turbidity meter

8/12/2019 turbidity meter

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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

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