1. Chemical Variables Measurements
Lecture for Licentiate Course in Measurement Science and Technology
Marion Hermersdorf
February
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2. Time Schedule 14:15h 45min Lecture Part 1 15:00h 15min Break
16:00h 15min Questions
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3. Table of Content Lecture Part 1 Lecture Part 2 Introduction
pH Measurements
Lecture Part 2
Humidity and Moisture Measurements
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4. 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
Small introduction to measurement principle (because I am first lecture in this course)
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5. Measurements in General
Introduction
The book defines …
A little bit fuzzy, because …
Book defines
I think this is not very clear
e.g.
force -> capacitance -> frequency
temperature -> resistance
length -> time
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6. Electronic Measurements
Introduction
Signal conversion and processing
Sensing element
Physical value
heat
resistance
NTC
temperature
Wheatstone Bridge
I define electronic measurements now this way
force
capacitance
Capacitive MEMS pressure element
Switched capacitor circuit
pressure
In the following are only the sensing elements discussed not the conversion and further signal processing.
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7. Definition of pH pH = pondus Hydrogenii, literally: hydrogen exponent
pH Measurement
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
- interpretation
- definition
- Historically just used to make it easier to talk about the hydrogen concentrations 10-7 moles/liter = pH 7
- pH has no unit
- [H+] is molar concentration of solvent protons in mol/liter
- aH+ is the ion activity
- gamma = activity coefficient
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8. pH Examples Water: Hydrochloric acid: Sodium hydroxide: pH Measurement
Some compounds form weak acids or bases;
Matter of dissociation grade
Balance is temperature dependent
Caustic = basicity
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9. Most widely performed measurement in chemical laboratories.
pH Measurements
pH Measurement
Most widely performed measurement in chemical laboratories.
Measurement principles:
Electrochemical Methods
Optical Methods
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10. 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
Chemical activity => electrical potential
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11. Glass Membrane Electrode
pH Measurement
Most widely used
Indicator and reference electrodes commonly combined into a single probe (combination electrode)
Glass indicator electrode
Reference electrode
First methods using the electrochemical principle
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12. Glass Indicator Electrode
pH Measurement
Glass membrane about 0.1 mm thick
Glass membrane acts as a transducer of the pH
Electrode = Silver wire
buffer solution = KCl
In glass :sodium or lithium, which act as electrical charge carriers
When the membrane glass is immersed in aqueous solution, a gel-like hydrated layer on the order of
100 nm thick forms at the surface of the glass.
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13. Reference Electrode
pH Measurement
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
Low resistive contact to sample (relative point)
Electrode
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14. Potential vs. pH ideally 59.16 mV per pH unit
pH Measurement
ideally mV per pH unit
Reference electrode introduces additional potential -> can be calibrated out
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15. Measurement Circuit
pH Measurement
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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16. Calibration pH Measurement
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17. Temperature Compensation
pH Measurement
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
The graph shows two calibration lines.
The original calibration was done with pH 4 and pH 10 buffers at temperature t1.
Suppose the temperature changes to t2 .
The pH analyzer automatically changes the slope by the ratio
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18. Selected Glass Membrane Electrodes
pH Measurement
Just to give an impression
Range in general 0-14
Accuracy within 0.1 to 0.03 pH
Cost around 100 to 300$
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19. Selected Glass Membrane Electrodes
pH Measurement
pHC2401 pH electrode
MI-506 Flexible pH Electrode
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20. pH-FET Measurement Principle
pH Measurement
Relatively recent development
Based on the use of an ion-selective field-effect transistor (ISFET)
pH-responsive membrane (instead of metal gate)
Advantages:
Inexpensive,
robust,
battery-powered,
pocket size
Especially used in food industry
pH responsive membrane show in picture (mistake) = gate electrode
FET show in picture
pH-responsive membrane material: silicon nitride, aluminum oxide, or tantalum oxide
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21. pH-FET Operation
pH Measurement
Voltage applied to reference electrode (relative to silicon substrate)
Charging of capacitor (electrode, solution, insulation layers and silicon substrate)
Drain source current influenced
A = geometrical factors
Vds = voltage applied to the drain
Vg = voltage applied to the reference electrode
VT = “threshold voltage” needed to produce mobile charge
C2 = capacitance of the gate region.
F = phase-boundarypotential at the interface between the sample and the pH-responsive insulating layer
constant drain current
also temperature compensation needed
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22. Selected ISFET Electrodes
pH Measurement
Overview
Accuracies and range similar to glass membrane method
Price higher
Device smaller and rugged
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23. Metal/Metal Oxide pH Sensors
pH Measurement
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
Metal oxide reduction
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24. Liquid Membrane Electrodes
pH Measurement
Ion-selective electrode
Membrane is selectively permeable to ions of interest
Shortly mention also …
Icon selective electrode (e.g. selective for potassium)
Done by membrane which is saturated with liquid ion exchanger (ion exchanger liquid: N-tridodecylamine)
For blood pH measurements
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25. 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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26. Indicator Dyes organic dye molecules are weak acids or bases
pH Measurement
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.
