Unit-II
Measurement of Acceleration, vibration, density and viscosity
Part –A
1. Define acceleration?
The ratio in which the velocity changes with respect to time, measured in
2
m/s
A=dv/dt,
2. What is seismic Instrument?
A system which consist of a mass- spring and damper combination housed
in a chamber is called seismic instrument.
3. What are the needs of accelerometer?
(i). for thr measurement of shock and vibration
(ii)For gross measurement of acceleration and vibration in vehicles like
aircraft and submarines
4. What are the methods of vibration measurement?
(i)
methods based on sensor
(ii)
mechanical measurement
(iii)
electrical measurement
(iv)
optical measurement
5. What are the types of accelerometer?
(i)
LVDT Accelerometer
(ii)
Seismic accelerometer
(iii)
Piezo-electric accelerometer
(iv)
Strain gauge accelerometer
(v)
Variable reluctance accelerometer
6. Give different modes of Seismic instruments?
(i)
Displacement mode: with large Mass and soft Spring
(ii)
Acceleration mode: with soft mass and stiff spring
7. Define Vibration?
A force which oscillates about some specific reference point, generally
continuous and time varying with some degree of repetitive nature is
called vibration, expressed in terms of Cycles /sec, strokes/minute, m/s,..
8. Give the classification of Calibration of vibrational pick-up?
(1) Constant Acceleration method
(i)
Tilting support method
(ii)
Centrifuge method
(2) Sinusoidal motion method
(3) Transient motion method
9. What are the choices of vibrational pick up?
(i)
mechanical impendence pick-up
(ii)
sensitivity
(iii)
frequency range
(iv)
phase characteristics
10. Define Density?
Density of a material is the ratio of weight of the material to the unit
volume. Density of liquids and gas depends on temperature and pressure.
Expressed in kg/m3, density id water is1.
11. Give some units of density?
Kg/m3, Kg/liter, g/cm3, ounce/inch3, pounds/inch3, pounds /gallon,
pounds/bushel, slug/feel3
12. What is API scale?
American petroleum institute introduced the method of expressing the
specific gravity called as API degree in 1921
Degree API=(141.5/SG at 60F)-131.5
13. What is baume Scale?
The Baume scale is actually two scale, one liquids heavier than water and
other liquids lighter that water, ◦Be
14. Give some purpose of density measurement?
To determine the mass and volume of products,
To evaluate the quality of product
To determine calorific value of fluids
15. Define viscosity
The resistance which arises from the lack of slipperiness of the of liquid
other things being proportional to the velocity of liquid. This lack of
slipperiness is called viscosity.
16. What is Newtonian fluid?
When shear stress is applied the fluid undergoes continuous deformation
and if the force-flow relation is linear, it obeys the Newton’s law of
viscosity and hence called Newtonian fluids.
17. What is non-Newtonian fluid?
If the force flow relation is of fluid non linear, it does not obey the
Newton’s law of viscosity and the fluid is called Non-Newtonian fluid.
18. Define Kinematic viscosity.
Kinematic viscosity is the ratio of absolute viscosity (μ)to density (ρ) of
the fluids.(ν)
19. what is specific viscosity?
It is the ratio of absolute viscosity of the fluid to absolute viscosity of the
standard fliud at constant temperature.
20. What is relative viscosity?
It is the ratio of absolute viscosity of fluid at given temperature to absolute
temperature of standard fliud at 20 degrees
21. Define Viscosity Index.
Its is an empirical number that indicates the effect of change of
temperature on viscosity of a fluid., the larger viscosity index indicates
lower sensitivity to temperature.
22. Define Consistency.
When continuous deformation occurs, the fluid tries to oppose with a
frictional resistance called as consistency.
23. Define Dynamic viscosity.
It is the ratio of Shear stress to velocity gradient
μ= τ/(dv/dy)
24. Define Co-efficeint of viscosity?
The force required per unit area to maintain unit difference of velocity
between two parallel plates.
F=ηA v/d ;η =Co-efficient of viscosity
25. Give some commonly used Specific gravity in industries?
API Degree, Baume Degree(Be), Balling Degree(Ba),brix degree(Br),
Skis, Richter, Tralles, Quevenne,Twadell
Part-B
1. Explain in detail about LVDT and Strain gauge Accelerometer. Give its
merit and demerits.
LVDT Accelerometer:
The LVDT accelerometer consist of two windings one primary and two
secondary, the core of the LVDT acts as the seismic mass and two flexible reeds
attached to the core provides the spring action.
The reads are attached to the housing or frame which is subjected to
vibration.
The accelerometer is fixed to the structure whose acceleration is ti be
measured. Due to the acceleration displacement of the seismic mass takes place.
