TI00459C/07/EN/16.20
71505775
2020-11-30

Products

Solutions

ServicesTechnical 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 phasesYour 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 signaltransmission• 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 orat place of installation

Viomax CAS51D

Function and system design
Measuring principle

The light from a pulsed, high-stability strobe lamp (item 3) passes through the measuring path 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.
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Measuring principle of nitrate sensor
Measuring receiver with filter
Cuvette gap
Strobe lamp
Lens
Reference receiver with filter
Beam splitter

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 2) 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 - themeasured 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 ofmunicipal wastewater treatment plants.
Nitrate measurement
The sensor is designed for measuring nitrate. As nitrite is also measured, it could also be regarded asan 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.

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Measuring path = open path through cuvette
Nitrate or substances that contribute to the spectral absorption coefficient (SAC)

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Viomax CAS51D

The sensor measures the light intensity of the 214 nm wavelength (measuring channel). At thiswavelength, nitrate and nitrite ions absorb light in proportion to their concentration, while the lightintensity in the reference channel remains virtually unchanged at 254 nm.
Interference factors, such as turbidity, fouling or organic hydrocarbons, are minimized.
The signal ratio between the reference wavelength and measurement wavelength constitutes themeasurement result. This ratio is converted to the concentration of nitrate using the calibration
curve programmed into the sensor.
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 themeasured 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.
SAC measurement
Many organic substances absorb light in the range of 254 nm. In the SAC sensor, absorption on themeasuring 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 thispurpose, 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)
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 theirabsorption 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• Color3)
4)
5)

COD = Chemical Oxygen DemandSpecified as COD. Corresponds to the quantity of oxygen that would be required to oxidize the substances if oxygen were the oxidizing agent.
Chemical Oxygen Demand (COD), Total Organic Carbon (TOC), Biochemical Oxygen Demand (BOD), Dissolved Organic Carbon (DOC)

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Viomax CAS51D

Trends:• Oxidizable substances, absorbing at 550 nm, corrupt the measurement result. In instances of thisnature, 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 instancesof 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.

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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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Example of a measuring system with flow assembly for clear waterTransmitter CM44x
Sensor Viomax CAS51D
Flow assembly
Holders (included in the flow assembly delivery)5

Viomax CAS51D

Input
Measured variables

NitrateNO3-N [mg/l], NO3 [mg/l]
SAC
SAC [1/m], COD [mg/l], TOC [mg/l], BOD [mg/l], DOC [mg/l], transmission [%]Measuring range

CAS51D-**A2 (2 mm gap)

0.1 to 50 mg/l NO3-N
0.4 to 200 mg/l NO3
Clear water and sludge activation

CAS51D-**A1 (8 mm gap)

0.01 to 20 mg/l NO3-N
0.04 to 80 mg/l NO3
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 1)
TOC/DOC 0 to 30 mg/l 1)
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 1)
TOC/DOC 0 to 150 mg/l 1)
Clear water, medium measuring range, drinking water,
wastewater treatment plant outlet, monitoring of bodies ofwater

CAS51D-**C3 (2 mm gap)

SAC 0 to 1000 1/m
COD/BOD 0 to 1500 mg/l 1)
TOC/DOC 0 to 600 mg/l 1)
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

0 to 4000 mg/l COD

Influent from milk-processing industry

0 to 10 000 mg/l COD

Influent from chemical industry

0 to 10 000 mg/l COD

Power supply
Electrical connection

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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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Viomax CAS51D

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SensorSensor 1

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Sensor 2

Sensor
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Sensor connection to sensor input (left) or via M12 connector (right)

Connecting the cable shield
Cable sample (does not necessarily correspond to the original cable supplied)

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Terminated cable

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Outer shield (exposed)
Cable cores with ferrules
Cable sheath (insulation)

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Inserting the cable
Grounding clip

The maximum cable length is 100 m (328.1 ft).

