Transcript Time Schedule

Chemical Variables Measurements
Lecture for Licentiate Course in Measurement Science and Technology
Marion Hermersdorf
February 15. 2006
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
Time Schedule
2
14:15h
45min
Lecture Part 1
15:00h
15min
Break
15:15h
45min
Lecture Part 2
16:00h
15min
Questions
Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
Content
Table of Content
Lecture Part 1
Introduction
pH Measurements
Lecture Part 2
Humidity and Moisture Measurements
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
Content
Table of Content – Lecture Part 1
• Introduction
• pH Measurements
• Definition of pH
• Electrochemical Methods of pH Measurement
• Glass Membrane Electrode
• pH-FET
• Metal/metal oxide electrodes
• Liquid Membrane Electrode
• Optical Methods of pH Measurement
• Indicator dyes
• Indicator paper
• Fiber-optic pH probes
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
Introduction
Measurements in General
The book defines …
A little bit fuzzy, because
…
e.g.
• force -> capacitance -> frequency
• temperature -> resistance
• length -> time
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
Introduction
Electronic Measurements
Physical
value
Signal
conversion and
processing
Sensing
element
resistance
heat
NTC
temperature
force
pressure
Capacitive MEMS
pressure element
Wheatstone Bridge
capacitance
Switched capacitor circuit
In the following are only the sensing elements discussed not the
conversion and further signal processing.
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
pH Measurement
Definition of pH
pH = pondus Hydrogenii, literally: hydrogen exponent
Most common interpretation:
pH is used to specify the degree of acidity or basicity (also
called causticity) of an aqueous solution.
Historical definition:
pH is defined as the negative logarithm of
the hydrogen ion concentration in solution
Later definition:
pH is defined as the negative logarithm of
the hydrogen ion activity in solution
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
pH  logH 

 
pH   logaH   logγ H
pH Measurement
pH Examples
• Water:
• Hydrochloric acid:
H2O  HCl 
 H3O  Cl
• Sodium hydroxide:
H2O  NaOH
 Na  2H2O  OH
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
pH Measurement
pH Measurements
Most widely performed measurement in chemical laboratories.
Measurement principles:
1. Electrochemical Methods
2. Optical Methods
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
pH Measurement
Electrochemical Methods of pH Measurement
Electrochemical measurement of pH utilizes devices that transduce the chemical
activity of the hydrogen ion into an electronic signal, such as an electrical
potential difference or a change in electrical conductance.
Methods:
Glass membrane electrode
pH-FET
Metal/metal oxide electrodes
Liquid membrane electrodes
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
pH Measurement
Glass Membrane Electrode
• Most widely used
• Indicator and reference
electrodes commonly combined
into a single probe
(combination electrode)
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
Glass indicator
electrode
Referenc
e
electrode
pH Measurement
Glass Indicator Electrode
• Glass membrane about 0.1 mm
thick
• Glass membrane acts as a
transducer of the pH
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
pH Measurement
Reference Electrode
• Stable and low resistance electrical
contact between the external
measuring circuit and the sample
• Different kinds of reference
electrodes:
• most widely used is the silver/silver
chloride electrode
• another commonly used reference
electrode is the calomel electrode
(HgCl) for high precision, limited
temperature
• Selection of reference electrode
dependent on:
• Type of solution
• Temperature range
• precision
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
pH Measurement
Potential vs. pH
• ideally 59.16 mV per pH unit
• Reference electrode introduces additional potential -> can be calibrated out
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
pH Measurement
Measurement Circuit
• Measured potential ranges in between a few 100 millivolts
• extremely high resistance of the measurement electrode's glass membrane
(100MΩ to more than 1000MΩ)
• voltmeter with extremely high internal resistance needed (high input impedance
amplifier with FET input stage)
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
pH Measurement
Calibration
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
pH Measurement
Temperature Compensation
• temperature coefficient of
approximately 0.3% per °C
• most pH meters have provision for
temperature compensation
• meters equipped with automatic
temperature compensation (ATC) use
a platinum resistance thermometer
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
pH Measurement
Selected Glass Membrane Electrodes
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
pH Measurement
Selected Glass Membrane Electrodes
pHC2401 pH
electrode
MI-506 Flexible pH Electrode
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
pH Measurement
pH-FET Measurement Principle
• Relatively recent development
• Based on the use of an ionselective field-effect transistor
(ISFET)
• pH-responsive membrane
(instead of metal gate)
• Advantages:
• Inexpensive,
• robust,
• battery-powered,
• pocket size
• Especially used in food industry
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
pH Measurement
pH-FET Operation
• Voltage applied to reference electrode (relative to silicon substrate)
Charging of capacitor (electrode, solution, insulation layers and silicon substrate)
Drain source current influenced
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
pH Measurement
Selected ISFET Electrodes
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
pH Measurement
Metal/Metal Oxide pH Sensors
• Metal electrodes coated with an oxide
• Operation at high temperatures and high
pressures
• Various shapes of electrode possible
• Based on reduction of the metal oxide :
• Near Nernstian response of -59mV per pH
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
pH Measurement
Liquid Membrane Electrodes
• Ion-selective electrode
• Membrane is selectively permeable to
ions of interest
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
pH Measurement
Optical Methods of pH Measurement
Use of organic dye molecules with pH-dependent spectral properties
Methods:
Indicator dyes
Indicator paper
Fiber-optic pH probes
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
pH Measurement
Indicator Dyes
• organic dye molecules are weak acids or bases
• loss or gain of a proton changes the electronic structure of the molecule
• measurable change in the manner in which the molecule interacts with light
• interaction can be the absorption of
light at a particular wavelength or
fluorescence
• pH of interest therefore dictates
selection of the particular dye
• Limitations of the human eye restrict
detectable changes in color of ±1 pH
unit.
