Endress+Hauser CAS51D Specifications

TI00459C/07/EN/15.19 71447514 2019-07-31

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Technical Information

Viomax CAS51D

Photometric sensor for SAC or nitrate measurement

Application

SAC measurement

• Organic load in WWTP inlet

• Organic load WWTP outlet

• Discharger monitoring

• Organic load in drinking water Nitrate measurement

• Nitrate measurement in natural bodies of water

• Monitoring nitrate content in WWTP outlet

• Monitoring nitrate content in aeration basins

• Monitoring and optimizing denitrification phases

Your benefits

• Affordable, environmentally friendly product

• No complicated sampling and treatment

• Chemical-free measurement

• Low-maintenance

• Data conditioning in the sensor

• Minimum sensitivity to interference during signal transmission

• Short response time

• Early, continuous detection of load peaks without delay

• Ready for use thanks to factory calibration

• Standardized communication (Memosens technology) enables "plug and play"

• Very long maintenance intervals thanks to compressed air cleaning

• Customer calibrations with 1 - 5 points (max.) - in the lab or at place of installation

Function and system design

Viomax CAS51D

Measuring principle

The light from a pulsed, high-stability strobe lamp (item 3) passes through the measuring path

(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.

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 1 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

Nitrate measurement

Within the measuring path, the medium in the cuvette (water, dissolved substances and particles) absorbs light across the entire spectrum. In the measuring wavelength range, the measured component

takes an additional amount of energy from the light.

For the calculation of the measured value, the ratio of the light signal of the measuring wavelength to the light signal of the reference wavelength is calculated in order to minimize the effect of turbidity and lamp aging.

This change in the ratio can be converted to determine the nitrate concentration or the SAC value. This dependency is non-linear.

Conclusion:

• Long measuring paths 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 measuring paths result in the total absorption of light - the measured values are no longer valid. The nitrate sensor with the 2 mm cuvette is recommended for media with high turbidity values (activated sludge application). The SAC sensor with the 2 mm cuvette is ideal for measuring the organic load in the inlet of municipal wastewater treatment plants.

The sensor is designed for measuring nitrate. As nitrite is also measured, it could also be regarded as an NOx 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.

1) Measuring path = open path through cuvette

2) Nitrate or substances that contribute to the spectral absorption coefficient (SAC)

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Viomax CAS51D

Interference factors, such as turbidity, fouling or organic hydrocarbons, are minimized.

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.

Cross-interference when measuring nitrate

SAC measurement

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

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

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.

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 C8H5KO4) 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

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)

The ratio to SAC (based on KHP) is calculated as follows: 1/m = 1.487 mg/l COD = 1.487 mg/l BOD = 0.595 mg/l TOC = 0.595 mg/l DOC

Many components that absorb light at 254 nm deviate significantly from KHP in terms of their absorption behavior. For this reason, an adjustment to the customer process is worthwhile.

Cross-interference when measuring SAC

The following have a direct impact on the measuring range:

• Turbidity

• Color Trends:

• Oxidizable substances, absorbing at 550 nm, corrupt the measurement result. In instances of this

nature, a comparison or calibration is necessary.

• Coloration that absorbs in the green spectral range increases the measured value.

• Oxidizable substances with spectral properties that differ to those of KHP (potassium hydrogen

phthalate) provide measurement results that can deviate from the factory calibration. In instances of this nature, a comparison or adjustment is necessary.

• A higher proportion of TS or greater turbidity reduces the upper end of the measuring range,

resulting in a smaller measuring range.

• 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 measured value due to turbidity.

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)

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Viomax CAS51D

Measuring system

A complete measuring system comprises:

• Viomax CAS51D

• Liquiline CM44x multi-channel transmitter

• Universal installation or flow assembly

• Flexdip CYA112 and Flexdip CYH112 holder or

• Flowfit CYA251 or 71110000 (for clear water)

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 2 Example of a measuring system with flow assembly for clear water

1 Transmitter CM44x 2 Sensor Viomax CAS51D 3 Flow assembly 4 Holders (included in the flow assembly delivery)

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Viomax CAS51D

Input

Measured variables

Measuring ranges

Nitrate

NO3-N [mg/l], NO3 [mg/l]

SAC

SAC [1/m], COD [mg/l], TOC [mg/l], BOD [mg/l], DOC [mg/l], transmission [%]

CAS51D-**A2 (2 mm gap) 0.1 to 50 mg/l NO3-N

0.4 to 200 mg/l NO Clear water and sludge activation

CAS51D-**A1 (8 mm gap) 0.01 to 20 mg/l NO3-N

0.04 to 80 mg/l NO Clear water (with a COD (KHP) content of up to 125 mg/l and up to 50 FNU turbidity based on mineral kaolin)

CAS51D-**C1 (40 mm gap) SAC 0 to 50 1/m

CSB/BSB 0 to 75 mg/l TOC/DOC 0 to 30 mg/l Clear water, low measuring range, drinking water

CAS51D-**C2 (8 mm gap) SAC 0 to 250 1/m

COD/BOD 0 to 375 mg/l TOC/DOC 0 to 150 mg/l Clear water, medium measuring range, drinking water, wastewater treatment plant outlet, monitoring of bodies of water

CAS51D-**C3 (2 mm gap) SAC 0 to 1000 1/m

COD/BOD 0 to 1500 mg/l TOC/DOC 0 to 600 mg/l Organic load in the inlet, influent control, industrial processes

1) equivalent KHP

The possible measuring range depends greatly on the properties of the medium.

Empirical values for typical COD measuring ranges

Inlet of municipal wastewater treatment plant

Influent from milk-processing industry 0 to 10 000 mg/l COD

Influent from chemical industry 0 to 10 000 mg/l COD

0 to 4000 mg/l COD

Power supply

The following connection options are available:

• via M12 connector (version: fixed cable, M12 connector)

• via sensor cable to the plug-in terminals of a sensor input on the transmitter (version: fixed cable,

end sleeves)

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