The wort/water shrinkage is the thermal expansion of water based on its temperature.
So when measuring out hot water by volume (say sparge at 75°C/167°F) then the actual volume of the water is 2.5% less then the same volume of water measured at 4°C (39.2°F).Therefore16 L (4.2 US Gal) at 75°C (167°F) is only 15.6 L at 4°C (39.2°F). Other software is designed for 3 vessel systems, which use a preheated vessel of water to fill their mash tun and to sparge with.
|Degrees Celsius (°C)||Degrees Fahrenheit (°F)||Specific Volume (L)|
Since all of the Grainfather brewing units use vessels that are designed to be filled with ground temperature water then heated to the temperature at which they are needed, this effect doesn’t come into play.
So for our software we don’t take this into account, whereas other software use a wort shrinkage factor of 4% to account for the thermal expansion at near boil.