Mechanical measurement basics

Measurement
Figure 1.0 Fundamental measuring process

• The word measurement is used to tell us the length, the weight, the
temperature, the colour or a change in one of these physical entities of a
material.
• Measurement provides us with means for describing the various physical and
chemical parameters of materials in quantitative terms.
• Measurement is the result of an opinion formed by one or more observers
about the relative size or intensity of some physical quantity.
• The opinion is formed by the observer after comparing the object with a
quantity of same kind chosen as a unit, called standard.
• The result of measurement is expressed by a number representing the ratio
of the unknown quantity to the adopted standard.
• This number gives the value of the measured quantity.
• For example 10 cm length of and object implies that the object is 10 times as
large as 1 cm; the unit employed in expressing length.

• The measurement standard is the physical
embodiment of the unit of measurement as well
as that of its submultiple value.
• This places a sizeable responsibility on the
observer, he may be an engineer or a technician,
to be certain that the standard used by him is
accurately known and commonly accepted.
• Further, the procedure and apparatus employed
for obtaining the comparison must be provable,
i.e., accuracy can be reproduced any where in the
world.

• The physical quantity or the characteristic
condition which is the object of measurement in
an instrumentation system is variously termed as
measurand,measurement variable,
instrumentation variable and process variable.
• The measurand may be
a fundamental quantity(length- mass and time) ,
a derived quantity (speed, velocity acceleration,
power, etc.) or
a quality like pressure, temperature etc.

• Pervariables or through variables
which-can be specified and measured at one point
in space.
 Examples are force, momentum, current and
charge.
• Transvariables or across variables
which need two points (usually one point is the
reference) to specify or measure them.
 Examples are displacement, velocity,
temperature and voltage.
What is Time then?????

Instrument
• The human senses cannot provide exact quantitative
information about the knowledge of events occurring
in our environments.
• The stringent requirements of precise and accurate
measurements in the technological fields have,
therefore, led to the development of mechanical aids
called instruments.
• Scientific instruments allow the human to observe and
measure aspects of the physical universe beyond the
range and precision of the unaided human senses.
• Instruments are the essential extensions of human
sensing and perception without which scientific
exploration of nature would be impossible.

• The instrument would sense a physical parameter
(pressure, temperature, velocity etc.), process and
translate it into a format and range which can be
interpreted by the observer.
• The instrument must also provide the controls by
which the operator can obtain, manipulate and
respond to the information.
• Apparently, the instrument designer aims to produce
a scientific instrument that not only works efficiently
but does so taking due account of the comfort,
safety, limitations and frailties of the human operator
when setting up, using and maintaining the
instrument.

Measurement methods
• Measurement is a process of comparison of
the physical quantity with a reference
standard.
• Depending upon the requirement and based
upon the standards employed, there are two
basic methods of measurement:
1. Direct measurements
2. Indirect measurements

1. Direct measurements
• The value of the physical parameter (measurand)
is determined by comparing it directly with
reference standards.
• The physical quantities like mass, length and time
are measured by direct comparison.
• Direct measurements are not to be preferred
because they involve human factors, are less
accurate and also less sensitive.
• Further, the direct methods may not always be
possible, feasible and practicable.

2. Indirect measurements
• The value of the physical parameter (measurand)
is more generally determined by indirect
comparison with secondary standards through
calibration.
• The measurand is converted into an analogous
signal which is subsequently processed and fed to
the end device that presents the result of
measurement.
• The indirect technique saves the primary or
secondary standards from a frequent and direct
handling.

Primary, secondary and tertiary
measurements
• The complexity of an instrument system
depends upon the measurement being made
and upon the accuracy level to which the
measurement is needed.
• Based upon complexity of the measurement
system, the measurements are generally
grouped into three categories namely the
primary, secondary and tertiary
measurements.

