1. Making Physical Measurements
Terry A. Ring
Department of Chemical Engineering
University of Utah

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

3. 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

4. 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 or Bias 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, goof ups

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

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

7. 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.
Minimum and Maximum Detectable Limit

8. 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.

9. Potential Problems Paralax Scale Interpretation Appropriate Scale
Appropriate Instrument
Appropriate detection limits
High Signal to Noise Ratio
Appropriate Accuracy and Precision
Significant Figures

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

11. 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

12. 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 – Placement
Flow Measurement
Level Measurement
Temperature Measurement
Pressure Measurement
Safety
Quantitative

13. 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

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

15. 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?

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

17. 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

18. 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