Endress+Hauser Viomax CAS51D, Operating Instructions Manual

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Device description Viomax CAS51D
8 Hauser
3.3 Operating principle
3.3.1 Measuring principle
The light from a pulsed, high-stability strobe lamp (item 3) passes through the
measurement section
1)
(item 2). A beam splitter (item 6) directs the light beam to the
two receivers (items 1 and 5). A filter upstream from the receivers only lets through light
in the measuring wavelength or reference wavelength.
1 2
3
4 5 6
A0013213
 4 Measuring principle of nitrate sensor
1 Measuring receiver with filter
2 Cuvette gap
3 Strobe lamp
4 Lens
5 Reference receiver with filter
6 Beam splitter
Within the measurement section, the medium in the cuvette (water, dissolved substances
and particles) absorbs light across the entire spectrum. In the measuring wavelength
range, the measured component
2)
takes an additional part of its energy from the light.
The ratio of the light signal of the measuring wavelength to the light signal of the
reference wavelength is used to calculate the measured value.
• If only water and turbidity are present, both signals are attenuated to the same degree
and the ratio remains constant.
• If the medium contains the measured component, the signal of the measuring
wavelength is attenuated more and the ratio changes.
This change in the ratio can be converted to determine the nitrate concentration or the
SAC value. This dependency is non-linear.
Conclusion:
• Long measurement sections are required for low concentrations of the measured
component.
For clear water measurements, this is achieved with the 8 mm cuvette for nitrate
measurement and the 40 mm cuvette for SAC measurement.
• For high turbidity values, longer measurement sections result in the total absorption of
light - the measured values are no longer valid.
For media with high turbidity values (activated sludge applications), the nitrate sensor
with the 2 mm cuvette returns good, reliable measured values.
The SAC sensor with the 2 mm cuvette is ideal for measuring the organic load in the
inlet of municipal wastewater treatment plants.
1) Measurement section = open path through cuvette
2) Nitrate or substances that contribute to the spectral absorption coefficient (SAC)