ME- 318 F MEASUREMENTS & INSTRUMENTATION LAB.
Sessional : 25 Marks
L T P Practical : 25 Marks
- - 2 Total : 50 Marks
Duration of Exam : 3 Hrs.
List of Experiments :
1. To Study various Temperature Measuring Instruments and to Estimate their Response times.
(a) Mercury – in glass thermometer
(b) Thermocouple
(c) Electrical resistance thermometer
(d) Bio-metallic strip
2.
To study the working of Bourdon Pressure Gauge and to check the
calibration of the gauge in a deadweight pressure gauge calibration set
up.
3.
To study a Linear Variable Differential Transformer (LVDT) and use it
in a simple experimental set up to measure a small displacement.
4. To study the characteristics of a pneumatic displacement gauge.
5. To measure load (tensile/compressive) using load cell on a tutor.
6. To measure torque of a rotating shaft using torsion meter/strain gauge torque transducer.
7. To measure the speed of a motor shaft with the help of non-contact type pick-ups (magnetic or photoelectric).
8. To measure the stress & strain using strain gauges mounted on simply supported beam/cantilever beam.
9. To measure static/dynamic pressure of fluid in pipe/tube using pressure transducer/pressure cell.
10. To test experimental data for Normal Distribution using Chi Square test.
11.
To learn the methodology of pictorial representation of experimental
data and subsequent calculations for obtaining various measures of true
value and the precision of measurement using Data acquisition system/
calculator.
12. Vibration measurement by Dual Trace Digital storage Oscilloscope.
13. To find out transmission losses by a given transmission line by applying
capacitive /inductive load.
14. Process Simulator.
Note:
1. At least ten experiments are to be performed in the Semester.
2.
At least seven experiments should be performed from the above list.
Remaining three experiments may either be performed from the above list
or designed & set by the concerned institution as per the scope of
the Syllabus.
Experiment No:1
Aim: To Study various Temperature Measuring Instruments and to Estimate their Response times.
(a) Mercury – in glass thermometer
(b) Thermocouple
(c) Electrical resistance thermometer
(d) Bi-metallic strip
Apparatus used: Mercury thermometer, Thermocouple setup, Platinum thermometer and Bi-metallic strip.
Theory:
(a) Mercury – in glass thermometer:
A
liquid-in-glass thermometer is widely used due to its accuracy for the
temperature range -200 to 600°C. Compared to other thermometers, it is
simple and no other equipment beyond the human eye is required. The LIG
thermometer is one of the earliest thermometers. It has been used in
medicine, metrology and industry. In the LIG thermometer the thermally
sensitive element is a liquid contained in a graduated glass envelope.
The principle used to measure temperature is that of the apparent
thermal expansion of the liquid. It is the difference between the
volumetric reversible thermal expansion of the liquid and its glass
container that makes it possible to measure temperature.
The liquid-in-glass thermometer comprises of
1. A bulb, a reservoir in which the working liquid can expand or contract in volume
2.
A stem, a glass tube containing a tiny capillary connected to the bulb
and enlarged at the bottom into a bulb that is partially filled with a
working liquid. The tube's bore is extremely small - less than 0.02 inch
(0.5 millimetre) in diameter
3.
A temperature scale is fixed or engraved on the stem supporting the
capillary tube to indicate the range and the value of the temperature.
It is the case for the precision thermometers whereas for the low
accurate thermometers such as industrial thermometer, the scale is
printed on a separate card and then protected from the environment. The
liquid-in-glass thermometers is usually calibrated against a standard
thermometer and at the melting point of water
4. A reference point, a calibration point, the most common being the ice point
5. A working liquid, usually mercury or alcohol
6.
An inert gas is used for mercury intended to high temperature. The
thermometer is filled with an inert gas such as argon or nitrogen above
the mercury to reduce its volatilization.
The
response of the thermometer depends on the bulb volume, bulb thickness,
total weight and type of thermometer. The sensitivity depends on the
reversible thermal expansion of the liquid compared to the glass. The
greater the fluid expansion, the more sensitive the thermometer. Mercury
was the liquid the most often used because of its good reaction time,
repeatability, linear coefficient of expansion and large temperature
range. But it is poisonous and so other working liquids are used.