• Thus, an indicator with a pKa of 5 will
display a color change if the solution
in which it is dissolved changed from
4 to 6 pH units.
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
pH Measurement
Indicator Papers
• simple, rapid, and inexpensive means of measuring pH
• strip of paper or plastic that has been impregnated with one or more absorption
indicator dyes
Litmus paper
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
pH Measurement
Fiber-Optic pH Probes
•
•
•
•
•
often referred to as optrodes
most sophisticated pH sensors
indicator dye at the tip of a light guide
Challenge and dependency of fixating dye at tip
Advantage:
• Usable in electrically noisy environment
• New methods and techniques developed in recent years
• Two main methods:
• Absorption optrodes
• Fluorescent indicator optrodes
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
pH Measurement
Absorption Optrodes Principle
• Measure the change in intensity of the light returned from the fiber tip
• Two fibers necessary
• Measurement at two wave lengths (one for reference)
• Ratio of the scattered intensities at the two wavelengths is related to the pH
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
pH Measurement
Fluorescent Indicator Optrodes
• single fiber to both interrogate and collect signal-carrying light
• amount of fluorescent pH indicator at the fiber tip must be maximized
• due to the relatively small light intensities, the detector is typically a
photomultiplier tube rather than a photodiode
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
Break
.. after the break:
humidity and moisture measurements
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
Content
Table of Content – Lecture Part 2
Humidity and Moisture Measurements
• Introduction
• Humidity measurements in gases
• Moisture measurements in liquids and solids
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
Humidity and Moisture
Introduction to Humidity and Moisture
Water and water vapor can be found everywhere
Humidity = water vapor in the air or any other gas
Moisture = water in liquids and solids
Humidity and moisture have great economic importance
• Storage of food and raw material
• Optimum manufacturing conditions
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
Humidity and Moisture
Expressions for Humidity and Moisture
• Vapor pressure
Ranges from a half to a few percent
• Absolute humidity
Mass of water vapor per unit volume
• Relative humidity
ratio of the actual vapor pressure and the saturation vapor pressure at a certain temperature
• Dewpoint temperature
is the temperature to which a gas must be cooled, at constant pressure, to achieve saturation
• Mixing ratio
mass of water vapor per unit mass of dry gas, usually expressed in grams per kilogram
• Mole fraction
ratio of the number of moles of water to the total number of moles
• Concentration of water in liquids/solids
Given in kg/kg or kg/volume
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
Humidity and Moisture
Characteristics of Humidity and Moisture
Saturation vapor pressure
When the saturation vapor pressure is reached, any further addition of water vapor
results in condensation. In the presence of air molecules at atmospheric pressure, the
saturation vapor pressure is about 0.4% higher (enhancement factor).
Equilibrium relative humidity
Condition where there is no net exchange of water vapor between a moisturecontaining material and its environment.
Water activity
the same condition like equilibrium relative humidity but expressed as a ratio instead
of a percentage
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
Humidity and Moisture
Fundamental Behavior of Water
Water changes:
•
•
•
•
•
•
•
•
length of organic materials
conductivity and weight of hygroscopic material and chemical absorbents
impedance of almost any material
color of chemicals
refractive index of air and liquids
velocity of sound in air
electromagnetic radiation in solids
thermal conductivity of gases, liquids, and solids
Water absorbs:
• infrared radiation
• ultraviolet radiation
• microwave radiation
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
Humidity and Moisture
Measurement Methods of Humidity and Moisture
Many different measurement methods.
