TI00459C/07/EN/17.24-00
71669891
2024-08-07

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Viomax CAS51D
Photometric sensor for measuring the spectral absorption
coefficient or nitrate content

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 aircleaning• 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, highly stable strobe lamp (item 3) passes through the measurement gap(item 2). A beam splitter (item 6) directs the light beam to the two receivers (items 1 and 5). A filter
in front of the receivers only lets through light in the measuring wavelength or reference
wavelength.
123654A0013213

112
3
4
5
6

Measuring principle of nitrate sensor
Measuring receiver with filter
Measurement gap
Strobe lamp
Optical window
Reference receiver with filter
Beam splitter

Within the measuring gap, the medium (water, dissolved ingredients, and particles) absorbs light
across the entire spectrum. In the measuring wavelength range, the measured component 1) 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 measurement paths 2) measurement gap are required for detecting low concentrations of themeasured component.
This is achieved in nitrate measurement with the measurement gap 8 mm (0.31 in) and in SAC
measurement with the 40 mm (1.57 in) measurement gap for clear water samples.
• For high turbidity values, longer measuring paths result in the total absorption of light - themeasured values are no longer valid.
For media with high turbidity (such as in activated sludge applications), the nitrate sensor with the2 mm (0.08 in) measurement gap is recommended. Alternatively, a nitrate sensor with the8 mm (0.31 in) measurement gap can be used with an appropriate sample preparation.
The SAC sensor with the 2 mm (0.08 in) measurement gap is ideal for measuring the organic loadin the inlet of municipal 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.

1)
2)

2

Nitrate or substances that contribute to the spectral absorption coefficient (SAC)
Measurement path = Open path length through theEndress+Hauser

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 measuringrange.
• 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 worthwhile in this case.
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 SACmeasurement 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 (DOC) = 0.4705 × c (KHP)• c (COD) = 1.176 × c (KHP)• c (BOD) = 1.176 × c (KHP)
The calculated relationships between COD, TOC, BOD, and DOC with SAC are as follows:• TOC = 0.595 (mg/l × m)× SAC (1/m)• DOC = 0.595 (mg/l × m)× SAC (1/m)• COD =1.487 (mg/l × m)× SAC (1/m)• BOD =1.487 (mg/l × m)× SAC (1/m)
Many components that absorb light at 254 nm deviate significantly from KHP in terms of theirabsorption behavior. For this reason, an adjustment based on the customer process is recommended.
The factors (F) stored in Liquiline can be adapted to the customer process (in the CAL menu). You
can determine the factor F(Liquiline) to be entered as follows:
F(Liquiline) = laboratory value/SAC(CAS51D) × 0.7909

3)
4)
5)

COD = Chemical Oxygen DemandSpecified as COD. Corresponds to the quantity of oxygen that would be required to oxidize the substances if oxygen was 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

Cross-interference when measuring SAC
The following have a direct impact on the measuring range:• Turbidity• ColorTrends:• 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 hydrogenphthalate) 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.
Measuring system

A complete measuring system comprises:• Viomax CAS51D sensor• Liquiline CM44x multi-channel transmitter• Universal installation or flow assembly:• Flexdip CYA112 and Flexdip CYH112 holder or• Flowfit CYA251 or• CAV01 (for clear water)

123 4A0055544

212344

Measuring system with flow assembly CAV01
Transmitter
Sensor Viomax CAS51D
HolderFlow assemblyEndress+Hauser

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 (0.08 in)measurement 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 (0.31 in)measurement 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 (1.57 in)measurement gap)

SAC 0 to 50 1/m
COD/BOD 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 (0.31 in)measurement 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 (0.08 in)measurement 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, discharger control, industrial processes

1)

equivalent KHP

The possible measuring range depends strongly on the properties of the medium.
Empirical values for typical COD measuring ranges

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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

5Viomax CAS51D

Power supplyThe following connection options are available:• Via M12 plug (version: fixed cable, M12 plug)• Via sensor cable to the plug-in terminals of a sensor input on the transmitter (version: fixed cable,end sleeves)85

Electrical connection

GY

85

86

PK

2

85

86

285

GY

1

PK

86

12DSSensor

98 97 88 87

Sensor 198 97 88 87

86

Sensor 1BNWH
GNYE

GNYE

98 97 88 87

Sensor 2Sensor
A0033092

3

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

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

Performance characteristics
Reference operating
conditions

20 °C (68 °F), 1013 hPa (15 psi)

Measurement error 6)

NitrateFor 0.1 to 50 mg/l NO3-N (measurement gap 2 mm (0.08 in)):
2 % of full scale value above 10 mg/l
0.4 % of full scale value below 10 mg/l
For 0.01 to 20 mg/l NO3-N (measurement gap 8 mm (0.31 in)):
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)

6)

6

The measurement 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

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

Installation
Mounting location

1.

Do not install the device in places where air pockets and foam bubbles form.

2.

Choose a mounting location that can be easily accessed at a later stage.

3.

Ensure that upright posts and assemblies are fully secured and vibration-free.

