Standard Reference Method - Measurement Method

Measurement Method

The ASBC and EBC measurements are now identical (both done at the same wavelength and in the same size cuvette) but the scaling is different. A photometer or spectrophotometer is used to measure the attenuation of light at 430 nm nanometers, as it passes through 1 cm of beer contained in a standard 1 cm by 1 cm cuvette. The absorption is the log of the ratio of the intensity of the light beam entering the sample to the intensity leaving. This difference is multiplied by 12.7 in the SRM system and 25 in the EBC (see below). For example, if the light intensity leaving is one one hundredth the light intensity entering the ratio is 100, the absorption is 2 and the SRM is 25.4. The scale factor derives from the original definition of SRM discussed in the next paragraph.

The SRM number was originally, and still is, defined by "Beer color intensity on a sample free of turbidity and having the spectral characteristics of an average beer is 10 times the absorbance of the beer measured in a 1/2 inch cell with monochromatic light at 430 nanometers." Modern spectrophotometers use 1 cm cuvettes rather than 1/2 inch ones. When a 1 cm cuvette is used, application of the Bouguer-Beer-Lambert law shows that the multiplier should be 12.7 rather than 10. When the SRM value for a beer or wort is larger than about 30 the log linear limit of some instruments using 1 cm cuvettes is approached. In such cases the sample is diluted with deionized water. Using Beer-Lambert again gives the mathematical definition of SRM in the general case as:

where is the dilution factor ( for undiluted samples, for 1:1 dilution etc.) and the absorbance at 430 nm in 1 cm.

The 430-nanometer wavelength corresponds to a deep blue light, and was chosen, as was the multiplier, to make values determined in the SRM system comparable to those determined using the Lovibond system in use at the time the SRM was adopted.

The SRM was adopted in 1950 by the American Society of Brewing Chemists which had recognized the need for an instrument based measurement of color unburdened by the difficulties of the Lovibond system which relies (it is still in use in many industries including brewing - malts are often labeled with the Lovibond color of laboratory worts prepared from them) on visual comparison of the sample to tinted glass discs. Beer colors measured in SRM and degrees Lovibond were, as noted above, approximately equal at the time of adoption of the SRM. Comparison of EBC (see below) and Lovibond data published by modern malsters makes it clear that the relationship between SRM and Lovibond is:

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