Endress+Hauser Viomax CAS51D, Operating Instructions Manual

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Viomax CAS51D Device description
Hauser 9
3.3.2 Nitrate measurement
The sensor is designed for measuring nitrate. As nitrite is also measured, it could also be
regarded as an NO
x
sensor.
Nitrate ions absorb UV light in the range of approx. 190 to 230 nm. Nitrite ions have a
similar absorption rate in the same range.
The sensor measures the light intensity of the 214 nm wavelength (measuring channel).
At this wavelength, nitrate and nitrite ions absorb light in proportion to their
concentration, while the light intensity in the reference channel remains virtually
unchanged at 254 nm.
Interference factors, such as turbidity, fouling or organic hydrocarbons, are factored out
mathematically.
The signal ratio between the reference wavelength and measurement wavelength
constitutes the measurement result. This ratio is converted to the concentration of nitrate
using the calibration curve programmed into the sensor.
3.3.3 Cross-interference when measuring nitrate
The following have a direct impact on the measuring range:
• Total solids (TS) and turbidity
• Sludge properties
• Nitrite
Trends:
• A higher proportion of TS or greater turbidity reduces the upper end of the measuring
range, resulting in a smaller measuring range.
• High COD
3)
levels reduce the upper end of the measuring range, resulting in a smaller
measuring range.
• Nitrite is measured as nitrate, thus resulting in a higher measured value.
The following can be deduced from the interdependencies cited above:
• Sludge floc causes scattering in the medium, resulting in the attenuation of both the
measuring and reference signal to varying degrees. This in turn can bring about a
change in the nitrate value due to turbidity.
• High concentrations of oxidizable substances
4)
in the medium may result in an increase
in the measured value.
• Nitrite absorbs light in a similar wavelength range to nitrate and is measured along with
nitrate. The dependency is constant: 1.0 mg/l nitrite is displayed as 0.8 mg/l nitrate.
• An adjustment to the customer process is always worthwhile.
3.3.4 SAC measurement
Many organic substances absorb light in the range of 254 nm. In the SAC sensor,
absorption on the measuring wavelength (254 nm) is compared with the largely
unaffected reference measurement at 550 nm.
KHP (potassium hydrogen phthalate C
8
H
5
KO
4
) is the established organic reference in SAC
measurement operations. That is why the sensor is calibrated in the factory using KHP.
The SAC value can be regarded as a trend indicator of the organic load in a medium. For
this purpose, it is converted to COD, TOC, BOD and DOC
5)
using predefined, adjustable
factors:
c (TOC) = 0.4705 * c (KHP)
c (COD) = 1.176 * c (KHP)
c (BOD) = 1.176 * c (KHP)
c (DOC) = 0.4705 * c (KHP)
3) COD = Chemical Oxygen Demand
4) Specified as COD. Corresponds to the quantity of oxygen that would be required to oxidize the substances if oxygen were the oxidizing agent.
5) Chemical Oxygen Demand (COD), Total Organic Carbon (TOC), Biochemical Oxygen Demand (BOD), Dissolved Organic Carbon (DOC)