Making Physical Measurements

1
Making Physical Measurements

• Terry A. Ring
• Department of Chemical Engineering
• University of Utah
• 22 August 2005

2
OVERVIEW

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

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 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

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
Digitization Error

• Analogue Signal
• mV, mA, Volts, Amp, Watts, other
• Amplification to Industrial Std Scales
• 4 to 20 mA
• 5 to -5 V
• Digitization
• Step size Scale/(2N)
• N bits in a byte of A-to-D board
• Calculation Precision 8, 10, 12 16 digits or 32
  digits of binary words are common

8
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.

9
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.

10
Experiments

• Literature work- understand theory/principles/con
  cepts from textbooks and references
• References- textbook- Perrys chemical
  engineers handbook- CRC handbook of
  chemistry/physics- Instrument Engineers
  Handbook- process measurement and analysis
  B.G. Liptak, ed.- Web sites

11
Potential Problems

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

12
WHAT PRECISION IS REQUIRED?

• Overall Precision
• Impact on Calculation
• Difficulty of Measurement

13
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

14
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
• Online Instrumentation
• Flow Measurement
• Level Measurement
• Temperature Measurement
• Pressure Measurement
• Safety

Quantitative
15
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

16
Analytical Instrumentation

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

17
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?

18
Analytical Strategies

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

19
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

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Audience Participation