LaMotte 2020wi Portable Turbidity Meters - Manual (Page 14)

reported as Jackson Turbidity Units (JTU). The lowest turbidity that
can be determined with this method is about 25 NTU. There are two
common methods for instruments to measure turbidity. Instruments
can measure the attenuation of a light beam passing through a
sample and they can measure the scattered light from a light beam
passing through a sample. In the attenuation method, the intensity
of a light beam passing through a turbid sample is compared with
the intensity passing through a turbidity-free sample at 180° from the
light source. This method is good for highly turbid samples. The most
common instrument for measuring scattered light in a water sample is
a nephelometer. A nephelometer measures light scattered at 90° to the
light beam. Light scattered at other angles may also be measured, but
the 90° angle defi nes a nephelometric measurement. The light source
for nephelometric measurements can be one of two types to meet EPA
or ISO specifi cations. The EPA specifi es a tungsten lamp with a color
temperature of 2,200–3,000 K. The units of measurement for the EPA
method are nephelometric turbidity units (NTU). The ISO specifi es a light
emitting diode (LED) with a wavelength of 860 ± 30 nm and a spectral
bandwidth less than or equal to 60 nm. The units of measurement for
the ISO method are formazin nephelometric units (FNU). The 2020we
meets the EPA specifi cation and the 2020wi meets the ISO specifi cation.
The nephelometric method is most useful for low turbidity.
The 2020we/wi is a nephelometer that is capable of measuring turbidity
by both the attenuation method and the nephelometric method. It uses
a detector placed at 180° to the light source for high turbidity samples.
It uses a detector placed at 90° to the light source for the nephelometric
method for low turbidity samples. The 2020we/wi has a signal averaging
option to improve the stability of readings on low turbidity samples.
The 2020we/wi has two different turbidity calibrations, formazin and
Japan Standard. The formazin calibration is the EPA and ISO approved
method of calibrating nephelometers. This calibration can be used with
user prepared formazin standards or commercially purchased formazin
standards. LaMotte Company approved AMCO TM standards labeled for
use with the 2020we/wi can also be used with the formazin calibration.
Stablcal ® standards below 50 NTU should not be used to calibrate the
2020we/wi.
The Japan Standard calibration is a calibration for a Japanese Water
Works standard. It is based on Japanese formulated polystyrene
turbidity standards. This calibration should only be used to meet
Japanese Water Works requirements. The Japanese polystyrene
standards can only be purchased in Japan. Formazin, AMCO and
Stablcal ® standards cannot be used with this calibration.
14