Post on 03-Jun-2018
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Standards
Certification
Education & Training
Publishing
Conferences & Exhibits
What is Turbidity?
John Daly
ISA NorCal President
South Fork Instruments, Inc.
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Objectives
Understand what Turbidity is
Investigate the various methods ofmeasurement
Compare those measurements
Calibration and how it relates.
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Definition
Turbidity is the phenomen where by a specific portion of
a light beam passing through a liquid medium is
deflected from undissolved particles.
What is Turbidity ?
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Scattered Light
Incident
Lightbeam
The deflection is a function of the size and shape of the particles
Size: Smaller Than 1/10 the
Wavelength of Light
Description: Symmetric
Size: Approximately 1/4 theWavelength of L ight
Description: Scattering Concentrated
in Forward Direction
Size: Larger Than the Wavelength of
Light
Description: Extreme Concentration
of Scattering in Forward Direction;
Development of Maxima and Minima
of Scattering Intensity at Wider
Angles
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Absorbance vs. Scattered Light
I0 IR
IS
I0 ID
Absorbance of light:
(Concentration)Dissolved Solids
Scattering of light:
(Turbidity)
Particulate and Solids
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Scattered Light - What are Particles ?
Particles may be anything creating heterogeneous
surfaces:
solids in liquids (suspension)
oil in water / water in oil (emulsion)
gas bubbles (foam)
dust in air (smoke)
droplets in air (fog, aerosol)
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Scattered Light
the type of the particles (absorbance)
the size of particles
the concentration (the number of particles)
the type and shape of particles
the wavelength of the light the angle of measurement
The diffusion of light caused by undissolved particles in the
medium to a lesser or greater degree of the deflection
depends on:
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Some History - Jackson Candle
Turbidity Scale
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Scattered Light - Formazine Standard
242 NHCFormazine =
1 FNU = 1 FTU = 1 NTU = 1 TU/F = 0,25 EBC
standard-formazin-solution = 4000 FNU
FNU = formazine nephelometric unit
FTU = formazine turbidity unit
NTU = nephelometric turbidity unitTU/F= turbidity units formazin
EBC = European Brewery Convention
ingredients: Hexamethylentetramine + Hydrazinsulfate
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Scattered Light - Comparability
A lot of suppliers with different sensors
hard to compare
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Dual Beam Forward Scattered Light
LIGHT
SOURCE
PROCESS
STREAM
WINDOWS
DIRECT BEAM
DETECTOR
SCATTER BEAM
DETECTOR
SCATTER BEAM
DETECTOR
COLIMATING
OPTICS
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90 degree Scatter- Nepholometry
Single Detector Nepholometry
Dual Detector Nepholometry
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One and Two Detector 90 degree
Measurement
Single Detector
(low turbidity)
Second shorter
path detector
(high turbidity)
2
3
Characteristic Lines
turbidity
intensity
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Backscatter Measurement
High Turbidity Systems 4000 FTU
Typically probe systems
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Disturbing Light Effects
disturbing light
- daylight
- other light
- heat sources (IR)
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Main Disturbance: Gas Bubbles
50-80% of the mistakes
in turbidity measurement
Many suppliers
have a debubbler
chamber
associated with
their installation
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Wiper
Spray cleaningSpray head
Sensor Cleaning Systems
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How does turbidity relate to Process?
Each process will have a specific turbidity
signature
Your process will have a different particulate size
distribution to Formazin so has to be correlated
Standards only allow the instrument to becalibrated to the same condition
Readings are TREND readings for your process
If the particle size distribution changes, so doesthe reading!
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Scattered Light - Comparability, Correlation
0
50
100
150
200
250
300
0 50 100 150 200
NTU (90)
F
TU
Formazine
Solids 1
Solids 2
Solids 3
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Scattered Light - Comparability,
Correlation - again
PPM readings against Nepholometry
0
20
40
60
80
100
120
140
160
0 10 20 30 40
PPM
NT
U
Ratio Measurement
(Nepholometer)Single Beam
Nepholometer
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TurbidityTurbidity as Function of Concentration
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TurbidityTypical Values
Measured Sample
Waste Water
Final outlet sewage treatment plant
Well Water
Potable water
Milk
Orange juice
Primary sludge
Activated sludge
Recirculated sludge
Digested sludge
Measured Value
70-2000 NTU
4-20 NTU
0.05 - 10 NTU
0.05 - 1.5 NTU
> 4000 NTU
300 - 900 NTU
6-3%(60 - 30 g/l)
3-7 g/l
6-8 g/l
5-8%(50-80 g/l)
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Turbidity - Examples of Applications
Filter Control
Centrifuge/Separator Control
Biomass in fermenter Cell Growth
Quality Control of final product
Oil in water
Water in oil
Catalyst concentration
Diesel in water
Oil in condensate
Leakage control heat exchanger
Fat Content in milk
Yeast dosage
CIP return line Interphase detection
Product recognition
Water control, In- and Outlet
Flocculant dosage
Sludge concentration
Pulp concentration
Content of solids
Dust in gases