• In the primary mode, the sought value of a
physical parameter is determined by comparing it
directly with reference standards.
• The requisite information is obtainable through
senses of sight and touch.
Examples are :
1. matching of two lengths when determining the length of an
object with a ruler
2. matching of two colours when judging the temperature of
red hot steel
3. estimating the temperature difference between the contents
of containers by inserting fingers
4. use of beam balance to measure (actually compare) masses
5. measurement of time by counting the number of strokes of
a clock

• The indirect methods make comparison with a
standard through use of a calibrated system, i.e.
an empirical relation is established between the
measurement actually made and the results that
are desired.
• For example, an indirect method may consist of
developing an electrical voltage proportional to a
physical variable to be measured, measuring that
voltage and then converting the measured
voltage back to the corresponding value of the
original measurand.
• Electrical methods are preferred in the indirect
methods due to their high speed of operation
and simple processing of the measured variable.

• The indirect measurements involving one
translation are called secondary Measurements
• The indirect measurements involving two
conversions are called tertiary measurements.
• The conversion of pressure into displacement
by means of bellows (Fig. 1.1a) and the
conversion of force into displacement by means
of springs (Fig, 1,16) are simple examples of
secondary measurements.

Fig. 1.1. Secondary measurements :
(i) bellows convert pressure into displacement,
(ii) springs convert force into displacement

• When a pressure above that of atmosphere is applied
to the open end of the bellows these expand and the
resulting displacement is a measure of applied
pressure.
…(1.1)
where P is the variable pressure (input), del is the bellows
displacement (output) and k1is the constant of proportionality.
• The displacement varies linearly with applied pressure
provided that the range of pressure variation is small.
• Likewise a spring stretches when a vertical force is
applied at its free end.
• Different forces give rise to different displacements and
so a measure of the spring deflection gives a unique
indication of the force.
….(1.2)

where F is the applied force (input), del is the
spring deflection (output) and k2 is a constant of
proportionality. The identity prescribed by
equation 1.2 is often written as
…(1.2a)
where S represents the spring stillness;
it is defined as that force which is necessary for
unit spring deflection.

• The pressure measurement by manometers and
the temperature measurement by mercury-in-
glass thermometers are other examples of
secondary measurements.
• In these instruments, the primary signal (pressure
or temperature) is first transmitted to a
transducer where its effect is translated into a
length change.
• The secondary signal of length change is then
transmitted to the observer’s eye.
• The observed length change is subsequently
converted into equivalent pressure or
temperature change through a calibration
process.

• The measurement of static pressure by a bourdon
tube pressure gauge (Fig. 1.2) is a typical example of
tertiary measurement.
Fig. 1.2. Tertiary measurement: measurement of pressure by
a bourdon tube pressure gauge

• When the static pressure (input signal) is
applied to bourdon tube, its free end deflects.
• The deflection which constitutes the
secondary signal is very small and needs to be
made larger for display and reading.
• The task is accomplished by an arrangement
of lever, quadrant, gearing and the pointer.
• The amplified displacement constitutes the
tertiary signal, and it is indicated by the
movement of the pointer against a graduated
scale.

• The measurement of the speed of a rotating shaft by
means of an electric tachometer fig. 1.3) is another
typical example of tertiary measurement.
• The angular speed of the rotating shaft is first
translated into an electrical voltage which is
transmitted.by a pair of wires to a voltmeter.
• In the voltmeter, the voltage moves a pointer on a
scale, i.e., voltage is translated into a length change.
• The tertiary signal of length change is a measure of the
speed of the shaft and is transmitted to the observer.
Fig. 1.3. Tertiary measurement: measurement of angular
speed by an electric tachometer

• The unit of a measuring system where
translation of a measurand takes place is
called transducer or translator.
• The term is usually applied to an
electromechanical device which converts the
measurand into a proportional electrical
output. The electrical, mechanical or any
other variable which is actually measured is
called the measured signal.

• In the example of a tertiary measurement cited
above, the measured signal is the voltage which is an
electrical analog of the speed of rotation of the unit
coupled to the tachometer.
• In a thermo-couple thermometer, the measured
signal is an electromotive force which is the electrical
analog of the temperature applied to the thermo-
couple.

• Likewise in a differential flow-meter, the
measured signal would be the differential
pressure which is the analog of the rate of
flow through an orifice plate.

