LaMotte 2020t BLE Mode D'emploi page 9

TURBIDITY
„ WHAT IS TURBIDITY?
Turbidity is an optical property that results when light passes through a liquid sample and
is scattered by particulate matter. The scattering of light results in a change in the direction
of the light passing through the liquid. If the turbidity is low, the particles may be invisible
to the naked eye and much of the light will continue in the original direction. As the quantity
of particles increases in samples with greater turbidity, the light strikes particles in solution
and is scattered backward, sideways and forward. Light scattered by the particles allows
the particles in the solution to be "seen" or detected just as sunlight allows dust particles
in the air to be seen. At high concentrations, turbidity is perceived as cloudiness, haze or
an absence of clarity. Turbidity is not specific to the types of particles in the sample. The
particles may be suspended or colloidal and can have inorganic, organic or biological origins.
In drinking water, turbidity may indicate a treatment problem or signal conditions with an
increased risk of gastrointestinal diseases. Because pathogens such as Cryptosporidium
and Giardia cause measurable amounts of turbidity, turbidity monitoring can hold the
key to assuring adequate water filtration. In 1998, the EPA published the IESWTR (interim
enhanced surface water treatment rule) mandating turbidities in combined filter effluent
should read at or below 0.3 NTU. By doing so, the EPA hoped to achieve a 2 log (99%)
removal of Cryptosporidium. There is presently consideration to lower this to 0.1 NTU. The
trend has been to check the calibration of on-line turbidimeters used to monitor drinking
water with hand-held field units. The optical design and low detection limit of the 2020t/i
BLE allows very accurate readings for such calibrations. Drinking water that is turbid is not
always harmful to human health but does impart an unpalatable appearance.
Turbidity in environmental waters reduces the amount of beneficial sunlight that reaches
submerged aquatic vegetation, raises surface water temperature, buries eggs and bottom
dwelling creatures, and can carry sediment and pesticides through the water system.
„ HOW IS TURBIDITY MEASURED?
Turbidity is measured by detecting and quantifying the scattering of light in a solution. The
amount of light that is scattered is influenced by particulate properties of color, shape, size
and reflectivity. Turbidity can be measured by various methods including visual methods and
instrumental methods. Visual methods are more suitable for samples with high turbidity.
Instrumental methods can be used for samples with turbidity at all levels.
Examples of visual methods are the Secchi Disk method and the Jackson Candle method.
The Secchi Disk method is often used in natural waters. A Secchi Disk with black and white
quadrants is lowered into the water until it can no longer be seen. It is then raised until it can
be seen again. The average of these two measurements is known as the "Secchi Depth".
The Jackson Candle method uses a long glass calibrated tube placed over a standardized
candle. Water is added or removed from the tube until the candle flame becomes indistinct.
The height of the water in the tube is measured with a calibrated scale and is reported
as Jackson Turbidity Units (JTU). The lowest level of turbidity that can accurately be
determined with this method is about 25 JTU.
Instrumental methods for measuring turbidity involve a combination of detection angles
and light sources to optimize accuracy in various samples and to meet regulatory
requirements. The 2020t BLE and 2020i BLE turbidimeters offer the option of three
calibration curves for measuring turbidity that is based on the characteristics of the sample.
In the nephelometric mode, which is the default mode, the detector that is located
90-degrees from the light source measures the scattered light from a light beam passing
through a sample. In the 2020t BLE, this configuration and the tungsten lamp, with a
color temperature of 2,200–3,000 °K, meet the requirements of EPA method 180.1. The
2020i BLE, 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 ISO compliant light source provides
extended lifetime and better precision and accuracy of turbidity readings on colored
samples and samples with high turbidity. 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 BLE or 0.00 to 10.00 Formazin
Nephelometric Units (FNU) for the 2020i BLE. 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
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