Mash Water Calculation
Grain weight x Ratio + Recoverable Dead space vol = Mash water volume
Grain weight = total weight of grain bill
Ratio = water to grist ratio most GF products are 2.7
Recoverable Dead Space Vol (RDSV) = the amount of water needed to reach the bottom of the grain basket G30 = 3.5L G40 & G70=6.9L S40=7L
The above is the calculation that all home brewing software use for calculating mash water. However, most of the theory is based on systems without a recoverable or dead space. Therefore, using too much water in the mash for small grain bills is easy. The typical water-to-grist ratio is between 2.0:1 - 4.0:1. This is because less than 2.0 the grains will be too dry to allow the enzymes in the base malts to convert the specialty malts and greater than 4.0 and the enzymes will be too dilute to convert the starches in all grains into fermentable sugars.
If we take a G30 example:
1.5kg grain bill
1.5 x 2.7 + 3.5 = 7.55L of water for the mash.
However, if we recalculate the water to grist (7.55/1.5), this is a new water to grist of 5.03:1, which is far too low for efficient conversion. This is because of the amount of the proportion of the RDSV to the grain volume. Therefore, when using minimal grain bills, we recommend lowering the water-to-grist ratio to 2.1:1-2.3:1.
If we take a G30 example again:
1.5kg grain bill
1.5 x 2.3 + 3.5 = 6.95L of water for the mash.
this equates to a recalculated water-to-grist of 4.6:1, which is not great but much better. So, a longer mash time should be expected in order to achieve a reasonable mash efficiency.
You will now have a significant amount of sparge water for sparging due to the change in water to grist. therefore, the pH of your sparge water becomes very important as you will likely extract tannins as there is a lower amount of sugars to keep the pH under 6.0 while sparging.
Note if you are doing minimal grain vol and small batch size this is less of a challenge.