AC & DC measurements
• It involves detection, acquisition, control &
analysis of data.
• reliability of a system is directly related to the
reliability of measurement.
• measuring instruments enable us to examine
physical events which are not apparent to our
senses.
• converts one kind of stimulus to which we are
insensitive, into another which can be detected
using eyes/ ears. E.g. Neon Tester, converts
current into light.
Definition of measurement
• An act / result of comparison between quantity
whose magnitude is unknown & a pre-defined
standard.
• Standards of measurements should be
accurately defined & commonly accepted.
• The apparatus & method used must be
provable.
Significance of measurement
• It converts one kind of stimulus to which we are insensitive to one that can
be detected by our eyes or ears. Eg. Neon tester.
• “When you can measure what you are speaking & express it in numbers,
you know something about it.” said Lord Kelvin. Without measurement we
can’t say what we are speaking about.
• The advancement of science & technology is dependent upon the
measurement technique.
• The nation’s progress is dependent upon how the measurements are made
& how data acquired by measurement is processed.
• The reliability of a system is dependent upon the measurement techniques
used.
Methods of measurement
• There are 2 types of measurement:
• Direct methods :
• Unknown quantity is directly compared against a standard.
• Result is expressed as a numeric number & unit.
• It is used for measurement of physical quantities like length,
mass & time.
• Indirect methods :
• direct methods are not always possible & feasible.
• Human involvement in Direct methods, increases chances of
errors & insensitive.
• in the indirect methods, a transducer is used which converts
the quantity to be measured in analogous form.
• the analogous signal is now processed and is fed to the end
devices.
Measurement terms:
• Instrument : device used for determining the value or
magnitude of a quantity or variable.
• Accuracy : closeness with which an instrument reading
approaches the true of the variable being measured.
• Precision : measure of reproducibility of measurements. It is a
measure of the degree to which successive measurements
differ from one another.
• Sensitive : ratio of the o/p signal or response of the instrument
to a change of input or measured variable.
• Resolution : smallest change in measured value to which
instrument will respond.
• Error : deviation from the true value of the measured variable.
• Accuracy refers to closeness with which instrument reading
approaches the true value whereas precision refers to the
reproducibility of the measurements.
Types of errors
• Gross errors :
• Occurs due to human mistakes like using the instruments, recording
& measurement of results.
• The responsibility of the mistake totally goes to the experimenter.
• Large no. of errors occur due to the carelessness of thr experimenter.
• These errors can be avoided by studying how to use the instrument &
where the instrument is suitable.
• In general, indicating instruments change conditions to some extent
when connected into a complete circuit, so that the measured quantity
is altered by the method employed.
• This type of error may also occur when the instrument is not set to 0
before the measurement is taken.
• Systematic errors ( fixed error):
• Occurs due to the instrument deflects / the effect of
change in environmental condition on instrument.
• It is of 2 types : instrumental error & Environmental error.
• Instrumental errors:
• It occurs due to 3 reasons:
• Inherent shortcoming.
• Misuse of instrument.
• Loading effects of the instruments.
• Environmental errors:
• Due to the effects of changes in temperature, humidity,
pressure or magnetic or electrostatic effect on instrument.
• Change in the elastic properties of the spring in a moving coil
mechanism.
• Avoided by using air conditioning, compensation circuit, use of
magnetic shields, sealing certain components in the instrument.
• (1) Static error :
• Caused by limitations of the measuring device or physical laws
governing its behavior.
• E.g. In micrometer, when excessive pressure is applied in
torquing the shaft.
• (2). Dynamic error :
• Caused b’coz the instrument is not responding fast enough to
follow the changes in a measured value. E.g. insensitive
instrument.
• Random errors:
• Occurs due to the unknown causes and even all error
have been accounted for.
• the magnitude & direction of these errors are not known.
• Can cause due to friction, noise and other phenomena.
• Few random errors usually occur but they become
important in high- accuracy work.
• Avoided by increasing the no of readings & by using
statistical means to obtain the best approximation of the
true value of the quantity under measurement.
• Limiting error :
• Given by manufacturer.
• Means that the manufacturers have guarantee that the value of
the circuit component differs by same value, also called as
guarantees error.
• 500ohm +10% means resistance falls between the limits of
450ohm & 550 ohm.
• Probable error :
• Defined as +/- 0.67453.
• Denoted by “r” where % is std. deviation of the no of readings
taken.
• Parallax errors means misalignment of instrument, i.e. keeping
instrument straight to our eyes and take readings, if not ; can
cause an error.
• Independent errors:
• Errors which are directly & in no circumstances compensate
each other in calculation of the result.
• Depending errors:
• Possibly compensate & reduce or even nullify the individual
errors. E.g. using incorrect deflecting coil
• Correlated errors :
• occurs due o functional relational between independent &
dependent errors.
• Controllable errors:
• Errors which can be controlled, cause of these errors is
definable.
• Incidental errors:
• These are not controllable b’coz they can’t be determined &
used in computation.
• Cause of errors are small transient functions in friction in the
measurement system or minute non-determinable
fluctuations in the conditions of environment.
Measurement standards
• It is physical representation of unit of measurement.
• it is applied to a piece of equipment having known measure of physical
quantity.
• These standards are used to determine the values of the other physical
quantities by comparison method.
• All standards are preserved at the Internal Bureau of weight & measures at
Paris.
• Depending upon the functions & applications, different types of standards of
measurement are:
• Internal standard :
• Defined by international agreement.
• Represent certain unit of measurement to the closest possible accuracy
attainable by the science & technology of measurement.
• These international standards are not available to ordinary users for
measurements & calibration.
• Primary standard:
• Its function is to calibrate & verify the secondary standards.
• These are absolute standards of high accuracy that can be
used as ultimate reference standards.
• Secondary standards:
• These are basic reference standards by measurement &
calibration laboratories in industries.
• Each industry has its own secondary standards.
• Each laboratory periodically sends its secondary standard to
the national standards laboratory for calibration &
comparison against primary standards.
• After comparison & calibration, the national standards
laboratory returns the secondary standards to the
particular industrial laboratory with a certification of
measuring accuracy in terms of a primary standards.
• Working standards :
• These are principle tools of a measurement laboratory.
• These are used to check & calibrate instrument for
accuracy & performance.