It deals with Analytical Instrumentation

1. Chapter 13 - Analytical
Instrumentation
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Chapter 14
Analytical Instrumentation

2. Chapter 13 - Analytical
Instrumentation
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pH Measurement and Control
• Many industrial applications require the control of the
concentration of acids and bases in a chemical solution
• The analytical process that controls this is called pH
control
• Within a chemical solution, the number of negative ions
compared to positive ions determines whether the solution
is acid or base

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Instrumentation
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pH Factors
• The relationship
between negative
and positive ions
is called
dissociation
(ionization)
constant

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Instrumentation
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Relationship
of pH and
Hydrogen Ion
Concentration

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Instrumentation
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pH Measurements
• Devices used to measure pH values detect the
concentration of hydrogen ions
• Early techniques used litmus paper which changed
color based upon the solution being acid or base
• Electronic sensors were developed to overcome
the shortcomings of paper-based tests

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Instrumentation
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Electronic Sensors
• Electronic sensors use two
electrodes and an amplifier to
measure pH
– The active or sensing probe
produces a voltage
proportional to the the
hydrogen-ion concentration
– The reference probe provides a
signal against which the
measuring electrode is
compared
Sensing Probe Reference Probe

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Instrumentation
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Controlling pH
• In a pH control system, either a solution is too
acidic or too alkaline
• The corrective ingredient added to a solution is
called a reagent
• One objective of a pH control system is to
minimize the amount of reagent added to the
solution, eliminating overshoot

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Instrumentation
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Batch Systems
• A batch system normally uses a
tank for the solution for storage
• Solution entering the tank is
called influent
• Solution exiting the tank that has
been treated is referred to as
effluent
• Batch pH control is used for
relatively small volumes

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Instrumentation
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Continuous Systems
• Many continuous systems use a
tank
• A reagent is added to correct the
pH of a solution using either an
on/off or proportional controller
• A disadvantage of using a tank is a
long delay time
• In applications requiring pH values
between 4 and 10, a static mixer
is used

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Instrumentation
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Conductivity
• Any process that involves liquids requires flow
• The liquid used in these processes is referred to as a process stream
• In many applications, the purity of water or the concentration level of
solutions is measured or controlled
• This can be done by measuring the conductivity of a solution
• Conductivity refers to the ability of material to pass electric current
Formula to determine conductance
G =
1
R

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Instrumentation
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Factors Affecting Conductivity
• Concentration of an ingredient dissolved in
water, ranging from zero to very high
• The type of electrolyte contained in a
dissolved ingredient
• The temperature of the liquid

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Instrumentation
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Conductivity Probes
• Two types of
probes used are:
– Electrode probe
– Inductive probe
Conductivity Probe

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Instrumentation
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Conductivity Inductive Probe
• The conductivity inductive
probe uses to toroidal coils
• One coil is connected to an
oscillator, the other coil is used
as a pickup
• The the current induced in the
pickup coil is directly
proportional to the conductivity
of the solution

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Instrumentation
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Conductivity Values

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Instrumentation
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Combustible Gases
• Some gases are dangerous if they escape from
their container
– Hydrogen
– CO
– Hydrogen sulfide
– Methane, propane, butane, ethane
• Analytical sensors are used to detect these gases

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Instrumentation
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Thermo-Conductivity Detector
• Every gas has the ability
to conduct heat
• The type of gas and its
concentration determine
the rate at which heat is
conducted
• The TCD uses a bridge ,
measuring chamber, and a
reference chamber

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Instrumentation
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Hydrocarbon Gases
• Combustion occurs when hydrocarbon
fuels are ignited
• When they burn completely, the by-
products are water and carbon dioxide
• CO2 and CO can be detected with an
infrared gas analyzer

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Instrumentation
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Optical Gas Analyzer
• An infrared gas analyzer
consists of six major
elements:
– Infrared light source
– Sample chamber
– A prism
– Two optical filters
– Two detectors
– Electronic converter

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Instrumentation
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Combustion Analyzers and Control
• Energy for many industrial process is produced by
combustion
• Combustion uses a combination of gases and fuel
• Two types of fuel
– Hydrocarbon
– Combustible gas

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Instrumentation
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Humidity
• Humidity is defined as the amount of moisture in
the air
• Humidity control is important in many industrial
applications
• Humidity affects hygroscopic materials.
Hygroscopic means the ability to absorb and retain
moisture

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Instrumentation
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Quantitative Measures of Humidity
• There are three different quantitative
measures of humidity
– Absolute
– Relative
– Dew Point

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Instrumentation
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Absolute Humidity
• Defined as the mass of water vapor present in a particular
volume of atmosphere
• Absolute humidity value is expressed as the ratio of the
mass of water vapor to the volume occupied by the air-
water vapor mixture
W =
Pw
Pa
W = Absolute Humidity
Pw = Mass Density of Water
Pa = Mass Density of Air

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Instrumentation
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Relative Humidity
• Defined as the
actual amount of
water vapor
present as
compared to the
maximum amount
of water vapor the
air can hold at a
given temperature

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Instrumentation
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Absolute Humidity Sensor
• The most common type of
device to measure
absolute humidity is the
aluminum oxide sensor
• Essentially, the aluminum
oxide sensor functions as a
capacitor with the value of
capacitance dependent
upon humidity

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Instrumentation
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Dew Point Measurements
• Dew Point is defined as the temperature at which the air
becomes saturated
• When air is cooled at a constant pressure, condensation of
vapor will occur at the dew point temperature
• Three common techniques to measure dew points are:
– Manual chilled mirror
– Adiabatic expansion sensing
– Optical chilled mirror

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Instrumentation
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Optical Chilled-Mirror Hygrometer
• Components include:
– Gold or rhodium-
plated copper mirror
– Thermoelectric cooler
– High-intensity LED
– Optical detector

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Instrumentation
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Relative Humidity Detectors
• Psychrometric Detector - uses two identical
thermometers called a dry bulb and a wet bulb
• Hygrometric Detector - measures the change in
dimension of hygroscopic materials
• Electronic Capacitance Detector - A common RH
sensor that is constructed as a small capacitor with
a hygroscopic polymer acting as the dielectric

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Instrumentation
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Sampling Measurement Systems
• Whenever the
temperature of the air
being tested is above
the level the sensor can
be exposed, it must
first be cooled before a
reading can be taken
• A sampling system
like the one on the
right offers a solution