Pre-boil vs. post boil gravity. Please help!

[ClemTiger0408]

I am at my wits' end here.

This is my second batch using batch sparge after about 20 BIABs. I was averaging 72% efficiency with BIAB.

My first batch sparge mash did not go great. 62% efficiency. I'll live with since it was my first time.

Today, I thought I had better results. My pre-boil efficiency was 73%. The only issue was I misunderstood beersmith directions and sparged with about a gallon too much water. I did not put this extra volume into boil kettle.

When the boil was over, I hit my volume exactly. But my gravity reading put me at 64% efficiency again! How is this possible?!

Stats:
13.5# 2 Row
.75# chocolate
1# biscuit
1# crystal 60
1# flaked oats
.5# dextrose

Stats (expected values from Beersmith):
Actual pH: 5.2
Expected pre-boil volume: 7.1, Actual: 7.2
Expected pre-boil gravity (@ 75%): 1.067 Actual: 1.065

Expected post-boil volume: 5.5gal Actual: 5.5gal
Expected post-boil gravity: 1.089 Actual: 1.078
65.6% efficiency


This is my first time making a beer this high of a gravity. Probably not the best idea for my second time batch sparging, but pipeline is pipeline.

What could have gone wrong here? I took the pre-boil gravity reading twice and both samples with a hydrometer and refractometer. Sample taken just before boil. I stirred well to make sure the dextrose was evenly distributed.

I have no idea how to fix this! Help!

 

[Oginme]

Does your post boil volume include the volume of any trub or wort left in the kettle?

Just doing a quick gravity points balance:

7.2 gal @ 65 points [ (1.065 - 1)*1000] gives 468 gravity points

468 gravity points / 5.5 gal = 85 points or a specific gravity of 1.085.

If your volume of 7.2 gal was the volume at 160 F (or there abouts), the corrected volume at room temperature would be 7.2 gal / 1.025 (volume expansion of water at 160 F) or 7.0 gal.

The same calculation would yield an ending specific gravity of 1.083

Either you are missing some wort or trub loss somewhere or one of your readings is off.

In my system, I do expect a slight drop in efficiency for higher gravity recipes. I typically do full volume BIAB for standard gravity recipes and get a mash efficiency between 85% and 86%. For higher gravity recipes, I do a batch sparge and end up at 82% mash efficiency.