LaMotte 2020t BLE Mode D'emploi page 10

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
BLE) or Formazin Nephelometric Ratio Units (FNRU, 2020i BLE).
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 – 2000 Attenuation
Units (AU, 2020t BLE) or Formazin Attenuation Units (FAU, 2020i BLE).
„ TURBIDITY UNITS
Traditionally, turbidimeters designed for use in the United State were made to the
specifications of EPA Method 180.1. This method defined 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 definition 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 effort 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 specifies 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 differentiate 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. Each
turbidity calibration mode has a default unit:
Nephelometric Mode: NTU (2020t BLE) or FNU (2020i BLE)
Ratiometric Mode: NTRU (2020t BLE) or FNRU (2020i BLE)
Attenuation Mode: AU (2020t BLE) or FAU (2020i BLE)
The meter will permit the user to choose to report results in any of the other available
calibration units (NTU, FNU, AU, FAU, NTRU or FNRU) other than the default unit. It also
allows the brewing industry to check process water in the nephelometric mode and choose
to have results reported in American Society of Brewing Chemists (ASBC) or European
Brewery Convention (EBC) units.
Acronyms Definitions
NTU Nephelometric Turbidity Units
FNU Formazin Nephelometric Units IR LED (usually) 860 nm,
AU Attenuation Units
Notes
Incandescent white light between
400 and 600 nm, 90° detection,
2020t BLE
bandwidth less than 60 nm, 90°
detection, 2020i BLE
Incandescent white light between
400 and 600 nm, 180° detection,
2020t BLE
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Regulatory Method
EPA 180.1
ISO 7027
Not applicable