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Tightening the screw(2 Nm (1.5 lbf ft))

The cable shield is grounded by thegrounding clip

Performance characteristics
Reference operating
conditions

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20 °C (68 °F), 1013 hPa (15 psi)

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Viomax CAS51D

Measured error 6)

NitrateWith 0.1 to 50 mg/l NO3-N (2 mm cuvette gap):
2 % of full scale value above 10 mg/l
0.4 % of full scale value below 10 mg/l
With 0.01 to 20 mg/l NO3-N (8 mm cuvette gap):
2 % of full scale value above 2 mg/l
0.2 % below 2 mg/l

SAC

Repeatability 6)

2 % of full scale value for standard measurement with potassiumhydrogen phthalate (KHP)

NitrateAt least ±0.2 mg/l NO3-N
SAC
0.5 % of end of measuring range (for homogeneous media)

Detection limits

Nitrate• CAS51D-AAA1
0.003 mg/l NO3-N• CAS51D-AAA2
0.013 mg/l NO3-N
SAC
In relation to the standard potassium hydrogen phthalate (KHP):• CAS51D-AAC1
0.045 mg/l COD• CAS51D-AAC2
0.3 mg/l COD• CAS51D-AAC3
1.5 mg/l COD

Determination limits

Nitrate• CAS51D-AAA1
0.01 mg/l NO3-N• CAS51D-AAA2
0.043 mg/l NO3-N
SAC
In relation to the standard potassium hydrogen phthalate (KHP):• CAS51D-AAC1
0.15 mg/l COD• CAS51D-AAC2
1.0 mg/l COD• CAS51D-AAC3
5.0 mg/l COD

Long-term drift

NitrateBetter than 0.1 mg/l NO3-N over one week
SAC
Better than 0.2 % of end of measuring range over one week

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The measured error contains all the uncertainties of the sensor and transmitter (measuring chain). It does not contain all the uncertainties
caused by the reference material and adjustments that may have been performed.

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Viomax CAS51D

Mounting
Mounting location• Choose a mounting location that can be easily accessed at a later stage.
• Ensure that upright posts and assemblies are fully secured and vibration-free.
• Select an installation location that produces a typical nitrate concentration / a typical SAC value
for the application in question.
• Do not install the sensor above aeration discs. Oxygen bubbles may accumulate at the cuvette gap
and distort the measured value.

‣ Align the sensor in such a way that the cuvette gap is rinsed

Orientation

with the flow of medium and air bubbles are removed.

A0013268

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Sensor orientation, arrow =direction of flow

Flexdip CYA112 wastewater assembly and Flexdip CYH112 holder

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Horizontal, fixed installation

The installation angle is 90°.
‣ Align the sensor in such a way that the
cuvette gap is rinsed with the flow of
medium and air bubbles are removed.

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A00132709Suspended vertically from a chain

The installation angle is 0°. Tried and tested
arrangement for operation in aerated zones.
‣ Ensure that the sensor is adequately cleaned.
There must be no buildup on the optical
windows.

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Viomax CAS51D

Flow assembly CAS51D 2-40 mm for small sample volumes
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Horizontally, in flow assembly, arrow points in the direction of flow

SensorFlow assembly
Medium outlet
Medium inflow
Window, required for aligning the sensor

Flowfit CYA251 flow assembly
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Horizontally, in flow assembly CYA251, arrow points in the direction of flow

SensorMedium outlet
Cap
Flow assembly
Medium inflow
Rinse connection

Environment
Ambient temperature range–20 to 60 °C (–4 to 140 °F)

Storage temperature–20 to 70 °C (–4 to 158 °F)

Degree of protection

IP 68 (1 m (3.3 ft) water column, 60 days, 1 mol/l KCl)

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Viomax CAS51D

Process
Process temperature range

5 to 50 °C (41 to 122 °F)

Process pressure range

0.5 to 10 bar (7.3 to 145 psi) (abs.)

Minimum flow

No minimum flow required.
For solids which have a tendency to form deposits, ensure that sufficient mixing is performed.

Mechanical construction

NPT ¾“NPT ¾“G1

Sensor with 2 mm gap width, dimensions in
mm (inch)

394 (15.51)
8 (0.31)14 (0.55)

2 (0.08)8 (0.31)50(1.97)

40 (1.57)

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12 (0.47)

20 (0.79)
14.5 (0.57)

388 (15.28)

14.5 (0.57)

12 (0.47)

20 (0.79)

G1

40 (1.57)

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78 (3.07)

Sensor78 (3.07)

Dimensions

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Sensor with 8 mm gap width, dimensions in
mm (inch)11

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Viomax CAS51D

14.5 (0.57)426 (16.77)

12 (0.47)

20 (0.79)

NPT ¾“G1

40 (1.57)

46 (1.81)

40 (1.57)

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Sensor with 40 mm gap width, dimensions in
mm (inch)

Flow assembly
70 (2.76)

70 (2.76)

30 (1.18)

Ø 6.5 (0.26)

60 (2.36)

86(3.39)

100 (3.94)**

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Dimensions in mm (inch)

