Standard Reference Method

Standard Reference Method or SRM is a system modern brewers use to measure color intensity, roughly darkness, of a beer or malted grain. This method involves the use of spectrophotometry to assign a number of degrees SRM to light intensity.

The SRM number is defined as 10 times the absorbance of a sample at 430 nanometers measured through a .5-inch cell. The 430-nanometer wavelength corresponds to a deep blue light, and is the wavelength at which beers appear most different from each other.

The standard was adopted in 1950 by the American Society of Brewing Chemists as an objective measurement of color unburdened by the difficulties of the Lovibond system; the measurement of the color of a beer in degrees SRM and degrees Lovibond are approximately equal and in practice can be used interchangeably to evaluate the color of intensity of beer.

Degrees SRM are related to the current EBC standard by:

$\mbox{EBC} = \mbox{SRM} \times 1.97$

$\mbox{SRM} = \mbox{EBC} \times .508$;

as a rule of thumb, EBC is approximately twice SRM.

Note that an earlier version of EBC color was based on absorbance at 530 nanometers, which permitted no direct conversion between the two systems. However, if one assumes a linear log absorbance spectrum (the Linner hypothesis from the realm of caramel color), and knows the Linner Hue Index, $H_L$, the absorbances are related by:

$A_{430} = A_{530} \times 10^{H_{L}/10}$

A formula for converting between the old EBC color value and SRM sometimes continues to appear in literature. It should not be used, as it is flawed and based on measurements which are no longer taken.

Color based on Standard Reference Method (SRM)

SRM/Lovibond	Example	Beer color	EBC
2	Pale lager		4
3	German Pilsener		6
4	Pilsner Urquell		8
6			12
8	Weissbier		16
10	Bass pale ale		20
13			26
17	Dark lager		33
20			39
24			47
29	Porter		57
35	Stout		69
40			79
70	Imperial stout		138

References

• Dictionary of Beer, Ed: A. Webb, ISBN 1-85249-158-2
• Home Brewing, Graham Wheeler, ISBN 1-85249-137-X

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