Making Physical Making Physical MeasurementsMeasurements

Terry A. RingTerry A. Ring

Department of Chemical Department of Chemical EngineeringEngineering

University of UtahUniversity of Utah

22 August 200522 August 2005

OVERVIEWOVERVIEW

• Course experiments – general Course experiments – general approach to making physical approach to making physical measurementsmeasurements

• Terminology Terminology

• CalibrationCalibration

• Types of InstrumentsTypes of Instruments

• Class ParticipationClass Participation

ExperimentsExperiments

• Preparation for an experimentPreparation for an experiment- organization – teamwork- organization – teamwork- time management- time management

• Equipment/apparatusEquipment/apparatus- keep detailed list of - keep detailed list of

equipment/chemicalsequipment/chemicals- know your equipment/capabilities- know your equipment/capabilities- most equipment expensive, do not - most equipment expensive, do not

abuse or neglectabuse or neglect

Important TermsImportant Terms• Error – the difference between the Error – the difference between the

“true value” and the observed “true value” and the observed (measured) value(measured) value

• Random error – fluctuations in the Random error – fluctuations in the measured value due to repeated measured value due to repeated measurementsmeasurements

• Systematic error – all measured Systematic error – all measured values are off by the same amount values are off by the same amount due to a) incorrect calibration b) due to a) incorrect calibration b) faulty equipment c) other causesfaulty equipment c) other causes

• Illegitimate error – erroneous Illegitimate error – erroneous method/technique, goofsmethod/technique, goofs

Random Error SourcesRandom Error Sources

• Judgement errors, estimate errors, Judgement errors, estimate errors, parallaxparallax

• Fluctuating ConditionsFluctuating Conditions• DigitizationDigitization• Disturbances such as mechanical Disturbances such as mechanical

vibrations or static electricty vibrations or static electricty caused by solar activitycaused by solar activity

• SamplingSampling

Systematic Error SourcesSystematic Error Sources

• Calibration of instrumentCalibration of instrument

• Environmental conditions different Environmental conditions different from calibrationfrom calibration

• Technique – not at equilibrium or at Technique – not at equilibrium or at steady state.steady state.

• SamplingSampling

22tmeasuremensamplingTotal

Important TermsImportant Terms

• Accuracy - a measure of how close the Accuracy - a measure of how close the result comes to the “true value” result comes to the “true value” (correctness). An indication of how well we (correctness). An indication of how well we control systemic errors.control systemic errors.

• Precision – a measure of how exactly the Precision – a measure of how exactly the result is determined (reproducibility) – no result is determined (reproducibility) – no relation to “true value”. An indication of relation to “true value”. An indication of how well we overcome or analyze random how well we overcome or analyze random errorserrors

• Limit of detection – smallest value which Limit of detection – smallest value which can be detected.can be detected.

Important TermsImportant Terms

• Discrepancy – the difference Discrepancy – the difference between values for the same between values for the same measurementmeasurement

• Uncertainty – an estimate of the Uncertainty – an estimate of the range in the error. Always range in the error. Always determined for a particular determined for a particular confidence level, i.e.confidence level, i.e. )%95( levelconfidenceyyy

ExperimentsExperiments• Literature workLiterature work

- understand theory/principles/concepts - understand theory/principles/concepts from textbooks and referencesfrom textbooks and references

• ReferencesReferences- textbook- textbook- Perry’s chemical engineers handbook- Perry’s chemical engineers handbook- CRC handbook of chemistry/physics- CRC handbook of chemistry/physics- Instrument Engineers’ Handbook- process - Instrument Engineers’ Handbook- process

measurement and analysis – B.G. measurement and analysis – B.G. Liptak, ed.Liptak, ed.- Web sites- Web sites

Potential ProblemsPotential Problems

• ParalaxParalax• Scale InterpretationScale Interpretation

• Appropriate ScaleAppropriate Scale• Appropriate InstrumentAppropriate Instrument

– Appropriate detection limitsAppropriate detection limits– Signal to Noise ratioSignal to Noise ratio– Appropriate Accuracy and PrecisionAppropriate Accuracy and Precision

• Significant FiguresSignificant Figures

WHAT PRECISION IS WHAT PRECISION IS REQUIRED?REQUIRED?