Fig: Liquid in Glass Thermometer
A mercury-in-glass thermometer,
also known as a mercury thermometer, consisting of mercury in a glass
tube. Calibrated marks on the tube allow the temperature to be read by
the length of the mercury within the tube, which varies according to the
heat given to it. To increase the sensitivity, there is usually a bulb
of mercury at the end of the thermometer which contains most of the
mercury; expansion and contraction of this volume of mercury is then
amplified in the much narrower bore of the tube. The response time of
the thermometer is nothing but as time constant or the time of
consideration for measuring particular temperature.
(b) Thermocouple:
An
electric current flows in a closed circuit of two dissimilar metals if
their two junctions are at different temperatures. The thermoelectric
voltage produced depends on the metals used and on the temperature
relationship between the junctions. If the same temperature exists at
the two junctions, the voltage produced at each junction cancel each
other out and no current flows in the circuit. With different
temperatures at each junction, different voltage is produced and current
flows in the circuit. A thermocouple can therefore only measure
temperature differences between the two junctions.
Fig: Thermocouple
Thermocouples
response time is measured as a “time constant.” The time constant is
defined as the time required for a thermocouple’s voltage to reach 63.2%
of its final value in response to a sudden change in temperature. It
takes five time constants for the voltage to approach 100% of the new
temperature value. Thermocouples attached to a heavy mass will respond
much slower than one that is left free standing because its value is
governed by the temperature of the large mass. A free standing (exposed
or bare wire) thermocouple’s response time is a function of the wire
size (or mass of the thermocouple bead) and the conducting medium. A
thermocouple of a given size will react much faster if the conducting
medium is water compared to still air.
(c) Electrical resistance thermometer:
Resistance
thermometers may be called as RTDs (resistance temperature detectors),
PRT's (platinum resistance thermometers), or SPRT's (standard platinum
resistance thermometers). These thermometers operate on the principle
that, electrical resistance changes in pure metal elements, relative to
temperature. The traditional sensing element of a resistance thermometer
consists of a coil of small diameter wire wound to a precise resistance
value. The most common material is platinum, although nickel, copper,
and nickel-iron alloys compete with platinum in many applications.
Platinum
Resistance thermometer consists of a fine platinum wire (platinum coil)
wound in a non-inductive way on a mica frame M (Figure 1). The ends of
this wire are soldered to points A and C from which two thick leads run
along the length of the glass tube (that encloses the set up) and are
connected to two terminals (P, P) fixed on the cap of the tube. These
are the platinum wire leads. Also, by the side of these leads, another
set of leads run parallel and are connected to the terminals (C, C)
fixed on the cap of the tube. These are called compensating leads and
are joined together inside the glass tube. The compensating leads and
the platinum wire are separated from each other by mica or porcelain
separators (D, D). The electrical resistance of the (P, P) leads is same
as that of the (C, C) leads.
Fig: Resistance Thermometer
A
time constant indicates the responsiveness of a resistance thermometer
to temperature change. A common expression is the time it takes a
thermometer to reflect 63.2% of a step temperature change in moving
water. Response speed depends on the mass of the thermometer and the
rate at which heat transfers from the outer surface to the sensing
element. A rapid time constant reduces errors in a system subject to
rapid temperature changes.
(d) Bi-metallic strip:
Bonding
two metals with dissimilar thermal expansion coefficients can produce
useful devices for detecting and measuring temperature changes. A
typical pair is brass and steel with typical expansion coefficients of
19 and 13 parts per million per degree Celsius respectively.
Fig: Bimetallic Strip
The
examples shown are straight strips, but bimetallic strips are made in
coils to increase their sensitivity for use in thermostats. One of the
many uses for bimetallic strips is in electrical breakers where
excessive current through the strip heats it and bends it to trip the
switch to interrupt the current.
