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which has an IR LED light source at 860 nm, uses the 90-degree detector to meet the
requirements of the ISO 2027 Standard. The nephelometric mode is best used for meeting
regulatory requirements on samples, such as drinking water, that are in the range of 0.00
to 40.00 Nephelometric Turbidity Units (NTU) for the 2020t or 0.00 to 10.00 Formazin
Nephelometric Units (FNU) for the 2020i. A signal averaging option improves the stability
of readings on low turbidity samples.
The ratiometric mode is typically used for natural waters and storm waters, or other
samples in the 0 – 1000 NTU/FNU range, to minimize interference from color in the
sample. The ratiometric mode takes readings from both the 90-degtree detector and the
180 degree detector. Results are reported as Nephelometric Turbidity Ratio Units (NTRU,
2020t) or Formazin Nephelometric Ratio Units (FNRU, 2020i).
In the attenuation mode, the detector is located 180-degrees from the light source. It
measures the attenuation of the light beam due to absorption and scatter. This mode
is best used to measure samples with high turbidity levels in the range of 40 – 4000
Attenuation Units (AU, 2020t) or Formazin Attenuation Units (FAU, 2020i).
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 that measured turbidity in the range of 0 – 40 NTU using a nephelometer.
According to the EPA a nephelometer was a turbidimeter that measures turbidity with
a 90° detector. If a sample had a turbidity that 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, which does not meet the defi nition of a
nephelometer, the results are often be reported as NTU.
Since the position of the detector and the light source is important information to include
when reporting and comparing turbidity results, there has recently been an eff ort by the
ASTM to use turbidity units which include this information. For EPA compliant meters,
measurements made with a 90° degree detector and an incandescent white light source
are reported as Nephelometric Turbidity Units (NTU). When an attenuation measurement
is made with a 180°detector, using the same light source, the results are reported as
Attenuation Units (AU). If a ratio of the measurements from both detectors is used to
calculate the turbidity, the results are reported in Nephelometric Turbidity Ratio Units
(NTRU).
ISO Method 7027, which specifi es an 860 nm light source, reports results in four turbidity
units. When the 90° degree detector is used, the results are reported as Formazin
Nephelometric Units (FNU). With an attenuation measurement made with a 180° detector,
the results are reported as Formazin Attenuation Units (FAU). And results that are a ratio
of measurements from the two angles are reported in Formazin Nephelometric Ratio
Units (FNRU). It is also possible to report the readings from each of the three ISO modes in
Nephelometric Turbidity Units (NTU).
It should be noted that all units are numerically equivalent if the meters are calibrated to
formazin. The units diff erentiate which detector and light source was used to make the
measurement. For example, 1 NTU = 1 AU = 1 NTRU = 1 FNU =1 FAU = 1 FNRU.
