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
• Terminology
• Calibration
• Types of Instruments
• Class Participation
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
2
2
 Total   sampling
  measuremen
t
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.
y  y  y (95% confidence level )
Experiments
• 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
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 Quantitative
– Grab sample
– Statistical Sampling
• Sample Preparation
– Splitting, extraction, decomposition
• On–line Instrumentation
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–
–
–
–
Flow Measurement
Level Measurement
Temperature Measurement
Pressure Measurement
Safety
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
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•
•
•
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
Audience
Participation