A
bimetallic strip is used to convert a temperature change into
mechanical displacement. The strip consists of two strips of different
metals which expand at different rates as they are heated, usually steel
and copper, or in some cases brass instead of copper. The strips are
joined together throughout their length by riveting, brazing or welding.
The different expansions force the flat strip to bend one way if
heated, and in the opposite direction if cooled below its initial
temperature. The metal with the higher coefficient of thermal expansion
is on the outer side of the curve when the strip is heated and on the
inner side when cooled.
Conclusion: Hence the study of various temperature measuring instruments and their response times is completed.
Experiment No:2
Aim: To
study the working of Bourdon Pressure Gauge and to check the
calibration of the gauge in a deadweight pressure gauge calibration set
up.
Apparatus used: Deadweight Pressure Gauge calibration set up
Theory: These
are used for measurement of pressure and vacuum and are suitable for
all clean and non-clogging liquid and gaseous media. Bourdon gauge
consists of a hollow metal tube with an oval cross section, bent in the
shape of a hook. One end of the tube is closed, the other open and
connected to the measurement region. If pressure (above local
atmospheric pressure) is applied, the oval cross section will become
circular, and at the same time the tube will straighten out slightly.
The resulting motion of the closed end, proportional to the pressure,
can then be measured via a pointer or needle connected to the end
through a suitable linkage.
Fig: Bourdan Tube Gauge
Working of the Bourdon Pressure Gauge:
In order to understand the working of the bourdon pressure gauge, we
need to consider a cross-section of the Bourdon tube, as shown in the
figure.
Fig: Working of Bourdon Gauge
Assume
that a pressure P, which is greater than the atmospheric pressure, acts
on at the pressure inlet of the gauge. According to the Pascal’s Law,
the pressure is transmitted equally in all directions. Therefore,
Pressure acting on the Inner Wall = Pressure acting on the Outer Wall.
Now,
Area of Outer Wall projected to the pressure = 2πRod
Therefore,
Force on Outer wall = Fo = Pressure x Area = 2PπRod
Similarly,
Force on Inner Wall = Fi = 2PπRid
Since, Ro>Ri then, Fo>Fi.
So,
the force that tries to unwind the tube is greater than the force that
tries to bend it further. Therefore, the tube unwinds due to the extra
pressure exerted on it. This unwinding is then recorded on a scale by
using a series of gears and a pointer.
Calibration
is the name of the term applied to checking the accuracy or the working
condition of the concerned device. So, the calibration of Bourdon
Pressure Gauge refers to the checking of its accuracy or reliability in
taking a reading. The apparatus used for this purpose is called the
Dead-Weight Gauge Tester.
Working of the Dead-Weight Gauge Tester: The working of this gauge tester can be understood easily with the help of the following diagram.
Fig: Dead-Weight Gauge Tester
In
this figure gauge A and B are the ones to be calculated. We can at any
stage disengage any gauge by closing the respective valve.
For the illustration purpose, we will just consider the calibration of Gauge A and assume that valve B remains closed.
Let
Weight of Plunger = W
Cross-sectional Area of the stem of Plunger = A
Therefore,
Pressure exerted on the fluid = P = W/A
Now,
according to Pascal’s Law, pressure is transmitted equally in all
direction. Therefore pressure encountered at the inlet of Gauge A is the
same as P
Now,
if Pressure registered by Gauge A = PA = P
within
experimental limits, then the gauge is working properly. If not, then
there is some problem which must be detected and accounted for.
Procedure:
1.
Fix the gauge to be tested on one end of the Dead-Weight Gauge tester
and make sure that the valve is fully opened. Meanwhile close the other
valve tightly so that no leakage of fluid is ensured.
2.
Next, gently place the plunger in the tester ensuring that the plunger
should not touch the edges of the bowl. Allow some time for the system
to attain equilibrium, than take the reading from the gauge. Record both
the applied and registered pressure in a table of values. Now, remove
the plunger and once again after some time record the reading on the
gauge. Record it in the table.
3.
Now place some weights on the plunger so that the applied pressure is
varied. Then, repeat the above mentioned procedure until there are at
least six readings. Record them all in the table.
