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1 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf Chemical Variables Measurements Lecture for Licentiate Course in Measurement Science and Technology.

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Presentation on theme: "1 Metrology Course –Lecture 1.ppt / 15.02.2006 / Hermersdorf Chemical Variables Measurements Lecture for Licentiate Course in Measurement Science and Technology."— Presentation transcript:

1 1 Metrology Course –Lecture 1.ppt / / Hermersdorf Chemical Variables Measurements Lecture for Licentiate Course in Measurement Science and Technology Marion Hermersdorf February

2 2 Metrology Course –Lecture 1.ppt / / Hermersdorf Time Schedule 14:15h45minLecture Part 1 15:00h15minBreak 15:15h45minLecture Part 2 16:00h15minQuestions

3 3 Metrology Course –Lecture 1.ppt / / Hermersdorf Table of Content Lecture Part 1 Introduction pH Measurements Lecture Part 2 Humidity and Moisture Measurements Content

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

5 5 Metrology Course –Lecture 1.ppt / / Hermersdorf Measurements in General The book defines … A little bit fuzzy, because … e.g. force -> capacitance -> frequency temperature -> resistance length -> time Introduction

6 6 Metrology Course –Lecture 1.ppt / / Hermersdorf Electronic Measurements In the following are only the sensing elements discussed not the conversion and further signal processing. Physical value Sensing element Signal conversion and processing temperature NTC resistance Wheatstone Bridge heat pressure Capacitive MEMS pressure element capacitance Switched capacitor circuit force Introduction

7 7 Metrology Course –Lecture 1.ppt / / Hermersdorf 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 pH Measurement

8 8 Metrology Course –Lecture 1.ppt / / Hermersdorf pH Examples Water: Hydrochloric acid: Sodium hydroxide: pH Measurement

9 9 Metrology Course –Lecture 1.ppt / / Hermersdorf pH Measurements Most widely performed measurement in chemical laboratories. Measurement principles: 1.Electrochemical Methods 2.Optical Methods pH Measurement

10 10 Metrology Course –Lecture 1.ppt / / Hermersdorf 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 pH Measurement

11 11 Metrology Course –Lecture 1.ppt / / Hermersdorf Glass Membrane Electrode Most widely used Indicator and reference electrodes commonly combined into a single probe (combination electrode) Glass indicator electrode Referenc e electrode pH Measurement

12 12 Metrology Course –Lecture 1.ppt / / Hermersdorf Glass Indicator Electrode Glass membrane about 0.1 mm thick Glass membrane acts as a transducer of the pH pH Measurement

13 13 Metrology Course –Lecture 1.ppt / / Hermersdorf 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 pH Measurement

14 14 Metrology Course –Lecture 1.ppt / / Hermersdorf Potential vs. pH ideally mV per pH unit Reference electrode introduces additional potential -> can be calibrated out pH Measurement

15 15 Metrology Course –Lecture 1.ppt / / Hermersdorf 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) pH Measurement

16 16 Metrology Course –Lecture 1.ppt / / Hermersdorf Calibration pH Measurement

17 17 Metrology Course –Lecture 1.ppt / / Hermersdorf 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 pH Measurement

18 18 Metrology Course –Lecture 1.ppt / / Hermersdorf Selected Glass Membrane Electrodes pH Measurement

19 19 Metrology Course –Lecture 1.ppt / / Hermersdorf Selected Glass Membrane Electrodes MI-506 Flexible pH Electrode pHC2401 pH electrode pH Measurement

20 20 Metrology Course –Lecture 1.ppt / / Hermersdorf pH-FET Measurement Principle 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 Measurement

21 21 Metrology Course –Lecture 1.ppt / / Hermersdorf 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 pH Measurement

22 22 Metrology Course –Lecture 1.ppt / / Hermersdorf Selected ISFET Electrodes pH Measurement

23 23 Metrology Course –Lecture 1.ppt / / Hermersdorf 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 pH Measurement

24 24 Metrology Course –Lecture 1.ppt / / Hermersdorf Liquid Membrane Electrodes Ion-selective electrode Membrane is selectively permeable to ions of interest pH Measurement

25 25 Metrology Course –Lecture 1.ppt / / Hermersdorf Optical Methods of pH Measurement Use of organic dye molecules with pH-dependent spectral properties pH Measurement Methods: Indicator dyes Indicator paper Fiber-optic pH probes

26 26 Metrology Course –Lecture 1.ppt / / Hermersdorf 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. pH Measurement

27 27 Metrology Course –Lecture 1.ppt / / Hermersdorf 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 pH Measurement

28 28 Metrology Course –Lecture 1.ppt / / Hermersdorf 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 pH Measurement

29 29 Metrology Course –Lecture 1.ppt / / Hermersdorf 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 pH Measurement

