Work in progress

Calculating Efficiency

Before we can look for efficiency shortcomings and their causes we need to determine the efficiency. Since a number of different definitions for efficiency exist, it is possible that the actual efficiency is in a acceptable range but was calculated based on a different efficiency formula that returned a lower result. Other error factors are incorrect gravity or volume measurements.

In this analysis we will use the brew house efficiency into the boil kettle. That efficiency is the same at the beginning and end of the boil since no extract is lost during the boil. Efficiency shortcomings after the boil (i.e. transfer to the fermenter) are obvious as they are proportional to the amount of wort that is left behind in the boil kettle. For a list of various efficiency definitions see the article Efficiency.

Measuring

Before the efficiency can be calculated, 3 things need to be measured on brew day:

• amount of grain used
• specific gravity of the collected or finished wort
• volume of the collected or finished wort

It is important to be reasonably accurate when measuring these values. The more precise the measurements are. the more precise the calculated efficiency will be. As an example: If the grain weight has an error of 5% (which is +/- 200g for a 4.0kg grist or 0.5 lb for 10lb), the calculated efficiency will also have a 5% error, i.e. a calculated efficiency of 70% could actually be between 65% and 75%. The same applies to volume and gravity measurements.

The following measurement errors can be seen as reasonable:

• 1% error for the grain weight. This means +/- 40g for a 4.00 kg grist or 0.1 lb for 10 lb
• 2% error for the specific gravity. This means +/- 0.25 for 12 Plato or +/- 1 point for 1.050 SG
• 2% error on the volume measurement. This means +/- 0.5 on 25l or +/- 0.5 qt on 6 gal

and with them the efficiency can be calculated with a 5% accuracy.

To calculate this efficiency two distinct approaches exist. The weight based approach calculates the weight of the extract that is present in the collected wort and puts it in relation to the total extract weight that was available in the grist. The gravity potential based approach puts the specific gravity that was achieved in relation to the maximum gravity potential that the grist has. Both approaches are explained below and either one can be chosen.