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The first thing I notice about a beer is the color. When I order a beer at the bar I know what to expect; if a beer has red in the title, it better be red in my glass. Nothing is worse than getting a beer and wondering, did they bring me the wrong thing because this doesn't look like the beer I thought I ordered; 'umm excuse me, is my Irish Red suppose to be murky brown?'
As a home brewer how can this be avoided? By knowing your ingredients, fine tuning your processes, and science. The standard reference method (SRM) numbers are defined as:
Beer color intensity on a sample free of turbidity and having the spectral characteristics of an average beer is 12.7 times the absorbance of the beer measured in a one inch cell with monochromatic light at 430 nanometers.What does that mean? A deep blue light is shot through a turbidity free sample in a glass with a thickness of one inch to determine amount of light absorbed by the beer. This is measured and multiplied by 12.7 to give us our final SRM
It could be simpler, right? Actually probably not. SRM uses one wavelength of light and thus only captures a single data point. This one data point however conveys 92% of the spectral information needed to determine the color of the beer. It is also a standard method meaning that with the right equipment measuring your home brew in America, and Sergi's home brew in Russia would be comparable. To convey 100% of the spectral information you actually need 81 measurements, that is a lot more work.
What does this mean to me? You might have noticed every color on the SRM chart is a shade of brown. It doesn't capture many of the colors a beer might actually be. If you add fruit, reddish malts, or basically anything else to your beer, the SRM measurement might be off by as much as 8%. What does that mean? not too much. 8% isn't a lot of deviation in my opinion; at 25 SRM it could be off by as much as 2SRM , at 12.5 SRM it would be only 1 SRM. If you're eyeballing it you might even be closer than the machine because your eye is a remarkable judge of color.
If your beer looks right, it probably is right, as much as most people will be concerned. If however your beer looks wrong no amount of standard deviation nonsense or science rambling will save you from the eye rolling and judgment of your friends.