1. Minimum range of operation
Over-specification can be expensive
2. Exposure of the sensor to the measurement environment
Danger of condensation
3. Accuracy needs
In general expected accuracies not better than 2% r.h. of 0.5°C
4. Response time
5. Calibration frequency
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
Humidity and Moisture
Measurement of Humidity in Gases
Gravimetric method
Precision humidity generator
Condensation dewpoint hygrometer
Psychrometer
Lithium chloride dewpoint meter
Resistive humidity sensor
Capacitive humidity sensor
Thermal conductivity humidity sensors
Coulometric method
Crystal Oscillator
Infrared method
Mechanical hygrometer
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
Humidity and Moisture
Gravimetric Method
• Most fundamental way of measuring the amount of water vapor in a moist gas
• Operation principle:
• The water vapor is frozen out by a cold trap
• Or absorbed by a chemical
• Advantages:
• very accurate 0.1% to 0.2% or 0.1°C dew point (used for primary standards)
• Disadvantages:
• difficult and laborious to use
• very expensive
• not portable
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
Humidity and Moisture
Precision Humidity Generator
•
Three practical methods:
1. Two flow method
One dry stream of air, one test stream (known temp.) -> humidity = rates of flows
2. Two temperature method
3. Two pressure method
•
Advantage:
•
•
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Accuracy close to gravimetric method
Disadvantage:
•
Stationary device
•
Expensive
•
Big device
Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
Humidity and Moisture
Condensation Dewpoint Hygrometer
• Air is cooled down until saturation temperature (constant pressure)
• Saturation temperature (dewpoint) is detected
• Practical means:
• A mirror/inert substance is cooled down
• Air is passed over
• Condensation is detected (visual, electrical or acoustical)
• Accuracies around 0.5°C
• Advantage:
• Contamination of the mirror
• Measurement of another condensable
vapor instead of water
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
LAB-EL DP-373
Humidity and Moisture
Psychrometer
• Principle:
• Two thermometers ventilated by the humid air
• One thermometer surrounded by a wet cloth
• The other thermometer measures the air temperature t
• The energy needed to evaporate water from the wet cloth to the air cools the “wet”thermometer down by tw
e: vapor pressure
ew: saturated vapor pressure
A: psychrometer coefficient
P: total atmospheric pressure
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
Humidity and Moisture
Lithium Chloride Dewpoint Meter (1)
• Principle:
A hygroscopic soluble salt, e.g. LiCl, added to water decreases the equilibrium
saturation humidity
• Implementation
• Sleeve fabric with a LiCl solution is put between two electrodes
• Electrodes heat up fabric until resistance between fabrics increases sharply (= dry
fabric)
• Cooling down => LiCl in fabric “sucks” in water out of humid gas => temperature of
fabric cools down very fast
• At one point the LiCl reaches equilibrium saturation relative humidity => this can be
measured by a “stabilized” temperature curve
• This temperature point (b) can be transformed into a relative humidity (see figure)
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
Humidity and Moisture
Lithium Chloride Dewpoint Meter (2)
• Advantages:
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• Disadvantages:
• Simple sensor
• Flow rates between 0.05 and 1 m/s
• Relative cheap
• Response time in order of minutes
• Rugged
• Lower limit at bout 11% r.h.
Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
Humidity and Moisture
Resistive Humidity Sensors (1)
• Principle:
• Relative humidity is a function of the
impedance/resistance of a hygroscopic medium
• Implementation:
• Noble metal electrodes
• Substrate coated with conductive hygroscopic
medium
• Medium absorbs water => resistance decreases
• AC excitation voltage for resistance
measurement to prevent polarization (30Hz to
10kHz)
• Resistance => impedance
• Rectify to dc voltage
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
Humidity and Moisture
Resistive Humidity Sensors (2)
• Advantages:
• No calibration needed
• Small
• Fast responding
• Do not dissipate heat
• Life expectancy >>5 years
• Disadvantages:
• Significant temperature dependency
• Condensation problem (=> new
improving developments)
• Historical
First sensor of this type in 1940:
Dunmore type
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
Humidity and Moisture
Capacitive Humidity Sensors (1)
• Principle:
Relative humidity is proportional to dielectric
constant of polymer or metal oxide
=> change in capacitance about 0.2 to
0.5pF for 1%r.h.