Observations & Calculations:
Sl No
|
Applied Pressure (P)
|
PA
|
Error
|
Neglecting Zero Error
| ||
Loading
|
Unloading
|
Mean
|
(PA-P)
| |||
1
|
0
|
9
|
9
|
9
|
9(Zero Error)
|
0
|
2
|
5
|
13
|
14
|
13.5
|
8.5
|
-0.5
|
3
|
10
|
18
|
18
|
18
|
8
|
-1
|
Conclusion: Hence the working of Bourdon Pressure Gauge and checking of calibration on a deadweight pressure gauge is completed.
Experiment No:3
Aim: To
study a Linear Variable Differential Transformer (LVDT) and use it in a
simple experimental set up to measure a small displacement.
Apparatus used: LVDT setup
Theory: The
letters LVDT are an acronym for Linear Variable Differential
Transformer, a common type of electromechanical transducer that can
convert the rectilinear motion of an object to which it is coupled
mechanically into a corresponding electrical signal. LVDT linear
position sensors are readily available that can measure movements as
small as a few millionths of an inch up to several inches, but are also
capable of measuring positions up to ±20 inches (±0.5 m). The
transformer's internal structure consists of a primary winding centered
between a pair of identically wound secondary windings, symmetrically
spaced about the primary. The coils are wound on a one-piece hollow form
of thermally stable glass reinforced polymer, encapsulated against
moisture, wrapped in a high permeability magnetic shield, and then
secured in cylindrical stainless steel housing. This coil assembly is
usually the stationary element of the position sensor. The moving
element of an LVDT is a separate tubular armature of magnet i cal l y
permeable material called the core, which is free to move axially
within the coil's hollow bore, and mechanically coupled to the object
whose position is being measured. This bore is typically large enough to
provide substantial radial clearance between the core and bore, with no
physical contact between it and the coil.
Fig: LVDT
The
device consists of a primary coil, two secondary coils, and a moveable
magnetic core which is connected to an external device whose position is
of interest. A sinusoidal excitation is applied to the primary coil,
which couples with the secondary coils through the magnetic core (ie.
voltages are induced in the secondary coils). The position of the
magnetic core determines the strength of coupling between the primary
and each of the secondary cores, and the difference between the voltages
generated across each of the secondary cores is proportional to the
displacement of the core from the neutral position, or null point.
Fig: LVDT Principle
Procedure:
1. Adjust the experimental setup for probe to zero position.
2. Verify all electrical connections.
3. Give the LVDT power supply on.
4. Record the displacement and output voltage.
Observations & Calculations:
Sl No
|
Displacement
|
Voltage
|
Error(V1-V2)
|
(V1)
| |||
(V2)
|
Conclusion: Hence the measurement of a small displacement using LVDT is ______.
Experiment No:4
Aim: To study the characteristics of a pneumatic displacement gauge.
Apparatus used: Model of a pneumatic displacement gauge.
Theory:
In
pneumatic type of devices, the displacement signal is converted to
pressure signal. The device shown below is pneumatic displacement gauge
and this is also known as flapper nozzle device.
Fig: Pneumatic Gauge
A
pneumatic displacement gauge system operates with air. The signal is
transmitted in form of variable air pressure (often in the range 3-15
psi, i.e. 0.2 to 1.0 bar) that initiates the control action. One of the
basic building blocks of a pneumatic displacement gauge system is the
flapper nozzle amplifier. It converts very small displacement signal (in
order of microns) to variation of air pressure. The basic construction
of a flapper nozzle amplifier is shown in above figure. Constant air
pressure (20psi) is supplied to one end of the pipeline. There is an
orifice at this end. At the other end of the pipe there is a nozzle and a
flapper. The gap between the nozzle and the flapper is set by the input
signal. As the flapper moves closer to the nozzle, there will be less
airflow through the nozzle and the air pressure inside the pipe will
increase. On the other hand, if the flapper moves further away from the
nozzle, the air pressure decreases. At the extreme, if the nozzle is
open (flapper is far off), the output pressure will be equal to the
atmospheric pressure. If the nozzle is blocks, the output pressure will
be equal to the supply pressure. A pressure measuring device in the
pipeline can effectively show the pressure variation. The
characteristics is inverse and the pressure decreases with the increase
in distance. Typical characteristics of a flapper nozzle amplifier is
shown in below figure. The orifice and nozzle diameter are very small.
