Lost efficiency post boil?

So I have a recipe (Haus Pale Ale) that I calculated 759 total points.

I have 13.25gal to the kettle at 1046. So 46/(759/13.25) = 80.3 % Mash Eff. Much better than the 63% last time.

Post boil I have 11.25gal at 1050. So 50/(759/11.25) = 74.1 %

Should these efficiency #'s not match or at least be very close? I was expecting 1054 post boil, which would have given me 80.0% which would seem to me like the right answer. I figure the only culprit could be my thermometer. My to kettle reading was 1046@81 (I subtract .002 due to my hydrometer reading 1002 in distilled water) so that gave me the 1046 and I read 1052@65 into the fermenters for the 1050 (still correcting for the .002)

I also got 10gal to the fermenter at 1050 so 50/(759/10.0) = 65.9% Brewhouse Eff.

All in all not bad for my second batch. +1 for the SS braid, worked a hell of a lot better than my false bottom. at 80% mash eff. I'm not sure why anyone would want to fly sparge.

did you account for the 4% volume shrinkage that occurs when you cool the wort from 212 to 60F?

Kai

Kaiser said:

did you account for the 4% volume shrinkage that occurs when you cool the wort from 212 to 60F?

Kai

Click to expand...

yea, I measure the post boil volume after I take the cooling coil out with a sanitized aluminum rod with marks scored into it. Not a huge deal since I was shooting for 11 gallons to the fermenters at 1052 (75% Brewhouse eff.) and got 10 at 1050. So I was not off by much. I'm just wondering if my logic is off thinking that pre and post boil efficiency should be the same.

yes, but did you substract 4% from the pre-boil volume?
How did you cool the sample for the preboil measurement?

Kai

Kaiser said:

yes, but did you substract 4% from the pre-boil volume?
How did you cool the sample for the preboil measurement?

Kai

Click to expand...

Ok I got you. the preboil 13.25gallons was at 168 or so I bet after sparge, So assuming a 3% reduction (I assume that 168 deg water is not as expanded as 212 deg water). That would give me 77.9% mash efficiency and 74.1% post boil efficiency. That certainly gets it closer, but I don't think it is the true culprit. I'm assuming it was a measurement error along the way, but thanks for pointing this out. Does anyone take this 4% into account when calculating mash efficiency?

evandam said:

Does anyone take this 4% into account when calculating mash efficiency?

Click to expand...

yes you have to. otherwise you need to change the ppg value that you assume for the malt bill.

Another problem is cooling the wort in a dish or bowl. This allows for to much evaporation and concentrates the wort further. I have seen as much as 2-3 points increase between a sample cooled in a covered jar and a sample cooled in an open bowl.

Here is the thread that I posted when I had your problem: https://www.homebrewtalk.com/f36/post-boil-gravity-doesnt-match-post-boil-gravity-80456/

Kai