The most efficient way to do a dunk sparge is to squeeze the bag prior to dunking in fresh water, then opening the bag and stirring well for about 5 minutes to get a uniform sugar concentration throughout the sparge wort, and finally, pull the bag and squeeze again. Highest efficiency is obtained when the initial drained volume (first runnings) is approximately the same as the sparge volume (second runnings.) The grain will absorb a significant amount of water from the mash, but not from the sparge, since the grain is already wet for the sparge. Because of the grain absorption, efficiency drops as the size of the grain bill increases, and the wort retained in the grains contains a larger percentage of the total sugar.
The chart below compares no-sparge BIAB efficiency vs. BIAB with dunk sparge (with equal first and second runnings.) The chart assumes a 5.5 gal post-boil volume, and 100% starch to sugar conversion (efficiency will be lower if conversion is less than 100%.) The different lines represent different amounts of retained water per lb of grain. How long you drain and/or how aggressively you squeeze will affect the grain absorption rate.
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The curves will shift to the left or right for different size batches, but the trends are the same.
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