Typical value of the orifice diameter is 0.01 inch (0.25 mm) and the
nozzle diameter 0.025 inch (0.6 mm). Typical change in pressure is 1.0
psi (66 mbar) for a change in displacement of 0.0001 inch (2.5 micron).
There is an approximate linear range in 3-15 psi, of the characteristics
of the amplifier, which is the normal operating range.
The
role of flapper nozzle lies in its ability to generate a large output
air pressure, by placing a small obstruction at the orifice (at the
nozzle) of an incoming pneumatic signal. This trainer has a flapper
nozzle, together with a pressure amplifier, suitably connected to a
spring damper, and a spring compensator. This trainer not only used to
draw the characteristics of a FLAPPER NOZZLE, but also highlights the
application of a FLAPPER NOZZLE itself.
The Flapper Nozzle trainer
is a pneumatic system. The air at fixed pressure enters a constriction
(a partial obstruction) in its delivery path and enters a nozzle. The
opening of the nozzle is larger than the constriction. When the flapper
is moved away (usually one thousandth of an inch) from the nozzle, the
pressure at the nozzle falls to a low value typically 2 to 3 psi. When
the flapper is moved close to the nozzle, the pressure at he nozzle
rises to the supply pressure. This pressure is now applied to a pressure
amplifier, which in turn moves a beam. The purpose of this beam is to
demonstrate the utility of a flapper nozzle experiment. The displacement
of this moving beam is proportional to the pressure developed due to
the positioning of the flapper from the nozzle.
Fig: Flapper Nozzle System
Conclusion: Hence the characteristics of a pneumatic displacement gauge are studied.
Experiment No:5
Aim: To measure load (tensile/compressive) using load cell on a tutor.
Apparatus used: Load cell on a tutor.
Theory: A
Load Cell is defined as a transducer that converts an input mechanical
force into an electrical output signal. Load Cells are also commonly
known as Load Transducers or Load Sensors.
Load
cell designs can be distinguished according to the type of output
signal generated (pneumatic, hydraulic, electric) or according to the
way they detect weight (bending, shear, compression, tension, etc.)
Hydraulic load cells are force -balance devices, measuring weight as a
change in pressure of the internal filling fluid. In a rolling diaphragm
type hydraulic load cell, a load or force acting on a loading head is
transferred to a piston that in turn compresses a filling fluid confined
within an elastomeric diaphragm chamber. As force increases, the
pressure of the hydraulic fluid rises. This pressure can be locally
indicated or transmitted for remote indication or control. Output is
linear and relatively unaffected by the amount of the filling fluid or
by its temperature. If the load cells have been properly installed and
calibrated, accuracy can be within 0.25% full scale or better,
acceptable for most process weighing applications. Because this sensor
has no electric components, it is ideal for use in hazardous areas.
Typical hydraulic load cell applications include tank, bin, and hopper
weighing. For maximum accuracy, the weight of the tank should be
obtained by locating one load cell at each point of support and summing
their outputs.
Pneumatic
load cells also operate on the force-balance principle. These devices
use multiple dampener chambers to provide higher accuracy than can a
hydraulic device. In some designs, the first dampener chamber is used as
a tare weight chamber. Pneumatic load cells are often used to measure
relatively small weights in industries where cleanliness and safety are
of prime concern. The advantages of this type of load cell include their
being inherently explosion proof and insensitive to temperature
variations. Additionally, they contain no fluids that might contaminate
the process if the diaphragm ruptures. Disadvantages include relatively
slow speed of response and the need for clean, dry, regulated air or
nitrogen.