• Implementation:
• Substrate (glass, ceramic or silicon)
• Between two electrodes a thin-film polymer
or metal oxide
• Coating with porous metal electrode =>
protection from contamination and
condensation
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
Humidity and Moisture
Capacitive Humidity Sensors (2)
• Advantages:
• Use of semiconductor processes
(signal conditioning circuit included)
• Small
• Low cost
• Widely used
• Disadvantages:
• Calibration needed (or laser trimmed)
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
Humidity and Moisture
Relative Humidity Accuracy vs. Dew Point Accuracy
Vaisila DryCap (+-2°C)
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
Humidity and Moisture
Thermal Conductivity Humidity Sensor
• Principle:
measure the absolute humidity by
quantifying the difference in thermal
conductivity of dry air and humid air
dry air has a greater capacity to sink
heat (e.g. desert)
• Implementation:
• Two matched NTC thermistors in a
bridge circuit
• One is hermetically encapsulated in
dry nitrogen
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
Humidity and Moisture
Coulometric Method
• Principle:
• A phosphorous pentoxide absorbs water
• The water is electrolyzed
• The resulting current is representing a defined amount of water
• 1mA = 0.0935μg H2O/s
• The sample stream of air must be very accurate
• Advantages:
• No calibration needed
• Especially suited for low humidity
• Disadvantages:
• 1 minute response times
• The sample stream of air must be very accurate
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
Humidity and Moisture
Crystal Oscillator
• Principle:
• Surface of a quartz crystal is coated with a hydroscopic material
• Resonant frequency of the quartz is a function of the mass of the quartz
• Alternately exposed to humid and dry air
• Advantages:
• Lowest humidity measurable
• Disadvantages:
• Expensive
• 1 minute response time
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
Humidity and Moisture
Infrared Method
• Principle:
• Water absorbs radiation in the infrared region
• Implementation:
• Gas is lead through a optical path between an infrared source and detector
• Another path through a reference gas
• Advantages:
• Wide range measurements
• Response time less than 1s
• Disadvantages:
• Pressure dependency
• Expensive
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
Humidity and Moisture
Comparison of Humidity Sensors
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
Humidity and Moisture
Measurement of moisture in Liquids and Solids
Gravimetric method
Karl Fischer method
Infrared techniques
Microwave absorbance
Nuclear magnetic resonance method
Neutron moderation
Time domain reflectory
Frequency domain technique
Thermal conductivity measurement
Water activity
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
Humidity and Moisture
Gravimetric Method
• Difference in weight before and after a drying process
• Assumption: loss of weight only based on water loss
• Problem with volatile components and crystal water
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
Humidity and Moisture
Karl Fischer Method
• Chemical method
• Karl Fischer reagent controlled added to liquid
• Electrodes measure the current through the liquid
• Sudden change in current indicated usage of all water
• Karl Fischer reagent is a mixture of
iodine
sulfur dioxide
pyridine
methanol
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
Humidity and Moisture
Infrared Techniques
• Reflectance of the surface indicate moisture
• Surface has to be representative
• Calibration for each material necessary
• Wave length bands of 1.45, 1.94 and 2.94μm
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
Humidity and Moisture
Microwave Absorbance
• Microwave absorbance is depending on the water incorporated’
• Water absorbs strongly in 1-2 GHz and 9 to 10GHz range
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
Humidity and Moisture
Nuclear Magnetic Resonance Method
• Hydrogen atoms in a permanent magnetic field have some defined orientation
• To change the orientation a defined amount of energy is needed
• At a right frequency the hydrogen atoms resonate
• The energy needed for the resonate state is proportional to the hydrogen/water
atoms/molecules
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
Humidity and Moisture
Other Methods
• Neutron Moderation
Neutron of high energy are slowed down by hydrogen atoms
• Time Domain Reflectory
Propagation velocity of electrical pulses
Water content in soils
• Frequency Domain Technique
Similar to TDR
• Thermal Conductivity Measurement
Thermal conductivity related to water content
Heat pulses and then cooling measured
• Water Activity
Material enclosed in measuring chamber developed after some time an equilibrium of
relative humidity.
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
Humidity and Moisture
Mechanical Hygrometer
• Principle
change of length of certain materials dependent of the humidity
• Use of human hair, textiles, or plastic fiber
• Accuracy up to 2% r.h. (in the range of 35% to 95%) commonly 5% r.h.
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
Questions ?
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf
Thank You !
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Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf