Option 1: Calculate specific gravity from blending two runnings with defined volumes.
Option 2: Calculate the second running volume needed for a blended specific gravity target.
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Parti-gyle brewing is an old English method for producing two or more beers from a single mashing process. Each running is called a gyle. The first running or gyle could be used to make a strong beer. Second or third runnings have a lower gravity. Two runnings can be combined to make a blended beer of an intermediate strength. The practical advantage of this approach is in making multiple beers - strong, intermediate, and possibly weak - from one mash.
The first option determines the specific gravity of a blended wort from combining a two runnings. This is based upon multiplying volume by gravity points (example: a 1.050 specific gravity is treated as 50 points). Next, the total gravity points are divided by the total volume to estimate the blended specific gravity. Volume in either gallons or liters will work as long as the same units are consistently used.
An example from the default values:
4 gallons * 70 (from 1.070) = 280 gravity points
6 gallons * 35 (from 1.035) = 210 gravity points
total gravity points = 280 + 210 = 490
estimated specific gravity = 490 / 10 gallons = 49 or 1.049 when expressed as specific gravity
Option two determines the volume of second runnings needed to produce a blended wort of a target specific gravity. The following formula is used:
R2Volume = (R1Volume * (R1GP - TargetGP)) / (TargetGP - R2GP)
The numerator of this formula determines the first running's total gravity points above the target value. These can be understood as excess gravity points that need dilution. The denominator is a dilution based on the specific gravity of the second runnings. This is similar to diluting wort to lower gravity except that the dilution volume has a specific gravity higher than water, which is 1.000. The difference between the target specific gravity and the gravity of the second running is essentially the water in the second running that can be used for dilution. Dividing the numerator (excessive gravity) by the denominator (water remaining after accounting for the second running gravity) yields the volume of second runnings needed for reaching the target.
Parti-gyle brewing methods sometimes use added sugars in the second running, such as adjunct sugar addition. These gravity additions must be included in the second running gravity for accurate calulations.
Strong, G. (2023, September). Alternative mashing techniques: Traditions of England, Germany, and the Czech Republic. BYO, p. 30 - 39.
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