5
For the ratio turbidity mode, the microprocessor of the instrument calculates the NTU value, from the
signals that reaches the two detectors, by using an effective algorithm. The optical system and measuring
technique compensate the color interference and also the LED intensity fluctuations, minimizing the
need of frequent calibration.
For the FNU and NTU non ratio modes the turbidity is calculated from the signal that reaches the
scattered light detector (90°).
In FAU mode the turbidity is calculated from the signal that reaches the direct light detector, while in
NTU ratio mode the turbidity is obtained from signal ratio on scatter and transmitted light detectors.
The non ratio methods are more sensitive to the LED intensity fluctuations.
The lower detection limit of a turbidimeter is determined by the “stray light”. Stray light is the light that
reaches the detectors and is not caused by light scattering from suspended particles.
The optical system of
HI88713
instrument 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 page 9 “General Tips for an Accurate Measurement” for sample preparation and measuring
techniques).
MEASUREMENT UNITS
The meter is using specific units according to the selected measurement mode. For NTU modes the
EBC unit option is available 1 EBC=0.245 NTU. When measuring in NTU modes a conversion to EBC
units is also available 0.245 NTU=1 EBC.