30 30 Metrology Course –Lecture 1.ppt / / Hermersdorf 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 pH Measurement

31 31 Metrology Course –Lecture 1.ppt / / Hermersdorf Break.. after the break: humidity and moisture measurements

32 32 Metrology Course –Lecture 1.ppt / / Hermersdorf Table of Content – Lecture Part 2 Humidity and Moisture Measurements Introduction Humidity measurements in gases Moisture measurements in liquids and solids Content

33 33 Metrology Course –Lecture 1.ppt / / Hermersdorf Introduction to 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 Humidity and Moisture

34 34 Metrology Course –Lecture 1.ppt / / Hermersdorf 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 Humidity and Moisture

35 35 Metrology Course –Lecture 1.ppt / / Hermersdorf 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 Humidity and Moisture

36 36 Metrology Course –Lecture 1.ppt / / Hermersdorf 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 Humidity and Moisture

37 37 Metrology Course –Lecture 1.ppt / / Hermersdorf 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 Humidity and Moisture

38 38 Metrology Course –Lecture 1.ppt / / Hermersdorf 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 Humidity and Moisture

39 39 Metrology Course –Lecture 1.ppt / / Hermersdorf 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 Humidity and Moisture

40 40 Metrology Course –Lecture 1.ppt / / Hermersdorf 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: Accuracy close to gravimetric method Disadvantage: Stationary device Expensive Big device Humidity and Moisture

41 41 Metrology Course –Lecture 1.ppt / / Hermersdorf 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 LAB-EL DP-373 Humidity and Moisture

42 42 Metrology Course –Lecture 1.ppt / / Hermersdorf 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 t w e: vapor pressure e w : saturated vapor pressure A: psychrometer coefficient P: total atmospheric pressure Humidity and Moisture

43 43 Metrology Course –Lecture 1.ppt / / Hermersdorf 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) Humidity and Moisture

44 44 Metrology Course –Lecture 1.ppt / / Hermersdorf Lithium Chloride Dewpoint Meter (2) 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 Humidity and Moisture

45 45 Metrology Course –Lecture 1.ppt / / Hermersdorf 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 Humidity and Moisture

46 46 Metrology Course –Lecture 1.ppt / / Hermersdorf 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 Humidity and Moisture

47 47 Metrology Course –Lecture 1.ppt / / Hermersdorf 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 Humidity and Moisture

48 48 Metrology Course –Lecture 1.ppt / / Hermersdorf Capacitive Humidity Sensors (2) Advantages: Use of semiconductor processes (signal conditioning circuit included) Small Low cost Widely used Disadvantages: Calibration needed (or laser trimmed) Humidity and Moisture

49 49 Metrology Course –Lecture 1.ppt / / Hermersdorf Relative Humidity Accuracy vs. Dew Point Accuracy Vaisila DryCap (+-2°C) Humidity and Moisture

50 50 Metrology Course –Lecture 1.ppt / / Hermersdorf 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 Humidity and Moisture

51 51 Metrology Course –Lecture 1.ppt / / Hermersdorf Coulometric Method Principle: A phosphorous pentoxide absorbs water The water is electrolyzed The resulting current is representing a defined amount of water 1mA = μg H 2 O/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 Humidity and Moisture

52 52 Metrology Course –Lecture 1.ppt / / Hermersdorf 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 Humidity and Moisture

53 53 Metrology Course –Lecture 1.ppt / / Hermersdorf 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 Humidity and Moisture

54 54 Metrology Course –Lecture 1.ppt / / Hermersdorf Comparison of Humidity Sensors Humidity and Moisture

55 55 Metrology Course –Lecture 1.ppt / / Hermersdorf 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 Humidity and Moisture

56 56 Metrology Course –Lecture 1.ppt / / Hermersdorf 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 Humidity and Moisture

57 57 Metrology Course –Lecture 1.ppt / / Hermersdorf 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 Humidity and Moisture

58 58 Metrology Course –Lecture 1.ppt / / Hermersdorf 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 Humidity and Moisture

59 59 Metrology Course –Lecture 1.ppt / / Hermersdorf Microwave Absorbance Microwave absorbance is depending on the water incorporated’ Water absorbs strongly in 1-2 GHz and 9 to 10GHz range Humidity and Moisture

60 60 Metrology Course –Lecture 1.ppt / / Hermersdorf 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 Humidity and Moisture

61 61 Metrology Course –Lecture 1.ppt / / Hermersdorf 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. Humidity and Moisture

62 62 Metrology Course –Lecture 1.ppt / / Hermersdorf 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. Humidity and Moisture

63 63 Metrology Course –Lecture 1.ppt / / Hermersdorf Questions ?

64 64 Metrology Course –Lecture 1.ppt / / Hermersdorf Thank You !


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