I consistently get extra lauter volume. (Adjust grain absorption value?)

I have very consistent results using BeerSmith2.  I am a happy customer who always recommends purchasing the product.  (“Insists” is probably more accurate.)  I do, however, have one VERY long standing wrinkle that I’ve been living with, but today I’m going to try and resolve it.

I always get 20min extra volume wort when lautering.  ~5 cups.

Every time.  Always.  Across all recipes, new or old, for my standard 3.5 to 4gal batch sizes.  All are single infusion Ales and I fly-sparge.  I always use whatever mill setting my local shop has when I purchase/grind my grain for my various big IPAs.  (Also: I refuse to enter a false value for [Deadspace] when I know exactly what the value is, because I can measure it precisely.  I'd rather just boil longer than enter fake info.)

I’ve tried everything under the sun except for the dreaded [Advanced Options] tab.  I seem to have no other option but to fuss with that and decrease the [Grain Absorption] value to account for the extra wort.

Anyone care to throw down some logic/math on what I should change the default value of [0.9600] to, for 5 cups extra wort?

Preface: I don't fly sparge.  I've done it twice with unsatisfactory results.  I went back to batch sparging, rather than fuss with a proper manifold.  That said, I'm willing to take a stab at the math and/or logic.

If you ALWAYS get ~5 cups regardless of grainbill, that doesn't lead to concluding that your grain absorption is wrong.  You should have more for bigger grainbills and less for smaller ones. 

To test this hypothesis you could do a batch-sparge batch, and MEASURE your absorption.  Or do a test with a pound or two in the kitchen with a strainer. 

(Water-in - first-runnings - deadspace )
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                weight of grain

Assuming the above doesn't rectify everything, I know you don't think that its your deadspace...but, that seems like the most likely cause for a FIXED volume error.  Is your deadspace truly a deadspace?  By that I mean is it space below a false bottom that is filled with wort ONLY?  If the "deadspace" can be filled with grain, then it isn't truly dead...more like a zombie.  The grain will take up some of the space, so the real loss ends up being quite a bit less than a plain water measurement. 

Ahhhh.  Master Po, you are wise.

I just added the [Total Grain] field to my main recipe page and zipped through my house beers, and there is quite a difference, even though I didn’t think there would be.  Min 9lb, Max 10lb (all 3.5 gal batches, but my Arrogant Bastard which is 3.2, because I don’t dry hop that one.) 

Looking back through my “one-pager” brew notes, I find extra pre-boil volumes thus:

- AleSmith IPA (9.88 lb)  .4, .4, .3        (avg. - .37 gal, 5.9 cups)
- Arrogant        (9.31 lb) .45, .3          (avg. - .38 gal, 6.1 cups)
- Celebration    (9.0 lb)  .2, .4, .4        (avg. - .33 gal, 5.3 cups)
- Hop Ottin’      (10.0 lb) .475, .35, .3  (avg. - .38 gal, 6.1 cups)

Overall average = .36 gal – or 5.76 cups (..too much pre-boil volume.)

But the variance may be more my estimate of % between gallon line markers in the brew kettle more than actuals.  I’m more apt to believe the averages, but the multiple digit ones may be more accurate because I used a ruler and figured it out a few times proper.  (…How embarrassing – to admit I’m that far off every single brew – FOR YEARS. )

My rough estimate has always been 20min extra boil, or 5.33 cups.  That was under shooting the 5.76 average a tad.

As far as my deadspace, I’ve never seen under the grainbed.  I assumed (…) the grains stayed mostly on top of the SS false bottom, and since the beer was running clear, that the deadspace was clear.  I suppose grain would get in through the mesh to a large extent and fill up the underside.  I suppose if that were the case it could fill up and clot up against the bulkhead ball valve.  With the SS false bottom supporting the entire grainbed, that area might still be filled up with loosely consolidated grain.

Holy powder puff, is it really just that simple?  Taking my .625 gal [Lauter Tun Deadspace] value and decreasing it by .36gal = .265?

Well... next Tuesday we will see.  Thank you for the kicker tom_hampton!