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TURBIDITY UNITS
Traditionally, turbidimeters designed for use in the United State were
made to the specifi cations of EPA Method 180.1. This method defi ned
the NTU, nephelometric turbidity unit, as a unit to measure turbidity
in the range of 0 – 40 NTU using a nephelometer. According to the
EPA a nephelometer was a turbidimeter that measured turbidity with a
90° detector. Also, if the turbidity was greater than 40 NTU, a dilution
was necessary to bring the sample into the 0 – 40 NTU range. Today,
many turbidimeters have additional detectors which increase the range
of the turbidity measurement, eliminate interferences and generally
improve the performance. Currently, many turbidimeters are capable of
measuring above 40 NTU by using detectors other than a 90° detector.
Even though they may use a 180° detector to measure the light that is
attenuated by high turbidity samples they may continue to report the
results as NTU.
Recently there has been an effort to use the units of turbidity
measurements to indicate which type of detector and light source was
used. For EPA compliant meters, measurements made with a 90° degree
detector and an incandescent white light source are reported as NTU.
When an attenuation measurement is made with a 180°detector, using
the same meter, the results are reported as AU, attenuation units. ISO
Method 7027, which specifi es a 860 nm light source, also uses two
turbidity units. When the 90° degree detector is used, the results are
reported as FNU, formazin nephelometric units. With an attenuation
measurement made with a 180°detector, the results are reported as
FAU, formazin attenuation units. It should be noted that all units are
numerically equivalent if the meters are calibrated to formazin and
that the units only designate which detector was used to make the
measurement. For example, 1 NTU = 1 AU = 1FNU = 1FAU.
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