my favorite because of the amount I learned from it and how it gets the brewer to totally understand recipe formulation and how the brewing process works etc.

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