Chapter 1: Principles of Measurement

1
Chapter 1 Principles of Measurement

• Agenda
• Engineering measurements
• Measurement Units
• Objectives of Measurement
• Process Measurement
• Instrument Classification
• Measurement Errors

2
Measurement Units

• Units of standards for physical quantities
• SI units (Système International dUnités)
  (metric units).
• Others British units
• Practice Mix

3
Objectives of Measurement

• Control Systems control the speed of a car you
  must first measure the speed, compare it
• to the objective, and then correct/adjust.

4
Instruments Classification

• Active/passive instruments
• Passive instruments the output is produced
  entirely by the quantity being measured. There
  are to the system
• Active instruments the quantity being measured
  simply modulates the magnitude of some . The
  comes from the
• Active instruments level of measurement ,
  gtlt passive instruments level of
  measurement ,

5
Instruments Classification

• Null/deflection-type instruments
• Null-type instruments the value of quantity
  being measured is made in terms of the
  needed to reach the position
• Deflection-type instruments the value of
  quantity being measured is made in terms of the
  of a pointer against a scale
• Null-type instrument high accurate, difficult to
  usegtltdeflection-type

6
Instruments Classification

• Monitoring/control instruments
• Monitoring instruments give only an or of
  the magnitude of the physical quantity measured.
  (null-type instrument, passive transducers)
• Control instruments must be in a suitable form
  for into the control system.

7
Instruments Classification

• Analog/digital instruments
• Analog instruments give an out put which varies
  as the quantity being, measured changes.
• Digital instruments give an output which varies
  in steps

8
Measurement Errors

• Systematic errors produce errors which are
  consistently on the .
• Causes
• System disturbance due to measurement,
  changes (modifying inputs)
• in instrument characteristics
• Overcome
• instrument at suitable intervals
• the effect of errors and correcting the
  measurements

9
Measurement Errors

• Random errors consist generally of small
  perturbations of the measurement
• Overcome a few measurements of the same
  quantity (not eliminate all ? describe
  measurements subject to RE in probabilistic terms

10
Example

• Suppose that the temperature of the class room is
  controlled at 30OC. What is the average error
  when repeated measurements gave the following
  results
• 30.5 31.0 31.0 31.5 31
• Suppose the constant level of water in a tank is
  measured a number of times and the following
  results obtained
• 1.010 1.015 1.010 1.010metres

11
Solution

• Average error
• (30.5-30) 3(31-30)(31.5-30)/5 1oC
• The average level (1.011.0151.011.0051.010)/5
  1.01m
• The results all lie within 0.005m of this
  average. Hence the level could be quoted as
  1.010.005m