Strain-gage
load cells convert the load acting on them into electrical signals. The
gauges themselves are bonded onto a beam or structural member that
deforms when weight is applied. In most cases, four strain gages are
used to obtain maximum sensitivity and temperature compensation. Two of
the gauges are usually in tension, and two in compression, and are wired
with compensation. When weight is applied, the strain changes the
electrical resistance of the gauges in proportion to the load. Other
load cells are fading into obscurity, as strain gage load cells continue
to increase their accuracy and lower their unit costs.
Fig: Load Cell
Procedure:
1. Make setup of load cell and tutor.
2. Place weight on the load cell.
3. Note down the reading given by tutor separately for compression and tension.
4. Take 8-10 readings by increasing weight.
5. Compare actual weight & weight given by tutor.
Conclusion: Actual tensile & compression loads are _______ & _________.
Tutor tensile & compression loads are _______ & _________.
Experiment No:6
Aim: To measure torque of a rotating shaft using torsion meter/strain gauge torque transducer.
Apparatus used: Torsion meter/strain gauge torque transducer.
Theory:
What is torque?
Torque
is the tendency of a force to rotate an object about an axis, fulcrum,
or pivot. (or) Torque is defined as a force around a given point,
applied at a radius from that point.
An
engine produces power by providing a rotating shaft which can exert a
given amount of torque on a load at a given rpm. The amount of torque
the engine can exert usually varies with rpm.
Facts about calculations:
- Power (the rate of doing work) is dependent on torque and rpm.
- Torque and rpm are the measured quantities of engine output.
- Power is calculated from torque and rpm, by the following equation: P = Torque x RPM
How to measure torque of a rotating shaft?
The power transmitted can be calculated from the torque, using the equation
P = ω T
Where,
P is the power (in watts),
T is torque (N m)
ω is angular speed (rad / s).
What is torsion meter?
The
deflection measuring system is called torsion meter. An instrument for
determining the torque on a shaft, and hence the horse power of an
engine by measuring the amount of twist of a given length of the shaft.
When a shaft is connected between a driving engine and driven load, a
twist (angular displacement) occurs on the shaft between its ends. This
angle of twist is measured and calibrated in terms of torque.
Construction of mechanical torsion meter: The
main parts of the mechanical torsion meter are as follows: A shaft
which has two drums and two flanges mounted on its ends as shown in the
diagram. One drum carries a pointer and other drum has a torque
calibrated scale. A stroboscope is used to take readings on a rotating
shaft.
Operation of mechanical torsion meter: One
end of the shaft of the torsion meter is connected to the driving
engine and its other end to the driven load. An angle of twist is
experienced by the shaft along its length between the two flanges which
is proportional to the torque applied to the shaft. A measure of this
angle of twist becomes a measure of torque when calibrated. The angular
twist caused is observed on the torque calibrated scale corresponding to
the position of the pointer. As the scale on the drum is rotating,
reading cannot be taken directly. Hence a stroboscope is used. The
stroboscope’s flashing light is made to fall on the scale and the
flashing frequency is adjusted till a stationary image is obtained. Then
the scale reading is noted.
What is strain gauge torque transducer?
The strain monitoring system is called torque meter (or) strain gauge torque transducer.
A
Torque sensor is a transducer that converts a torsional mechanical
input into an electrical output signal. Torque Sensor, are also commonly
known as a Torque Transducer.
Torque
is measured by either sensing the actual shaft deflection caused by a
twisting force, or by detecting the effects of this deflection. The
surface of a shaft under torque will experience compression and tension,
as shown in figure below.
Fig: Strain Gauge Torque Transducer
To
measure torque, strain gage elements usually are mounted in pairs on
the shaft, one gauge measuring the increase in length (in the direction
in which the surface is under tension), the other measuring the decrease
in length in the other direction.
A
strain gage can be installed directly on a shaft. Because the shaft is
rotating, the torque sensor can be connected to its power source and
signal conditioning electronics via a slip ring. The strain gage also
can be connected via a transformer, eliminating the need for high
maintenance slip rings. The excitation voltage for the strain gage is
inductively coupled, and the strain gage output is converted to a
modulated pulse frequency as shown in figure. Maximum speed of such an
arrangement is 15,000 rpm.
Fig: Strain Gauge Working
Conclusion: Hence the torque of a rotating shaft is _______.
Experiment No:7
Aim: To measure the speed of a motor shaft with the help of non-contact type pick-ups (magnetic or photoelectric).
Apparatus used: Optical pick up
Theory: Besides
specific measurement requirements, application conditions determine the
choice of the appropriate sensor technology. Because of their ability
to withstand harsh environments and abrasive conditions, non-contact
magnetic sensors should be used for the most critical functions inside
the engine compartment. For rotational speed and position detection, to
compensate for position tolerances and position drifts of the mechanical
connection without degraded performance, magnetic sensors with a large
control tolerance field are used.
To
control the speed of a prime mover, speed controls compare actual speed
to desired, or set, speed. The speed sensor most often used to detect
prime mover speed is the magnetic pickup (MPU). When a magnetic material
(usually a gear tooth driven by the prime mover) passes through the
magnetic field at the end of the magnetic pickup, a voltage is
developed. The frequency of this voltage is translated by the speed
control into a signal which accurately depicts the speed of the prime
mover. The gap between the end of the MPU and the gear tooth is set at
0.25 to 1.02 mm (0.010 to 0.040 inch) at the closest point. The MPU will
be damaged if it touches the moving gear. A properly installed MPU will
provide as much as 50 Vac (rms); most Woodward controls require a
minimum of 1.5 Vac at the lowest speed. Voltage decreases as the MPU is
moved farther from the gear. If the gap between the pickup and the gear
cannot be measured directly, it can be determined by counting the number
of turns the pickup is backed away from the gear. One full turn
counterclockwise will move the MPU out 0.0555 inch (1.5 mm for the
metric model).
Procedure:
There
are electric tachometer consists of a transducer which converts
rotational speed into an electrical signal coupled to an indicator. The
transducer produces an electrical signal in proportion to speed. The
signal may be in the analog form or in the form of pulses. Tachometer or
pickups of this type produce pulses form a rotating shaft without being
mechanically connected to it. As the energy produced by these devices
is not sufficient to actual an indicator directly, amplifiers of
sufficient sensitivity are employed. The various types of non-contact
pick-ups are optical pick ups or photoelectric or photoconductive cell.
• Electromagnetic pick up
• Capacitive pick up
Here
we will measure the speed by optical pick up. As they don’t have moving
parts so speed up to 3 million rpm. These are available in a variety of
designs using the principle of shaft rotation to interrupt a beam of
light falling on a photoelectric or photo conductive cell. The pulse
thus obtained are first amplified & then either fed to an electric
counter, or shaped to an along signal and connected to the indicator. A
bright white spot is made on the rotating shaft. A beam of light
originating from the tachometer case hits the white spot & the
reflected light falls on photoconductive cell inside the case, producing
pulse in transes torised amplifier, which is turn, causes the
indicator to deflect which is measure of speed of the shaft.
Observations & Calculations:
Formula used: - Speed (rpm) = Frequency x Diameter of Disk / No. of segments.
Now,
1. Connect the CKT & CRO with the required apparatus & switch on the supply.
2.
Adjust the speed of the motor by the knob and wait for some time till
the motor attains the maximum speed at corresponding knob position.
3. Measure the frequency from out put wave on CRO.
4. Find the speed of the motor.
Calculations: - At knob position (A)
RPM = (frequency) x diameter of disc/No. of teeth of segments
N = RPM = f x d / T Where f = 1/t
Where
t = time period of one cycle of out put wave &
f = 1.8 x 2ms = 3.6 x 10-3 s [on CRO] and
d = 56.5mm.
Therefore, R.P.M = 2.79 x 102 x 56.5/ 60 = 262 rpm
Conclusion: Hence the Speed of position ‘A’ = 262 rpm
Experiment No:8
Aim: To measure the stress & strain using strain gauges mounted on simply supported beam/cantilever beam.
Apparatus used: Strain gauge Kit, cantilever beam weights, multimeter.
Theory:
When external forces are applied to a stationary object, stress and strain are the result. Stress
is defined as the object's internal resisting forces, and strain is
defined as the displacement and deformation that occur. For a uniform
distribution of internal resisting forces, stress can be calculated by dividing the force (F) applied by the unit area (A). Strain
is defined as the amount of deformation per unit length of an object
when a load is applied. Strain is calculated by dividing the total
deformation of the original length by the original length (L).
Fig: Stress - Strain Concept
Fundamentally, all strain gauges
are designed to convert mechanical motion into an electronic signal. A
change in capacitance, inductance, or resistance is proportional to the
strain experienced by the sensor. If a wire is held under tension, it
gets slightly longer and its cross-sectional area is reduced. This
changes its resistance (R) in proportion to the strain sensitivity (S)
of the wire's resistance. When a strain is introduced, the strain
sensitivity, which is also called the gauge factor (GF), is given by:
GF= (∆R/R)/(∆L/L)
There
are many types of strain gauges. Among them, a universal strain gauge
has a structure such that a grid-shaped sensing element of thin metallic
resistive foil (3 to 6µm thick) is put on a base of thin plastic film
(15 to 16µm thick) and is laminated with a thin film.
Fig: Strain Gauge
The
strain gauge is tightly bonded to a measuring object so that the
sensing element (metallic resistive foil) may elongate or contract
according to the strain borne by the measuring object. When bearing
mechanical elongation or contraction, most metals undergo a change in
electric resistance. The strain gauge applies this principle to strain
measurement through the resistance change. Generally, the sensing
element of the strain gauge is made of a copper-nickel alloy foil. The
alloy foil has a rate of resistance change proportional to strain with a
certain constant.
Procedure:
1. Arrange the cantilever beam, ammeter and voltmeter as shown in figure.
2. After this, put the weight on the rod of cantilever beam.
3. Measure the digital display reading for a particular weight.
4. Measure the value of ammeter (along) and voltmeter reading (micro-volt)
5. Increase the strength of weight.
6. Repeat the steps for increased weight.
7. Measure all dimensions of scale of cantilever.
Observations & Calculations:
Stress=F/A=Wg/A
Strain=∆L/L
GF= (∆R/R)/(∆L/L)
Depending
upon the beam used in apparatus force stress and strain values varies
accordingly with simply supported or cantilever beam terminology.
Conclusion: Hence stress=________ & strain=________.
Experiment No:9
Aim: To measure static/dynamic pressure of fluid in pipe/tube using pressure transducer/pressure cell.
Apparatus used: Pressure transducer Kit, multimeter etc.
Theory:
Pressure
is defined as force per unit area that a fluid exerts on its
surroundings. A pressure measurement can be described as either static
or dynamic. The pressure in cases where no motion is occurring is
referred to as static pressure. Examples of static pressure include the
pressure of the air inside a balloon or water inside a basin. Often
times, the motion of a fluid changes the force applied to its
surroundings. Such a pressure measurement is known as dynamic pressure
measurement. For example, the pressure inside a balloon or at the bottom
of a water basin would change as air is let out of the balloon or as
water is poured out of the basin.
Because
of the great variety of conditions, ranges, and materials for which
pressure must be measured, there are many different types of pressure
sensor designs. Often pressure can be converted to some intermediate
form, such as displacement. The sensor then converts this displacement
into an electrical output such as voltage or current. The three most
universal types of pressure transducers of this form are the strain
gage, variable capacitance, and piezoelectric.
Fig: Pressure Transducer
Procedure:
1. Firstly arrange the pressure transducer, Multimeter, Voltmeter.
2. After that increase the pressure in the pressure transducer.
3. Set the readings of pressure transducer on a particular reading.
4. Now note the display reading on Kit.
5. Also note the voltmeter & ammeter readings.
6. Repeat the numbers of reading with different pressure on transducer.
7. Compare the value of pressure applied on transducer & display readings.
Observations & Calculations:
Theoretically, P=ρg∆H
Where,
ρ=density of water in pipe
g=acceleration due to gravity
∆H=change in head
Conclusion: Hence the pressure of the fluid in pipe is _______.
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