UNIT-5: Measurements and measurement systems:
• Definition, significance of measurement, generalized
• Definitions and concept of accuracy, precision,
calibration, threshold, sensitivity, hysterisis, repeatability,
linearity, loading effect, system response-times delay.
• Errors in measurement, classification of errors.
• Transducers, transfer efficiency, primary and secondary
transducers, electrical, mechanical, electronic transducers,
advantages of each type transducers.
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• Measurement means determination of anything that exists in some amount.
• If those things that exist are related to mechanical engineering, then the
determination of such amounts are referred to as mechanical measurements.
• An engineer is not only interested in the measurement of physical variables but
also concerned with their control.
• These two functions are closely related because one must be able to measure a
variable such as temperature, or flow in order to control it.
• The accuracy of control is essentially dependent on the accuracy of measurement.
Hence, a good knowledge of measurement techniques is necessary for the design
of control systems.
Definition of Measurement :
• Measurement is defined as the process or the act of obtaining a quantitative
comparison between a predefined standard and an unknown magnitude.
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SIGNIFICANCE OF MEASUREMENT SYSTEM
• Measurement provides the fundamental basis for research and development.
Development is the final stage of the design procedure involving the
measurement of various quantities pertaining to operation and performance of
the device being developed.
• Measurement is also a fundamental element of any control process, which
requires the measured discrepancy between the actual and the desired
• Many operations require measurement for proper performance. For example :
In modern central power stations, temperatures, pressures, vibrational
amplitudes etc., are monitored by measurement to ensure proper performance.
• Measurement is also a bias of commerce, because the cost of the products are
established on the basis of amounts of materials, power, expenditure of time
and labour, and other constraints.
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THE GENERALIZED MEASURING SYSTEM
• Most measuring systems fall within the framework of a general
arrangement consisting of three phases or stages:
• Stage 1. A detection-transduction, or sensor-transducer, stage
• Stage 2. An intermediate stage, which we shall call the signal-
• Stage 3. A terminating, or readout-recording, stage
• Each stage consists of a distinct component or group of
components that performs required and definite steps in the
measurement. These are called basic elements: their scope is
determined by their function rather than by their construction. Figure 1.2
outline the significance of each of these stages.
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First, or Sensor-Transducer, Stage
• The primary function of the first stage is to detect or to sense the measurand.
• At the same time, ideally, this stage should be insensitive to every other possible
• For instance, if it is a pressure pickup, it should be insensitive to say,
• if it is a strain gage, it should be insensitive to temperature;
• if a linear accelerometer, it should be insensitive to angular acceleration; and so
• Unfortunately, it is rare indeed to find a detecting device that is completely
• Unwanted sensitivity is a measuring error, called noise.
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Second, or Signal-Conditioning, Stage
• The purpose of the second stage of the general system is to
modify the transduced information so that it is acceptable
to the third, or terminating stage.
• In addition, it may perform one or more basic operations,
such as selective filtering to remove noise, integration,
differentiation, as may be required.
• Probably the most common function of the second stage is
to increase either amplitude or power of the signal, or both,
to the level required to drive the final terminating device.
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Third, or Readout-Recording, Stage
• The third stage provides the information sought in a form
comprehensible to one of the human senses or to a
• If the output is intended for immediate human recognition,
it is, with rare exception, presented in one of the following
– As a relative displacement, such as movement of an indicating hand or
displacement of oscilloscope trace
– In digital form, as presented by a counter such as an automobile odometer,
or by a liquid crystal display (LCD) or light-emitting diode (LED) display
as on a digital voltmeter
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• To illustrate a very simple measuring system, let us consider the familiar
tire gage used for checking automobile tire pressure. Such a device is
shown in Fig. (a).
• It consists of a cylinder and piston, a spring resisting the piston
movement, and a stem with scale divisions.
• As the air pressure bears against the piston, the resulting force
compresses the spring until the spring and air forces balance.
• The calibrated stem, which remains in place after the spring returns the
piston, indicates the applied pressure.
• The piston-cylinder combination constitutes a force-summing
apparatus, sensing and transducing pressure to force.
• As a secondary transducer, the spring converts the force to a
displacement. Finally, the transduced input is transferred without signal
conditioning to the scale and index for readout.
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• Transfer Efficiency:
• The first contact that a measuring system has with the measurand is through
the input sample accepted by the detecting element of the first stage . This
act is usually accompanied by the immediate transduction of the input into
an analogous form.
• The medium handled is information. The detector senses the information
input. Iin, and then transduces or converts it to a more convenient form. Iout,.
The relationship may be expressed as
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THE PRIMARY AND THE SECONDARY TRANSDUCER
• Consider the Bourdon lube pressure gage as shown in
• The primary detector-transducer element consists of a
circular tube of elliptical cross section.
• When pressure is introduced the elliptical cross section
tends to become circular section. This causes the free
end A to move outward and the resulting motion is transmitted
by the link to sector gear and in-turn to pinion causing the
pointer to move over the scale.
• In this example Bourdon tube serves as the primary detector-
transducer, changing pressure to linear displacement.
• The link, sector gear and pinion acts as secondary transducer
and as an amplifier to give magnified output.
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• A component whose output energy is supplied entirely or almost entirely by its input
signal is called a passive transducer.
• The output and input signals may involve energy of the same form (say both mechanical)
or there may be energy conversion from one form to another (say mechanical to
• In other words, a passive transducer requires an auxiliary source of energy.
Example : Bonded wire strain gage.
• An active transducer has an auxiliary source of power which supplies a major part of the
output power while the input signal supplies only an insignificant portion.
• In other words, active transducers are self powered.
• Further, in these transducers, there may or may not be a conversion of energy from one
form to another.
• Example : electronic amplifiers, piezoelectric transducer.
Active transducers are those which require
electric current (a source of energy) for
working, while passive transducers are those
which does not need an external source.
Passive transducers directly produce electric
signals without an external energy source.
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Classification of first stage devices
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