SRM, or the Standard Reference Method, is a widely adopted method of measuring the color of your beer. The values of color range from a 1 for a light pilsner to over 40 for a dark stout. The BJCP style book uses the SRM color to identify the color that a beer of a select style should meet, and it is also a common metric that can be used for cloning a beer. Having an accurate way to measure this can help you achieve a better product and take another step toward producing award wining beer.
SRM is ten times the log of the transmission ratio of blue light (420 nm to be exact) The definition of SRM shows that dilution of the sample can be used to scale the SRM value. (1) Furthermore, Beer's Law (The scientist, not the beverage) shows that transmission is logarithmically related in the same way that SRM is calculated. (2)
There are methods in practice that use beers with known SRM values at different dilutions to make a set of SRM reference. That sounds like a lot of work, and a waste of beer to me.
What I propose is using a dark liquid that almost everyone has in their house, and diluting it until it matches the color of the beer. The amount of water needed to dilute the liquid can then be measured and from that the SRM value can be determined.
So what is this mystery liquid? Soy Sauce! The color content is very close to that of beer, and only a very small amount is needed for the test. Both Kikoman's low sodium and regular have the same apparent attenuation of light. Testing indicates that at a dilution of 38:1 the liquid has an SRM of 8.
Two glass 250ml Graduated Cylinders make a convenient way to measure the amount of added water, and to view the beer. The graduated cylinder also makes a continent container for specific gravity measurements.
For 1/4tsp of soy sauce the ml of water required for SRM 1-20 is as follows:
For 1tsp of soy sauce the ml of water required for SRM 21-40 is as follows: