I started batch sparging about five or six batches ago. I use a square 5 gal Coleman cooler set up like Denny’s larger cooler. I have my grain milled at my LHBS, so I don’t know what setting is used on the mill. The LHBS has high turnover of product so I am sure that it is as fresh as most/any.
The first four batches had an absorbtion rate of between .10 and .13, which seems about on par with most of the information avail on batch sparging. But the last two batches have had a rate of .27.
Yesterday’s brew was with:
I mashed in with 9.06 qts water (1.25 qts/lb) which should have left aprox 8 qts avail in my first running. I need a boil volume of 4 gals, so this would work out great to divide the two runnings in half. After I recirculated, the run stopped at 6.5 qts. I thought maybe it stuck, so when I looked inside to stir there was no visable liquid. I tried anyway and got no additional liqiud. I added the 8 qts of water for the second running, and after the recirculation I drew off all 8 qts before it stopped again.
Since this same thing happened the last session, I was very careful to correctly measure the water at each step especially the first 9.06 qts.
In the end, every thing turned out just fine because I forgot to allow for the space that my 3.75 lbs DME would take up in the pot, if I had gotten the full 4 gal boil volume, I would have been too full. But, I am still wondering what is causing my absorbtion rate to be so high.
Did the last two batches have less grain? You may have dead space in the bottom of the tun that you aren’t taking into account, and this will look proportionally larger when you’re using less grain/water.
Wort out = (wort in) * (1 - absorption rate) - dead space
Did you change your units? Around 0.13 gallon/lb is typical, which would be about 0.5 quart/lb. Regardless, with 7.25 lb of grain, I’d expect about 3.5 qt to be retained. 9 - 3.5 = 5.5 qt, so you actually got out about a quart more than expected.
No, my mash tun has zero dead space.
This amount of grain is typical for my system. I use a 4 gal boil volume in a 5 gal pot in my kitchen. In order to maximize the amount of grain, I adjust all of my recipes to contain 7 - 7.5 lbs grain and make up the rest with an appropriate base/light extract (LME or DME).
DUH !!! I was doing the wrong math! The first few batches I was expecting to loose .12 gallons/lb and the last two batches I have been expecting to loose .12 %. ** Thanks a10t2.**
From now on I must remember to RDWHAHB after the boil begins, not during recipe formulation and planning!