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Turbidity Units of Measure |
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| Absorption |
Spectral absorption is expressed by Lambert-Beer’s law and is based on compiled work by Pierre Bouguer and Johann Heinrich Lambert in the 1700’s, as well as August Beer a century later.
Absorbance units are commonly seen as 'A', 'A.U.' or 'ABS', all of which represent the same units.
Other units such as Concentration Units (C.U.), Optical Density (O.D.) can be measured at various pathlengths. Absorbance, being linear in most circumstances, can then be easily correlated to any preferred unit of measure.
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| Scattered Light |
Around 1500, Leonardo da Vinci originally speculated the color of the sky was due to light scattering off of water particles in the air. It was not until the late 1800's that scientific work by John Tyndall and John Strutt 3rd Baron Rayleigh (Lord Rayleigh), and later work by Gustav Mie, has extended our understanding of the phenomena.
Today Rayliegh Scatter and Mie Scatter are the two basic theories used to express electromagnetic light interaction with particulate. The definition of units has evolved with the scientific methods employed.
Early turbidity meters simply used the absorption method (relative turbidity, percent transmittance, detectable distance etc.). Jackson "Candle" Turbidity Units (JTU) used a graduated cylinder, PPM of Silica as a reference, and a candle flame, but did not entirely account for all light scattering properties of potential samples.
In 1926, formazin solutions began to be used, as they appeared more consistent and more easily reproducible. Benchmarking these measurements with primary turbidity reference standards was further encouraged to better represent actual scatter properties and create a more uniform standard reference. Units based on primary solutions such as formazin are now commonly used and are the basis for EPA Formazin Turbidity Units (FTU). FTU is non descript of measurement angle and can be made at any angle of detection, although both the EPA and ASTM dictate measurement angles of 90°.
Nephelometric Turbidity Units (NTU) specifically calls for a 90° measurement technique and are also formazin based, although safer polymer bead suspensions are now commercially available, and recognized as an acceptable alternative.
Typical series of Formazin turbidity standards shown in NTU/FTU.
In addition, there are more industry specific units of measure that have been used such as Helms Units (formazin). Brewing organizations such as the European Brewing Convention (EBC) and the American Society of Brewing Chemists (ASBC-FTU) employ their own formazin based turbidity unit scales. While EBC units are commonly used for both forward and side-scatter measurements, ASBC FTU is strictly a 90° measurement.
Parts Per Million of standard substance, such as PPM/DE (Kieselguhr), are also in use as they represent an average mean particle size desirable for a majority of applications. Because of the sensitivity and particle selectivity aspect of different scatter angles and the unique design of various individual turbidity instruments, correlations between instruments of different measuring techniques, or different manufacturers are difficult and often impossible unless the turbidity constituents are uniform and homogeneous. Precise repeatability of the instruments is essential.
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Real World Turbidity |
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