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).
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.
Originally Posted by Rugg1132
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.