Mash vs. Brewhouse efficiency

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SpanishCastleAle

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Saturday I brewed an Oatmeal Stout with the following grain bill (this is a 6 gal. batch):
8# Maris Otter
1# Roasted Barley
1# Crystal 60L
1# Flaked Oats
then I added .25# Molasses well into the boil (so the Molasses was NOT in my preboil gravity reading but WAS in my OG reading).

When I took my preboil/mash gravity reading I got 1.042 in 7.75 gallons. But then my OG was only 1.052 in 6 gallons (remember this number has the Molasses where the preboil gravity reading does NOT have it). So it looks like my Mash Efficiency is ~81% but my Brewhouse Efficiency is only ~76%.

My question is where is this efficiency going? This recipe used 2 oz. of whole hops so I assume all the wort that they absorb is one place I'm losing efficiency. But where else? I must admit that between the inaccuracy if gravity readings and volume measurements that there could be efficiency 'losses' there.

FWIW, I did a Glucan rest but still had a slow-ish sparge...not stuck but slow to the point that the grain bed couldn't 'keep up' with the usual flow rate. But I went from mash out to boil in about an hour so nothing bad. I usually do 5 gal. batches so this was a little more grain than I'm used to...plus the gummy Oats.
 
A brewhouse vs. mash efficiency is going to be due to your deadspace in transferring from your boil kettle to your fermenter (and as you mentioned the wort soaked up by the hops). Also you must be very sure of the volume you have in the end.
 
Also you have to be sure of your hydrometer readings - often the preboil wort is pretty hot and you have adjust your reading for the temperature. Even then, the farther away from 60°F the wort is the more inaccurate your reading will be.
 
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