Making Physical Measurements Terry A. Ring Department of Chemical Engineering University of Utah 25 August 2008

OVERVIEW Course experiments – general approach to making physical measurements Terminology Calibration Types of Instruments Preliminary Lab Conferences

Experiments 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

Important Terms Error – the difference between the “true value” and the observed (measured) value 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 Illegitimate error – erroneous method/technique, goofs

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

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

Important Terms 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.

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

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

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

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

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

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

Types of Analysis Content Analysis – What is in it? Qualitative Semi-Quantitative Quantitative Distribution Analysis – Where is it? Process Analysis – When does it occur? Structural Analysis –What is its structure?

Analytical Strategies Sampling Sample Preparation Analytical Principle Analytical Procedure Decomposition Methods Separation Methods Enrichment Methods Measurement Methodology 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

Preliminary Lab Conference 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 Chemical and Equipment Safety