4.

Align the device so that the measurement gap is rinsed by the flow of medium.

5.

Do not install the sensor above aeration discs. Oxygen bubbles may accumulate on the optical
windows of the sensor, leading to inaccurate measurements.

6.

Select an installation location that produces a typical nitrate concentration/a typical SAC value
for the application in question.

‣ Align the sensor in such a way that the measurement gap isOrientationrinsed with the flow of medium and air bubbles are removed.

A0013268

4Endress+Hauser

Sensor orientation, arrow =direction of flow

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Viomax CAS51D

Flexdip CYA112 wastewater assembly and Flexdip CYH112 holder

A0013267

5Horizontal, fixed installation

A0013270

6The installation angle is 90°.
‣ Align the sensor in such a way that themeasurement gap is rinsed with the flow of
medium and air bubbles are removed.

Suspended 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 of the sensor.

Flow assembly CAV01

12 34A0055564

71234Horizontal, in flow assembly CAV01, arrow indicates the flow direction
Sensor Viomax CAS51D
Flow assembly
Medium inlet
Medium outlet

Flowfit CYA251 flow assembly
123

456A0032901

81234568

Horizontally, in flow assembly CYA251, arrow points in the direction of flow
Sensor Viomax CAS51DRinse connection
Medium outlet
Flow assemblyCap
Medium inlet

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Viomax CAS51D

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, 24 hours, 1 mol/l KCl)

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) absolute

Minimum flow

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

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9Viomax CAS51D

Mechanical construction

NPT ¾“NPT ¾“G1

Dimensions of sensor with measurement gap
2 mm (0.08 in). Unit: mm (in)394 (15.51)
8 (0.31)14 (0.55)

2 (0.08)8 (0.31)50(1.97)40 (1.57)

A0013208

A0013193

1012 (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)

978 (3.07)

Sensor78 (3.07)

Dimensions

 10

Dimensions of sensor with measurement gap
8 mm (0.31 in). Unit: mm (in)Endress+Hauser69 (2.72)

Viomax CAS51D

14.5 (0.57)426 (16.77)

12 (0.47)

20 (0.79)

NPT ¾“G140 (1.57)

46 (1.81)

40 (1.57)

A0031311 11

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Dimensions of sensor with measurement gap
40 mm (1.57 in). Unit: mm (in)11

Viomax CAS51D

Cleaning unit

1300 (11.81)

295 (11.6)

122

16 (0.63)

20.5(0.81)
CA

A0013292

 1212AC

Dimensions in mm (inch)

Adapter 8 mm (0.31 in) with 300 mm (11.81 in) hose (only for connection 8 mm (0.31 in))Connection 6 mm (0.24 in) or 6.35 mm (0.25 in))Sensor (measurement gap 2 mm (0.08 in) or 8 mm (0.31 in))
SAC sensor (measurement gap 40 mm (1.57 in))

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
Current certificates and approvals for the product are available at www.endress.com on the relevant
product page:1.

Select the product using the filters and search field.

2.

Open the product page.

3.

Select Downloads.

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.

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Viomax CAS51D

Ordering information
Product pagewww.endress.com/cas51d

Product Configurator

1.

Configure: Click this button on the product page.

2.

Select Extended selection.
 The Configurator opens in a separate window.

3.

Configure the device according to your requirements by selecting the desired option for eachfeature.
 In this way, you receive a valid and complete order code for the device.

4.

Accept: Add the configured product to the shopping cart.
For many products, you also have the option of downloading CAD or 2D drawings of theselected product version.

5.

Scope of delivery

CAD: Open this tab.
 The drawing window is displayed. You have a choice between different views. You candownload these in selectable formats.

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

Accessories
The following are the most important accessories available at the time this documentation wasissued.
Listed accessories are technically compatible with the product in the instructions.

Device-specific accessories

1.

Application-specific restrictions of the product combination are possible.
Ensure conformity of the measuring point to the application. This is the responsibility of theoperator of the measuring point.

2.

Pay attention to the information in the instructions for all products, particularly the technicaldata.

3.

For accessories not listed here, please contact your Service or Sales Center.

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
CAV01• Flow assembly• Material: POM-C• Product Configurator on the product page: www.endress.com/cav01
Technical Information TI01797C

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13Viomax CAS51D

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
Cleaning
Cleaning brushes• Cleaning brushes to clean the measurement gap (for all gap sizes)• Order number: 71485097
Compressed air cleaning for CAS51D• Pressure: 1.5 to 2 bar (21.8 to 29 psi)
• Measurement gap 2 mm (0.08 in) or 8 mm (0.31 in):• 6 mm (0.24 in) (with 300 mm (11.81 in) hose and 8 mm (0.31 in) adapter)
Order number: 71485094• 6.35 mm (0.25 in)Order number: 71485096• Measurement gap 40 mm (1.57 in):
6 mm (0.24 in) (with 300 mm (11.81 in) hose and 8 mm (0.31 in) adapter)
Order No. 71126757
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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