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Chemical Variables Measurements. Lecture for Licentiate Course in Measurement Science and Technology. Marion Hermersdorf February 15. 2006. Time Schedule. 14:15h 45min Lecture Part 1 15:00h 15min Break 15:15h 45min Lecture Part 2 16:00h 15min Questions. Table of Content. Content.

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chemical variables measurements
Chemical Variables Measurements

Lecture for Licentiate Course in Measurement Science and Technology

Marion Hermersdorf

February 15. 2006

1 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

time schedule
Time Schedule

14:15h 45min Lecture Part 1

15:00h 15min Break

15:15h 45min Lecture Part 2

16:00h 15min Questions

2 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

table of content
Table of Content

Content

Lecture Part 1

Introduction

pH Measurements

Lecture Part 2

Humidity and Moisture Measurements

3 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

table of content lecture part 1
Table of Content – Lecture Part 1

Content

  • 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

4 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

measurements in general
Measurements in General

Introduction

The book defines …

A little bit fuzzy, because …

  • e.g.
  • force -> capacitance -> frequency
  • temperature -> resistance
  • length -> time

5 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

electronic measurements
Electronic Measurements

Introduction

In the following are only the sensing elements discussed not the conversion and further signal processing.

Signalconversion and processing

Sensingelement

Physical value

resistance

heat

NTC

Wheatstone Bridge

temperature

capacitance

force

Capacitive MEMSpressure element

Switched capacitor circuit

pressure

6 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

definition of ph
Definition of pH

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

7 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

ph examples
pH Examples

pH Measurement

  • Water:
  • Hydrochloric acid:
  • Sodium hydroxide:

8 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

ph measurements
pH Measurements

pH Measurement

Most widely performed measurement in chemical laboratories.

Measurement principles:

  • Electrochemical Methods
  • Optical Methods

9 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

electrochemical methods of ph measurement
Electrochemical Methods of pH Measurement

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

10 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

glass membrane electrode
Glass Membrane Electrode

pH Measurement

  • Most widely used
  • Indicator and reference electrodes commonly combined into a single probe(combination electrode)

Referenceelectrode

Glass indicatorelectrode

11 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

glass indicator electrode
Glass Indicator Electrode

pH Measurement

  • Glass membrane about 0.1 mm thick
  • Glass membrane acts as a transducer of the pH

12 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

reference electrode
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

13 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

potential vs ph
Potential vs. pH

pH Measurement

  • ideally 59.16 mV per pH unit
  • Reference electrode introduces additional potential -> can be calibrated out

14 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

measurement circuit
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)

15 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

calibration
Calibration

pH Measurement

16 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

temperature compensation
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

17 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

selected glass membrane electrodes
Selected Glass Membrane Electrodes

pH Measurement

18 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

selected glass membrane electrodes19
Selected Glass Membrane Electrodes

pH Measurement

pHC2401 pH electrode

MI-506 Flexible pH Electrode

19 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

ph fet measurement principle
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

20 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

ph fet operation
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

21 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

selected isfet electrodes
Selected ISFET Electrodes

pH Measurement

22 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

metal metal oxide ph sensors
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

23 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

liquid membrane electrodes
Liquid Membrane Electrodes

pH Measurement

  • Ion-selective electrode
  • Membrane is selectively permeable to ions of interest

24 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

optical methods of ph measurement
Optical Methods of pH Measurement

pH Measurement

Use of organic dye molecules with pH-dependent spectral properties

Methods:

Indicator dyes

Indicator paper

Fiber-optic pH probes

25 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

indicator dyes
Indicator Dyes

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.

26 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

indicator papers
Indicator Papers

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

Litmus paper

27 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

fiber optic ph probes
Fiber-Optic pH Probes

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

28 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

absorption optrodes principle
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

29 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

fluorescent indicator optrodes
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

30 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

break
Break

.. after the break:

humidity and moisture measurements

31 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

table of content lecture part 2
Table of Content – Lecture Part 2

Content

Humidity and Moisture Measurements

  • Introduction
  • Humidity measurements in gases
  • Moisture measurements in liquids and solids

32 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

introduction to humidity and moisture
Introduction to Humidity and Moisture

Humidity and Moisture

Humidity and moisture have great economic importance

  • Storage of food and raw material
  • Optimum manufacturing conditions

Water and water vapor can be found everywhere

Humidity = water vapor in the air or any other gas

Moisture = water in liquids and solids

33 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

expressions for humidity and moisture
Expressions for Humidity and Moisture

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

34 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

characteristics of humidity and moisture
Characteristics of Humidity and Moisture

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

35 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

fundamental behavior of water
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

36 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

measurement methods of humidity and moisture
Measurement Methods of Humidity and Moisture

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

37 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

measurement of humidity in gases
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

38 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

gravimetric method
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

39 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

precision humidity generator
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

40 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

condensation dewpoint hygrometer
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

41 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

psychrometer
Psychrometer

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 pressureew: saturated vapor pressureA: psychrometer coefficientP: total atmospheric pressure

42 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

lithium chloride dewpoint meter 1
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)

43 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

lithium chloride dewpoint meter 2
Lithium Chloride Dewpoint Meter (2)

Humidity and Moisture

  • Disadvantages:
    • Flow rates between 0.05 and 1 m/s
    • Response time in order of minutes
    • Lower limit at bout 11% r.h.
  • Advantages:
    • Simple sensor
    • Relative cheap
    • Rugged

44 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

resistive humidity sensors 1
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

45 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

resistive humidity sensors 2
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

46 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

capacitive humidity sensors 1
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

47 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

capacitive humidity sensors 2
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)

48 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

relative humidity accuracy vs dew point accuracy
Relative Humidity Accuracy vs. Dew Point Accuracy

Humidity and Moisture

Vaisila DryCap (+-2°C)

49 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

thermal conductivity humidity sensor
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

50 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

coulometric method
Coulometric Method

Humidity and Moisture

  • 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

51 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

crystal oscillator
Crystal Oscillator

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

52 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

infrared method
Infrared Method

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

53 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

comparison of humidity sensors
Comparison of Humidity Sensors

Humidity and Moisture

54 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

measurement of moisture in liquids and solids
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

55 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

gravimetric method56
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

56 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

karl fischer method
Karl Fischer 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

57 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

infrared techniques
Infrared Techniques

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

58 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

microwave absorbance
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

59 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

nuclear magnetic resonance method
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

60 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

other methods
Other Methods

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.

61 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

mechanical hygrometer
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.

62 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

questions
Questions ?

63 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf

thank you
Thank You !

64 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf