Instructor: Dr Alina Mirza
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Commonly Encountered Electrical Units
Functional Elements of an Instrument
Instrument Classification
Performance Characteristics
Static Characteristics
Dynamic Characteristics
Methods of Measurement
Types of Errors
Loading Effects
Dr Alina Mirza - Dept of EE MCS, NUST
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Instrument Classification
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Active / Passive Instruments
Null / Deflection type Instruments
Monitoring / Control Instruments
Analog / Digital Instruments
Absolute / Secondary Instruments
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The instrument in which output is produced
entirely by the quantity being measured, are
called passive instruments
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The instrument in which quantity to be measured
just activates the magnitude of some external
input source which in turn produced the
measurement, are called active instruments.
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The instruments in which a zero or null
indication leads to the determination of the
magnitude of the measurand, are called null
type instruments
Pressure gauge with weights
DC Potentiometer
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The instruments in which measurand
produces some effect due to which pointer
deflects, are called
deflection type
instruments.
Dr Alina Mirza - Dept of EE MCS, NUST
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The instruments which are used to monitor the
process, indicating the value of measurand, are
called monitoring instruments.
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Audio/ video indication of measurand.
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Examples : Voltmeter, Ammeter, thermometers.
Dr Alina Mirza - Dept of EE MCS, NUST
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The instruments which are used in automatic
control systems are called control
instruments.
Such instruments have an electrical output
and are used in the feedback path to measure
the output and feedback the information to
the controller.
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The instrument which gives the output which
varies in continuous fashion as the
measurand changes, is called analog
instrument e.g. voltmeter, ammeter etc.
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The instrument which gives the output which
varies in discrete steps and take only finite
values in a given range, is called digital
instrument e.g. computers, microcontrollers
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The instrument which gives the magnitude of
the measurand in terms of a physical
constant of the instrument is called an
absolute
instrument
e.g.
tangent
Galvanometer
The instrument in which reading shown by
the
instrument
gives
directly
the
measurement of measurand is called
secondary instrument e.g. ammeter,
voltmeter, thermometer etc.
Dr Alina Mirza - Dept of EE MCS, NUST
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Selecting a proper instruments for a
particular measurand needs knowledge of the
performance characteristics of an instrument.
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Two categories
▪ Static Characteristics
▪ Dynamic Characteristics
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The various performance characteristics are
obtained in one form or the other by a
process called calibration.
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Calibration is a process of making an
adjustment or marking a scale , so that the
readings of an instrument agree with the
accepted or certified standard.
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The calibration process involves steps like:
1. Visual inspection for various defects
2. Installation according to the specifications
3. Zero adjustment
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Calibration record – error measurement
Calibration Curve – graphical
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Set of criteria defined for the instruments,
which are used to measure the quantities
which are slowly varying with time or mostly
constant, is called static characteristics.
It includes accuracy, precision, error,
sensitivity, threshold, resolution, linearity,
zero drift, stability, linearity etc.
Dr Alina Mirza - Dept of EE MCS, NUST
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It is the degree of closeness with which
the instrument reading approaches the
true value of the measurand.
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It indicates true value of the measurand.
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It denotes the closeness with which
individuals measurements are departed or
distributed about average of measured
values.
It is the measure of consistency or
repeatability of measurements.
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Example 1.1:
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It is the algebraic difference between the
indicated value and the true value of the
measurand
Types
▪ Absolute Error
▪ Relative Error
▪ Percentage Relative Error
▪ Error in percentage of full scale reading
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Absolute Error
𝒆 = 𝑨𝒕 − 𝑨𝒎
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Relative Error
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Percentage relative error
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Relative Percentage Error Accuracy
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Percentage of full scale deflection
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Example: An ammeter reads 8.3 A and the
true value of the current is 8.5 A. Determine
the absolute and relative percentage error?
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Example: A voltmeter reads 111.5 V. the error
taken from an error curve is 5.3%. Find the
true value of the voltage.
Dr Alina Mirza - Dept of EE MCS, NUST
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It is the ratio of the change in the output of
the instrument to a change in the value of
measurand.
It denotes the smallest change in the
measurand value to which the instrument
responds.
Dr Alina Mirza - Dept of EE MCS, NUST
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Inverse Sensitivity = Deflection Factor
Units: mm/μA, mm/Ω
Example: A particular ammeter require a change
of 2 A in its coil to a change in deflection of the
pointer by 5mm. Determine its sensitivity and
deflection factor?
Dr Alina Mirza - Dept of EE MCS, NUST
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The smallest measurable input change is
called resolution
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It is the ability to reproduce the input
characteristic symmetrically and linearly
Graph of output across input is called
calibration curve
Linearity is defined as maximum deviation of
actual calibration curve (output) from
idealized straight line, expressed as
percentage of actual reading
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Zero Drift: It is defined as the deviation in the
instrument output with time, from its zero
value, when measurand is constant
Factors of drift.
▪ Electrical field,
Magnetic field, temperature
changes, changes in atomic structure, wear &
tear, corrosion etc.
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Stability is to retain instrument performance
throughout its specified operating life.
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For a certain range of input values, there is no
change of output. This range is called dead
space
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