Hanna Instruments HI 98713 - Manual (Page 4)

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Turbidity of the water is an optical property that causes light to be scattered and absorbed, rather than
transmitted. The scattering of the light that passes through a liquid is primarily caused by the
suspended solids. The higher the turbidity, the greater the amount of scattered light. Because even
the molecules in a very pure fluid scatter light to a certain degree, no solution will have zero
turbidity.
The
ISO 7027 standard specifies the key parameters for the optical system to measure turbidity for
drinking and surface water, using the formazin based metric method.
The
HI 98713 Portable Turbidimeter is designed to meet or exceed the criteria specified by the
ISO 7027 standard.
The light beam that passes through the sample is scattered in all directions. The intensity and pattern
of the scattered light is affected by many variables like wavelenght of the incident light, particle size,
shape, refractive index and color.
The Hanna’s
HI 98713 is based on a state-of-the-art optical system that guarantee both high
performance and reliable results.
This optical system includes an infrared LED, a scattered light detector (90
0 ) and a transmitted light
detector (180 0 ). The microprocessor of the instrument calculates from the signals that reaches the
two detectors, the FNU value, using an effective algorithm. This algorithm corrects and compensates
for interferences of color, making the
HI 98713 turbidimeter color-compensated.
The optical system and measuring technique allow the compensation of LED intensity fluctuations,
minimizing the need of frequent calibration.
The lower detection limit of a turbidimeter is determined by the so called “stray light”. Stray light
is the light detected by the sensors, that is not caused by light scattering from suspended particles.
The optical system of
HI 98713 turbidimeter is designed to have very low stray light, providing
accurate results for low turbidity samples. However, special care must be taken when measuring low
turbidities (see General Tips for an Accurate Measurement for sample preparation and measuring
techniques).
PRINCIPLE OF OPERATION PRINCIPLE OF OPERATION PRINCIPLE OF OPERATION PRINCIPLE OF OPERATION PRINCIPLE OF OPERATION M E A S U R E M E N T U N I T S M E A S U R E M E N T U N I T S M E A S U R E M E N T U N I T S M E A S U R E M E N T U N I T S M E A S U R E M E N T U N I T S
Many methods were used to measure turbidity over the years. The Jackson Candle Turbidimeter was
used to measure turbidity as Jackson turbidity units (JTU). The Secchi Disk is commonly used to
measure turbidity in lakes and other deep waters (mg/L SiO
22222
). Both methods are visual and are not
considered very accurate. To obtain more accurate readings a formazin based meter should be used
as a turbidity reading instrument.
The
HI 98713 turbidimeter reports the measurements only in FNU (Formazin Nephelometric
Units). The conversion table between these measurement units is shown bellow:
U T J U T N / U T F / U N F O i S
2
) L / g m (
U T J 1 9 1 0 5 . 2
U N F
U T F
U T N
3 5 0 . 0 1 3 1 . 0
O i S
2
) L / g m ( 4 . 0 5 . 7 1