190 likes | 321 Views
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.
E N D
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