Making Physical Measurements Terry A. Ring Department of Chemical Engineering University of Utah 22 August 2005

OVERVIEW Course experiments – general approach to making physical measurements Course experiments – general approach to making physical measurements Terminology Terminology Calibration Calibration Types of Instruments Types of Instruments Class Participation Class Participation

Experiments Preparation for an experiment - organization – teamwork - time management Preparation for an experiment - organization – teamwork - time management Equipment/apparatus - keep detailed list of equipment/chemicals - know your equipment/capabilities - most equipment expensive, do not abuse or neglect Equipment/apparatus - keep detailed list of equipment/chemicals - know your equipment/capabilities - most equipment expensive, do not abuse or neglect

Important Terms Error – the difference between the “true value” and the observed (measured) value Error – the difference between the “true value” and the observed (measured) value Random error – fluctuations in the measured value due to repeated measurements Random error – fluctuations in the measured value due to repeated measurements Systematic error – all measured values are off by the same amount due to a) incorrect calibration b) faulty equipment c) other causes Systematic error – all measured values are off by the same amount due to a) incorrect calibration b) faulty equipment c) other causes Illegitimate error – erroneous method/technique, goofs Illegitimate error – erroneous method/technique, goofs

Random Error Sources Judgement errors, estimate errors, parallax Judgement errors, estimate errors, parallax Fluctuating Conditions Fluctuating Conditions Digitization Digitization Disturbances such as mechanical vibrations or static electricty caused by solar activity Disturbances such as mechanical vibrations or static electricty caused by solar activity Sampling Sampling

Systematic Error Sources Calibration of instrument Calibration of instrument Environmental conditions different from calibration Environmental conditions different from calibration Technique – not at equilibrium or at steady state. Technique – not at equilibrium or at steady state. Sampling Sampling

Important Terms Accuracy - a measure of how close the result comes to the “true value” (correctness). An indication of how well we control systemic errors. Accuracy - a measure of how close the result comes to the “true value” (correctness). An indication of how well we control systemic errors. Precision – a measure of how exactly the result is determined (reproducibility) – no relation to “true value”. An indication of how well we overcome or analyze random errors Precision – a measure of how exactly the result is determined (reproducibility) – no relation to “true value”. An indication of how well we overcome or analyze random errors Limit of detection – smallest value which can be detected. Limit of detection – smallest value which can be detected.

Important Terms Discrepancy – the difference between values for the same measurement Discrepancy – the difference between values for the same measurement Uncertainty – an estimate of the range in the error. Always determined for a particular confidence level, i.e. Uncertainty – an estimate of the range in the error. Always determined for a particular confidence level, i.e.

Experiments Literature work - understand theory/principles/concepts from textbooks and references Literature work - understand theory/principles/concepts from textbooks and references References - textbook - Perry’s chemical engineers handbook - CRC handbook of chemistry/physics - Instrument Engineers’ Handbook- process measurement and analysis – B.G. Liptak, ed. - Web sites References - textbook - Perry’s chemical engineers handbook - CRC handbook of chemistry/physics - Instrument Engineers’ Handbook- process measurement and analysis – B.G. Liptak, ed. - Web sites

Potential Problems Paralax Paralax Scale Interpretation Scale Interpretation Appropriate Scale Appropriate Scale Appropriate Instrument Appropriate Instrument –Appropriate detection limits –Signal to Noise ratio –Appropriate Accuracy and Precision Significant Figures Significant Figures

WHAT PRECISION IS REQUIRED? Overall Precision Overall Precision Impact on Calculation Impact on Calculation Difficulty of Measurement Difficulty of Measurement

Calibration Should use primary standards if possible Should use primary standards if possible Calibrate as close to measuring conditions as possible Calibrate as close to measuring conditions as possible Sometimes performed at Sometimes performed at –the factory –Professional laboratories Laboratory standards ice bath, constant temp bath Laboratory standards ice bath, constant temp bath Tabulated properties and relationships boiling water at barometric pressure triple point of water Tabulated properties and relationships boiling water at barometric pressure triple point of water Linear vs Non-linear Calibration curves Linear vs Non-linear Calibration curves

Types of Instruments Off-line Off-line Process Instrumentation Density Density Measurement Weight Weight and Misc. Sensors Analytical Analytical Instrumentation –Issues Sampling Sampling –Grab –Grab sample –Statistical –Statistical Sampling Sample Sample Preparation –Splitting, –Splitting, extraction, decomposition On–line On–line Instrumentation –Flow –Flow Measurement –Level –Level Measurement –Temperature –Temperature Measurement –Pressure –Pressure Measurement –Safety Quantitative

Liquid Density Measurement Hydrometers (based upon buoyancy) Hydrometers (based upon buoyancy) Pycnometer (based on weight) Pycnometer (based on weight) Weighing a fixed volume Weighing a fixed volume Oscillating Coriolis Densitometers Oscillating Coriolis Densitometers Hydrostatic Densitometers Hydrostatic Densitometers Radiation Densitometers – liquid/sludge Radiation Densitometers – liquid/sludge Vibrating Densitometers – Liq/sludge/gas Vibrating Densitometers – Liq/sludge/gas

Analytical Instrumentation Viscometers Viscometers Spectrophotometers Spectrophotometers –IR –UV-Visible Chromatographs Chromatographs Ion-selective Electrodes Ion-selective Electrodes Mass Spectrometers Mass Spectrometers Inductively Coupled Plasma Spectrometer Inductively Coupled Plasma Spectrometer Many, many more Many, many more

Types of Analysis Content Analysis – What is in it? Content Analysis – What is in it? –Qualitative –Semi-Quantitative –Quantitative 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 structure? Structural Analysis –What is its structure?

Analytical Strategies Sampling Sampling Sample Preparation Sample Preparation Analytical Principle Analytical Principle Analytical Procedure Analytical Procedure –Decomposition Methods –Separation Methods –Enrichment Methods Measurement Methodology Measurement Methodology Measurement Results Measurement Results –Accuracy –Precision

CONCLUSIONS  KNOW YOUR EQUIPMENT  Know its limitations and strengths  CHOOSE THE RIGHT PRECISION  CALIBRATE AS MUCH AS POSSIBLE  UNDERSTAND THE LIMITATIONS OF YOUR EQUIPMENT  LEARN THE TERMINOLOGY

Audience Participation