Everybody knows from the school chemistry labs
Change color depending on pH value
Also fluorescence
Table: …
Interesting: eye limitation
equilibrium constants Ka
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27. Indicator Papers simple, rapid, and inexpensive means of measuring pH
pH Measurement
simple, rapid, and inexpensive means of measuring pH
strip of paper or plastic that has been impregnated with one or more absorption indicator dyes
Based on the indicator dyes
Litmus paper
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28. Fiber-Optic pH Probes often referred to as optrodes
pH Measurement
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
More sophisticated
Still indicator dye (tip of light guide)
Challenge to fixate dye
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29. Absorption Optrodes Principle
pH Measurement
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
Own light source now
Two wave length (reference)
Scattered light => amplifying necessary
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30. Fluorescent Indicator Optrodes
pH Measurement
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
Fluorescent dye
Laser to excite the dye
Max dye concentration
PMT photomultiplier tube
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31. Break .. after the break: humidity and moisture measurements
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32. 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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33. 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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34. 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
vapor pressure kPa
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35. 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 moisture-containing 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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36. Fundamental Behavior of Water
Humidity and Moisture
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
Sensors are based on at least one of these characteristics
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37. Measurement Methods of Humidity and Moisture
Many different measurement methods.
Minimum range of operation
Over-specification can be expensive
Exposure of the sensor to the measurement environment
Danger of condensation
Accuracy needs
In general expected accuracies not better than 2% r.h. of 0.5°C
Response time
Calibration frequency
Select method by defining on following requirements
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38. Measurement of Humidity in Gases
Humidity and Moisture
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
Introduce “heavy” measurement methods
Then small scale and low cost
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39. Gravimetric Method
Humidity and Moisture
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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40. Precision Humidity Generator
Humidity and Moisture
Three practical methods:
Two flow method
One dry stream of air, one test stream (known temp.) -> humidity = rates of flows
Two temperature method
Two pressure method
Advantage:
Accuracy close to gravimetric method
Disadvantage:
Stationary device
Expensive
Big device
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41. Condensation Dewpoint Hygrometer
Humidity and Moisture
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
LAB-EL DP-373
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42. Psychrometer Principle: Two thermometers ventilated by the humid air
Humidity and Moisture
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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43. Lithium Chloride Dewpoint Meter (1)
Humidity and Moisture
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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44. Lithium Chloride Dewpoint Meter (2)
Humidity and Moisture
Advantages:
Simple sensor
Relative cheap
Rugged
Disadvantages:
Flow rates between 0.05 and 1 m/s
Response time in order of minutes
Lower limit at bout 11% r.h.
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45. Resistive Humidity Sensors (1)
Humidity and Moisture
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
resistive - impedeance
hygroscopic medium: conductive polymer, salt, treated substance
=> polymere relative humidity sensors
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46. Resistive Humidity Sensors (2)
Humidity and Moisture
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
Dunmore sensor on the very right
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47. Capacitive Humidity Sensors (1)
Humidity and Moisture
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
Also based on polymer => sometime confusion with resistive method
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48. Capacitive Humidity Sensors (2)
Humidity and Moisture
Advantages:
Use of semiconductor processes (signal conditioning circuit included)
Small
Low cost
Widely used
Disadvantages:
Calibration needed (or laser trimmed)
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49. Relative Humidity Accuracy vs. Dew Point Accuracy
Humidity and Moisture
Just to give and idea about 2% r.h. accuarcy
Vaisila DryCap (+-2°C)
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50. Thermal Conductivity Humidity Sensor
Humidity and Moisture
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
because heat is retained by water vapor in the atmosphere
Desert air cools down fast during night
Humid air at the sea stays warm over night
absolute humidity
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51. Coulometric Method Principle: Advantages: Disadvantages:
Humidity and Moisture
Principle:
A phosphorous pentoxide absorbs water
The water is electrolyzed
The resulting current is representing a defined amount of water
1mA = μg 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
Shortly some more less widely used methods:
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52. Crystal Oscillator Principle: Advantages: Disadvantages:
Humidity and Moisture
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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53. Infrared Method Principle: Implementation: Advantages: Disadvantages:
Humidity and Moisture
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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54. Comparison of Humidity Sensors
Humidity and Moisture
Primary Standards. These systems rely on fundamental principles and base units of measurement.
Transfer/fundamental. Instruments in this category operate on fundamental principles and can provide good, stable, and repeatable results, but if they are not properly used, the instruments can give incorrect results.
Secondary devices. These devices are nonfundamental and must be calibrated against a transfer standard or other fundamental system.
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55. Measurement of moisture in Liquids and Solids
Humidity and Moisture
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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56. Gravimetric Method
Humidity and Moisture
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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57. Karl Fischer Method Chemical method
Humidity and Moisture
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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58. Infrared Techniques Reflectance of the surface indicate moisture
Humidity and Moisture
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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59. Microwave Absorbance
Humidity and Moisture
Microwave absorbance is depending on the water incorporated’
Water absorbs strongly in 1-2 GHz and 9 to 10GHz range
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60. Nuclear Magnetic Resonance Method
Humidity and Moisture
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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61. Other Methods Neutron Moderation Time Domain Reflectory
Humidity and Moisture
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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62. Mechanical Hygrometer
Humidity and Moisture
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.
Last but not least
Still in use in some museum
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63. Questions ?
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64. Thank You !
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