• Needless-to-say, majority of measurement systems are
tertiary systems and they include the whole range of
mechanical, electrical, pneumatic, electro-mechanical
and electro-pneumatic instruments.
• Where as the input to a measuring system is known as
measurand, the output is called measurement.
• For example in a bourdon tube gauge, the applied pressure
(input to the measurement system) is the measurand. The
output from the system is the movement of the pointer
against a calibrated scale, and this pointer movement
becomes the measurement.
• Likewise in an electric tachometer, the angular speed is the
mesurand and the movement of pointer (length change) is the
measurement.

Contact and non-contact type
measurements
• Measurements may also by described as
1. contact type where the sensing element of the
measuring device has a contact with the medium
whose characteristics are being measured and
2. non-contact type where the sensor does not
communicate physically with the medium.
The optical, radioactive and some of the
electrical/electronic measurements belong to this
category.

Generalized measurement system
Fig. 1.4. Generalized measurement system

• The principal function of an instrument is the
acquisition of information by sensing and
perception, the processing of that information
and its final presentation to a human
observer.
• For the purpose of analysis and synthesis, the
instruments are considered as systems, i.e.
assemblies of interconnected components
organized to perform a specified function.
• The different components are called elements
and they perform certain definite and
required steps in the act of measurement.

• The following basic components can be identified
in a generalized measurement system (Fig. 1.4).
• The scope of the different elements is
determined by their functioning rather than by
their construction.
1. Primary sensing element
2. Variable conversion or transducer element
3. Manipulation element
4. Data transmission element
5. Data processing element
6. Data presentation element

1. Primary sensing element
• Its an element that is sensitive to the measured
variable.
• The sensing elements sense the condition, state
or value of the process variable by extracting a
small part of energy from the measurand and
then produce an output which reflects this
condition, state or value of the measurand.
• Because of this energy extraction or utilization,
the measured quantity is always disturbed by the
act of measurement and that makes a perfect
measurement theoretically impossible.
• Good instruments are designed to minimize this
loading effect.

2. Variable conversion or transducer
element
• Its an element that converts the signal from one physical form into another
without changing the information content of the signal.
• The signal after transduction is more suitable for purpose of measurement
and control.
• The transduction may be from mechanical, electrical or optical to any other
related form.
• Some examples of transducers and the conversions associated with them
are
1. bourdon tube and bellow’s which transform pressure into displacement
2. proving ring and other elastic members which convert force to
displacement
3. rack and pinion which convert the linear to rotary motion and vice versa
4. obstruction flow meters which transform flow to pressure
5. thermocouples which convert information about temperature difference
to information in the form of emf.

3. Manipulation element
• Its an element that operates on the signal
according to some mathematical rule without
changing the physical nature of the variable.
• For the odometer of an automobile, the above
mathematical operation takes the form.

4. Data transmission element
• Its an element that transmits the signal form one
location to another without changing its information
content.
• Data may by transmitted over long distances (from one
location to another) or short distances (from a test
centre to a nearby computer).
• Further, the transmission element may be as simple as
a shaft and gearing assembly or as complicated as a
telemetry system for transmitting signals from missiles
to ground equipment.
• Direct transmission via cables, called land-line
telemetry, generally employs either current, voltage,
frequency, position or impulses to convey the
information.

5. Data processing element
• Its an element that modifies the data before it is displayed or finally
recorded.
Data processing may be used for such purposes as:
1. corrections to the measured physical variables to compensate for
scaling, non-linearity, zero offset, temperature error etc.
2. perform repeated calculations that involve addition, subtraction,
multiplication or division of two or more physical variables and
their associated constants.
3. collect information regarding average, statistical and logarithmic
values
4. convert the data into useful form, e.g., calculation of engine
efficiency from speed, power input and torque developed
5. separate out signals buried in noise, generate information for
displays, and a variety of other goals

6. Data presentation element
• Its an element that provides a record or indication of the output from the
data processing element.
• In a measuring system using electrical instrumentation, an exciter and an
amplifier are also incorporated into the circuit (Fig. 1.5).
• The exciter is a source of electrical energy for the transducer.
• The amplifier serves to amplify the voltage from the transducer if this
voltage is small.
• The display unit may be required to serve the following functions :
1. Transmitting: to convey the information concerning the measured
quantity over some distance to a remote point
2. Signalling : to give a signal that.the desired value has been reached
3. Registering : to indicate by numbers or by some other symbol the value
of some quantity
4. Indicating: to indicate the specific value with an indicating hand over a
suitably calibrated scale
5. Recording : to produce a written continuous record ofthe measurand
against

• It is a usual practice to regroup the various
elements of a generalized measurement
system into three stages; each comprising a
distinct component or grouping of
components.

