A unit used to measure the clarity of water and beer. The ISO¹ refers to the units as FNU (Formazin Nephelometric Units). The technique is the same as that for the NTU, but the calibration uses microspheres of the polymer formazin.
Directions for the AOAC test for the “Haze (Total) of Beer after Chilling” states² “1000 formazin turbidity units (FTU) on empirical formazin turbidity scale represents reflectance of insol. reaction products of 0.0725 g hydrazine sulfate with 0.7250 g hexamethylenetetramine dild to 1 L.”
1. ISO 7027:1999
Water Quality -- Determination of turbidity.
2. Association of Official Analytical Chemists.
Official Methods of Analysis of the Association of Official Analytical Chemists. 14th ed.
Arlington, VA: AOAC, Inc. 1984.
10.010(c), page 192.
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Last revised: 24 February 2011.