Campbell OBS-3A Specifikace stáhnout pdf (Strana 45)

Section 7. Optics and Turbidity

Measurements

Turbidity is the cloudy appearance of a liquid produced by light scattered from

suspended matter. It is an apparent optical property that depends on the size,

color, and shape of scattering particles, and the instrument used to measure it.

In accordance with standard method 2130B and ISO 7027, turbidity is usually

measured with a 90

-scatterance nephelometer and reported in nephelometric

turbidity units (NTUs). Turbidity standards are discussed in Section 6.

FIGURE 6.3-1. Optical Particle Detectors

Light transmission in water is attenuated by scattering (deflection by water

molecules, and suspended matter) and absorption, which converts light to heat.

Attenuation, absorption, and scattering are inherent properties of water that are

affected only by impurities such as color and suspended organic matter.

Optically pure water is not readily available; however deionized water that has

passed through a 0.2 μm filter is adequate for most practical purposes.

There are dozens of turbidimeter designs, however most are configured in one

of the ways shown in FIGURE 6.3-1. These include: forward-scatterance, 90

scatterance, and backscatterance nephelometers. Some instruments combine

two or more of these configurations and blend signals to produce a useful

output. The transmissometer measures attenuation, an inherent optical

property but is not approved for turbidity measurements except by ISO 7027.

OBS sensors have superior linearity in turbid water but a transmissometer is

more sensitive at low concentrations (<~25 mg/l). Data from turbidimeters

made by different companies should be compared cautiously. Inconsistencies

between instruments results from variations in light sources, detectors, optical

configurations and turbidity standards.

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Campbell OBS-3A Specifikace Strana 45