Fig 1.5 Electro-mechanical measurement system
Electro-mechanical Measurement System

Input stage (Detector-transducer)
• It is acted upon by the input signal (a variable
to be measured) such as length, pressure,
temperature, angle etc. and which transforms
this signal in some other physical form.
• When the dimensional units for the input and
output signals are same, this functional
element/stage is referred to as the
transformer.

Intermediate stage (Signal conditioning)
• It includes :
1. signal amplification to increase the power or amplitude
of the signal without affecting its waveform.
• The output from the detector-transducer element is
generally too small to operate an indicator or a
recorder and its amplification is necessary.
• Depending upon the type of transducer signal, the
amplification device may be of mechanical,
hydraulic/pneumatic, optical and electrical type.
• The reverse of amplification is termed attenuation, i.e.,
reduction of the signal amplitude while retaining its
original form

2. signal filtration to extract the desired
information from extraneous data.
• Signal filtration removes the unwanted noise
signals that tend to obscure the transducer signal.
• Depending upon nature of the signal and
situation, one may use mechanical, pneumatic or
electrical filters
• 3. signal modification to provide a digital signal
from an analog signal or vice versa, or change
the form of output from voltage to frequency
or from voltage to current

4. data transmission to telemeter the data for
remote reading and recording
• The act of signal amplification, attenuation,
demodulation and filtration etc. is often referred
to as signal processing or signal conditioning.
• Manufacturers often provide the required signal
conditioning, acquisition and conversion
equipment into one instrument known as data
logger.
• The logger output is in digital form suitable to
operate a printer or paper punch, be recorded by
magnetic tape, or be fed directly to digital
processing equipment.

What measurements does this data
logger support?
The H22-001 data logger supports the
following measurements: 4-20mA, AC
Current, AC Voltage, Air Velocity, Amp
Hour (Ah), Carbon Dioxide,
Compressed Air Flow, DC Current, DC
Voltage, Differential Pressure, Gauge
Pressure, Kilowatt Hours (kWh),
Kilowatts (kW), Power Factor (PF),
Pulse Input, Relative Humidity,
Temperature, Volatile Organic Comp.,
Volt-Amp Reactive, Volt-Amp Reactive
hour, Volt-Amps (VA), Water Flow,
Watt Hours (Wh) and Watts (W)

Output stage (Data presentation)
• It constitutes the data display record or control.
• The data presentation stage collects the output from the signal-
conditioning element and presents the same to be read or seen and
noted by the experimenter for analysis.
• This element may be of
1. visual display type such as the height of liquid in a manometer or
the position of pointer on a scale
2. numerical readout on an electrical instrument
3. graphic record on some kind of paper chart or a magnetic tape
4. audible alarms indicating that the values have reached
unacceptable levels
• When the measurement system forms an integral part of a control
system, the output may not be necessarily displayed.
• The essential requirements of data presentation element are: very
small inertia and friction, minimum drag on the system and a fast
response

EXAMPLE
• Consider mercury-in-glass thermometer which
represents a system for the measurement of
temperature:
(i) Mercury acts as transducer; it converts
information about temperature (invisible) into
information in the form of volume change
(visible)

(ii) The volume change is very small and needs
to be made larger for facility in display and
reading. It is achieved through a small diameter
capillary tubing which performs the function of
a signal conditioner.

(iii) The position of meniscus of mercury in the
capillary tube is read on a graduated scale
engraved on the stem or placed beside it. The
scale constitutes the display element.

Mechanical measurement basics

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