9
PRINCIPLE OF OPERA
TION
6. PRINCIPLE OF OPERATION
Water turbidity is an optical property that causes light to be scattered and absorbed, not transmitted.
The scattered light passing through a liquid is primarily caused by suspended solids. The higher the
turbidity, the greater the amount of scattered light.
The light beam that passes through the sample is scattered in all directions by the particles. The
intensity and pattern of the scattered light are affected by wavelength of the incident light, particle
size, shape, refractive index and color.
The optical system consists of an infrared LED, a scattered light detector (90°) and a transmitted light
detector (180°). Using an internal algorithm, the instrument calculates from the signals reaching the
two detectors the FNU value. This algorithm corrects and compensates for color interferences.
LED
EMITTED
LIGHT
CUVETTE
SCATTERED
LIGHT DETECTOR
TRANSMITTED
LIGHT DETECTOR
The optical system and measuring technique allow the compensation of LED intensity fluctuations,
minimizing the need of frequent calibrations.
6.1. LOW-TURBIDITY MEASUREMENTS
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.
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optical system 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 ACCURATE MEASUREMENTS).
7. MEASUREMENT UNITS
The Jackson Candle Turbidimeter measures turbidity as Jackson turbidity units (JTU) and the
Secchi Disk measures turbidity in SiO
2
mg/L. Both are visual methods considered less reliable.
For more accurate readings a formazin-based meter should be used as a turbidity reading instrument.
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measures in FNU only. Reference conversion table is detailed here:
JTU
FNU / FTU / NTU
SiO
2
(mg/L)
JTU
1
19
2.50
FNU / FTU / NTU
0.053
1
0.13
SiO
2
(mg/L)
0.4
7.5
1