• Overall PrecisionOverall Precision

• Impact on CalculationImpact on Calculation

• Difficulty of MeasurementDifficulty of Measurement

CalibrationCalibration

• Should use primary standards if possibleShould use primary standards if possible• Calibrate as close to measuring conditions as Calibrate as close to measuring conditions as

possiblepossible• Sometimes performed at Sometimes performed at

– the factorythe factory– Professional laboratoriesProfessional laboratories

• Laboratory standards Laboratory standards ice bath, constant temp bathice bath, constant temp bath

• Tabulated properties and relationshipsTabulated properties and relationshipsboiling water at barometric pressureboiling water at barometric pressuretriple point of watertriple point of water

• Linear vs Non-linear Calibration curvesLinear vs Non-linear Calibration curves

Types of InstrumentsTypes of Instruments• Off-line Process InstrumentationOff-line Process Instrumentation

• Density MeasurementDensity Measurement• Weight and Misc. SensorsWeight and Misc. Sensors• Analytical InstrumentationAnalytical Instrumentation

– Issues Issues • SamplingSampling

– Grab sampleGrab sample– Statistical SamplingStatistical Sampling

• Sample PreparationSample Preparation– Splitting, extraction, decompositionSplitting, extraction, decomposition

• On–line InstrumentationOn–line Instrumentation– Flow MeasurementFlow Measurement– Level MeasurementLevel Measurement– Temperature MeasurementTemperature Measurement– Pressure MeasurementPressure Measurement– SafetySafety

QuantitativeQuantitative

Liquid Density Liquid Density MeasurementMeasurement

• Hydrometers (based upon buoyancy)Hydrometers (based upon buoyancy)

• Pycnometer (based on weight)Pycnometer (based on weight)

• Weighing a fixed volumeWeighing a fixed volume

• Oscillating Coriolis DensitometersOscillating Coriolis Densitometers

• Hydrostatic DensitometersHydrostatic Densitometers

• Radiation Densitometers – liquid/sludgeRadiation Densitometers – liquid/sludge

• Vibrating Densitometers – Vibrating Densitometers – Liq/sludge/gasLiq/sludge/gas

Analytical InstrumentationAnalytical Instrumentation

• ViscometersViscometers• SpectrophotometersSpectrophotometers

– IRIR– UV-VisibleUV-Visible

• ChromatographsChromatographs• Ion-selective ElectrodesIon-selective Electrodes• Mass SpectrometersMass Spectrometers• Inductively Coupled Plasma Inductively Coupled Plasma

SpectrometerSpectrometer• Many, many moreMany, many more

Types of AnalysisTypes of Analysis

• Content Analysis – What is in it?Content Analysis – What is in it?– QualitativeQualitative– Semi-QuantitativeSemi-Quantitative– QuantitativeQuantitative

• Distribution Analysis – Where is it?Distribution Analysis – Where is it?

• Process Analysis – When does it occur?Process Analysis – When does it occur?

• Structural Analysis –What is its Structural Analysis –What is its structure?structure?

Analytical StrategiesAnalytical Strategies

• SamplingSampling• Sample PreparationSample Preparation• Analytical PrincipleAnalytical Principle• Analytical ProcedureAnalytical Procedure

– Decomposition MethodsDecomposition Methods– Separation MethodsSeparation Methods– Enrichment MethodsEnrichment Methods

• Measurement MethodologyMeasurement Methodology• Measurement ResultsMeasurement Results

– AccuracyAccuracy– Precision Precision

CONCLUSIONS KNOW YOUR EQUIPMENTKNOW YOUR EQUIPMENT

Know its limitations and strengthsKnow its limitations and strengths

CHOOSE THE RIGHT PRECISIONCHOOSE THE RIGHT PRECISION

CALIBRATE AS MUCH AS POSSIBLECALIBRATE AS MUCH AS POSSIBLE

UNDERSTAND THE LIMITATIONS OF UNDERSTAND THE LIMITATIONS OF YOUR EQUIPMENTYOUR EQUIPMENT

LEARN THE TERMINOLOGYLEARN THE TERMINOLOGY

Audience Participation