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Continuous sparging needs more water than batch or no-sparging approaches. Batch and no-sparge calculations assume that the grain bed will be run dry, with no free wort being left in the mash tun. For this situation, Beechum and Conn (2020) give the estimate of .12 gallons per pound (1 liter per kilogram) of water absorbed. In contrast, continuous sparging processes will keep the grains floating until a run-off gravity of 1.008 is reached. The sparge is then stopped, resulting in some non-absorbed water being left behind in the mash tun. In my system, this is about .24 gallons per pound (2 liters per kilogram). Another way to understand this relationship is that the continuous sparge process must maintain a thickness about the same as a thick mash (1.0 quart or .25 gallon per pound) and this will require some additional water compared to other forms of sparging.
The boil off evaporation percentage is calculated as (1 - (final boil volume / pre-boil volume)) * 100. My system boils off about 1.0 gallon/hour with a medium intensity boil. An example batch would be starting at 6.5 gallons and ending at 5.5 gallons. The proportion boiled off is 1 - (5.5 / 6.5) = 1 - .85 = .15 or 15% expressed as a percentage. However, some systems can boil off between 1.5 to 2.5 gallons per hour, which would be about 23% to 42% per hour for an approximately five gallon batch. The default value of 20% seems realistic for most homebrewing systems.
The hop losses were empirically determined from boiling 10 g of CTZ whole cone hops in 500 ml of water for 1 minute. This absorbs 100 ml. The relationship is that 1 g absorbs 10 ml, which is 10X the weight in metric units. This can be understood as reversing the dehydration process used in fresh hops. When hops are dried to an 8 to 10% water content, the hops will absorb water equal to about 10 times the dry weight during the boil.
When dry hopping, the dry hop weight could be included in the calculations.
The wort volume will shrinkage about 4% as it is cooled. This means that hot wort volume measurements are inflated slightly due to expansion. The present calculations assume a cool beginning and ending, so expansion and contraction are not calculated. Keep in mind that hot measurements will be slightly overestimated.
Beechum, D. & Conn, D. (2020, January-February). Sparge ho! The many ways of rinsing grain (or not). BYO, 79-81.
Carr, N. (2020). How to calculate water loss during brewing
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