Variable length

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Viomax CAS51D

Cleaning unit

1300 (11.81)

295 (11.6)

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20.5(0.81)C

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Dimensions in mm (inch)

Adapter 8 mm with 300 mm hose (only for 8 mm connection)
Connection 6 mm or 6.35 mm (¼")
Sensor (2mm oder 8 mm gap)
SAC sensor (40 mm gap)

WeightApprox. 1.6 kg (3.53 lbs) (without cable)

Materials

SensorOptical windows
O-rings

Process connections• G1 and NPT ¾'• Clamp 2" (depending on sensor version)/DIN 32676

Stainless steel 1.4404 (AISI 316 L)
Quartz glass
EPDM

Certificates and approvals
 markThe product meets the requirements of the harmonized European standards. As such, it complies
with the legal specifications of the EU directives. The manufacturer confirms successful testing of the
product by affixing to it the  mark.

EAC

The product has been certified according to guidelines TP TC 004/2011 and TP TC 020/2011 which
apply in the European Economic Area (EEA). The EAC conformity mark is affixed to the product.

Ordering information
Product page

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www.endress.com/cas51d

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Product Configurator

On the product page there is a Configure button to the right of the product image.
1.

Click this button.
 The Configurator opens in a separate window.

2.

Select all the options to configure the device in line with your requirements.
 In this way, you receive a valid and complete order code for the device.

3.

Export the order code as a PDF or Excel file. To do so, click the appropriate button on the rightabove the selection window.
For many products you also have the option of downloading CAD or 2D drawings of theselected product version. Click the CAD tab for this and select the desired file type using
picklists.

Scope of delivery

The delivery comprises:• Sensor in the version ordered• Operating Instructions

Accessories
The following are the most important accessories available at the time this documentation was
issued.

‣ For accessories not listed here, please contact your Service or Sales Center.
Device-specific accessories

Assemblies
Flexdip CYA112• Immersion assembly for water and wastewater• Modular assembly system for sensors in open basins, channels and tanks• Material: PVC or stainless steel• Product Configurator on the product page: www.endress.com/cya112
Technical Information TI00432C
Flowfit CYA251• Connection: See product structure• Material: PVC-U• Product Configurator on the product page: www.endress.com/cya251
Technical Information TI00495C
Flow assembly for CAS51D• For small flow volumes• Connection: hose, OD 6 mm
• Material: PVC-U• Two brackets for CAS51D• Order number: 71110000
HolderFlexdip CYH112• Modular holder system for sensors and assemblies in open basins, channels and tanks• For Flexdip CYA112 water and wastewater assemblies• Can be affixed anywhere: on the ground, on the coping stone, on the wall or directly onto railings.
• Stainless steel version• Product Configurator on the product page: www.endress.com/cyh112
Technical Information TI00430C

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Compressed air cleaning
Compressed air cleaning for CAS51D• Connection: 6 or 8 mm (metric) or 6.35 mm (¼")• Order numbers for sensor with 2 mm gap or 8 mm gap:
• 6 mm (with 300 mm hose and 8 mm adapter)
Order No.: 71110787• 6.35 mm (¼")
Order No.: 71110788• Order numbers for sensor with 40 mm gap:
• 6 mm (with 300 mm hose and 8 mm adapter)
Order No.: 71126757• 6.35 mm (¼")
Order No.: 71126758

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Compressed air cleaning for CAS51D

Cleaning for sensors with 2 mm and 8 mm gap
Cleaning for sensors with 40 mm gap
Adapter, 8 mm
300 mm hose (Ø = 6 mm)
Coupling, 6 mm or 6.35 mm (¼") for 2 mm and 8 mm gap
Coupling, 6 mm or 6.35 mm (¼") for 40 mm gap

Compressor• For compressed air cleaning• 230 V AC, order number: 71072583• 115 V AC, order number: 71194623
Standard solutions

Nitrate standard solutions, 1 liter• 5 mg/l NO3-N, order number: CAY342-V10C05AAE• 10 mg/l NO3-N, order number: CAY342-V10C10AAE• 15 mg/l NO3-N, order number: CAY342-V10C15AAE• 20 mg/l NO3-N, order number: CAY342-V20C10AAE• 30 mg/l NO3-N, order number: CAY342-V20C30AAE• 40 mg/l NO3-N, order number: CAY342-V20C40AAE• 50 mg/l NO3-N, order number: CAY342-V20C50AAE
KHP standard solution
CAY451‐V10C01AAE, 1000 ml parent solution 5 000 mg/l TOC

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