Chapter 1: Principles of Measurement

1
Chapter 1 Principles of Measurement

• Agenda
• Engineering measurements
• Measurement Units
• Applications of Measurement
• Instruments Classification
• Measurement Errors

2
Measurement Units

• Standard units for the measurement of physical
  quantities
• SI units (Système International dUnités)
  (metric units).
• Others British units
• Practice Mix

3
Applications of Measurement

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

4
Components of Measurement System
Physical variable
Signal processing
Converter
Sensor
Signal transmission
Signal utilization
5
Instruments Classification

• Active/passive instruments
• Passive instruments the output is produced
  entirely by the quantity being measured. There
  are not other energy inputs to the system
• Active instruments the quantity being measured
  simply modulates the magnitude of some external
  power source. The energy in the output signal
  comes from the external power source
• Active instruments high level of measurement
  resolution, expensive gtlt passive instruments
  low level of measurement resolution, cheap

6
Instruments Classification

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

7
Instruments Classification

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

8
Instruments Classification

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

9
Measurement Errors

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

10
Measurement Errors

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

11
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

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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