What does the acronym SRM stand for and how is it calculated?
SRM = Standard Reference Method
It was adopted by the Society of Brewing Chemistists in the 1950s as their standard method of defining beer color.
In the lab, it involves the use of a spectrometer to measure the absorbency of a specific wave length of light (i.e. 430 nanometers).
SRM is computed from these values by SRM=12.7 x D x A(430)
where D is the dilution factor (D = 1 for undiluted samples, D = 2 for 1:1 dilution etc.) and A(430) the absorbency at 430 nm in 1 cm.
This method was adopted because it is roughly equivalent to the Lovibond scale that was in use at the time.
One can do a quick and dirty estimate simply by computing the Malt Color Units (MCU).
This is simply the weight of the grain in pounds multiplied by the color of the grain in degrees Lovibond and divided by the wort volume in gallons.
When using multiple grains then the total MCU would simply be the summation of the individual values of each type of malt used.
A more accurate relation is:
SRM color = 1.4922 x (MCU)exp(0.6859)
This is for SRM values of less than 50.