At null displacement of the core when the coil is excited the output in the
secondary coils will be equal and opposite and so the net result will be zero.
When the core displaces the emf generated in both the coils will vary and the
difference in these two output is proportional to the acceleration applied
Adv: vibrations at high frequencies can be measured
High resolution
Strain gauge Accelerometer:
When a cantilever beam is attached with mass at its free end is subjected
to vibration hence the beam is strained and the resulting strain is proportional to
acceleration.
Two strain gauges is attached to the beam and the system is filled with the
damping oil. This accelerometer is fitted to the structure whose acceleration to
be measured. due to acceleration the vibrational displacement of the body
occurs causing the beam to strain. the strain gauge mounted on the beam os also
strain and the resistance changes. The measure of change in resistance gives the
value of acceleration which is obtained using Wheatstone bridge.
2. Explain in detail about piezo electric and variable reluctance accelerometer.
Variable reluctance Accelerometer.
The variation in the magnetic reluctance induces the voltage in the
surrounding coil due to the movement of the mass.
When the accelerometer is mounted on the accelerating body, the
supporting spring(ss) and the mass(A/M) undergoes a deflection proportional to
the acceleration and the proximity with respect to the differential pair of core coil
of the picks changes flux pattern so emf is generated which after rectification and
flitering gives the measure of acceleration.
Piezo-electric accelerometer:
Based on piezo electric effect. when a crystal is subjected to mechanical
stress, a voltage is generated proportional to applied stress.
When the accelerometer is exposed to acceleration the spring loaded test
mass stress the mass by a force F=ma, resulting voltage is generated across the
crystal. This voltage is picked up by the electrodes which gives the measure of
acceleration.
3. Say the types of vibration measurement. Explain seismic instrument?
Methods of vibrational measurement
(i)
based on sensor
(ii)
mechanical method
(iii)
electrical method
(iv)
optical method
Seismic instrument:
The seismic instrument enables the measurement of velocity , vibration
and acceleration. It constitutes of mass-damper-spring combination inside a chamber.
This housing chamber is rigidly connected to the vehicle whose vibration/velocity/
acceleration is to be measured. The seismic system consist of a proof mass M supported
by means of a spring and provided with a damping arrangement of a dash-pot type.
It has only single degree of freedom and the mass moves along
perpendicular to platform axis only, the absolute motion of the platform is represented by
x1(t) and the mass movement by xm(t), when the platform is at rest no movement of the
mass so output is zero, when subjected to acceleration the mass displaces and this
displacement is proportional to acceleration.
4. How relative motion is measured using seismic instruments
For measuring relative motions the measuring system if attached to a fixed point.
The instrument consist of a rotating recording drum together with a inking pen. This pen
is coupled with a vibrating body of the system and a spring is used to keep the follower
pin pressed against the vibrating body. The thrust excreted by the spring together with the
mass of the acceleration body determines the maximum acceleration of the body.
When the vibrating object moves with certain acceleration in the downward
direction the spring force has to overcome the inertia force of the follower pin and the
system of levers in order to keep the pin pressed against the vibrating body so that it will
not disengage . in order to ensure that the effect of spring on the vibrating object is
minimum. The variation in the length of the spring should be small as possible
5. What is vibrational pick-up? How it is calibrated?
6. Give some commonly used specific density units in industries.
1. For petroleum products the American petroleum Institute(API)
introduced unit of API degree
API degree=(1.5/SG at 60deg)-131.5
2. The brewing and sugar industries use balling degrees to express the
percentage of worth or weight percentage of dissolved solids at 60F
3. the tanning and tanning extract industry express Sg in Barkometer
degree(Br).
4. the density of the acids and heavy syrups is expressed in baume degress.
5. the sugar industry uses Brix degree(Br)
6. Dairy and dairy product uses Quevenne deg.
7. Alcohol industry uses Sikes , Richter, tralles. Also uses unit of “proof”
8. Sugar, Tanning. And acid industries also uses twadell degrees.
7. Explain pressure head type and bridge type density meter?
Pressure head type density meter
 the pressure at the bottom of the tank of constant liquid column is
proportional to density
 the weight of the given volume of the fluid is proportional to density.
This method compares the hydrostatic pressure due to the heights of liquids in
two tanks. One is reference tank consists of liquid of constant height and
density. Other tank maintains the height constant by overflow so the
manometer can be directly calibrated in the terms of the density of the liquid
of the measuring tank. The relation
Density =K1+K2h
K1=(h1/h2)density of ref, K2=(1/h2)density of unknown.
Bridge type gas densitometer
The fluid bridge is mounted in the vertical plane. The reference gas enters
through R, splits and enters into two branches and passed though two thermal detectors
D1 & D2 and the temperature is noted and this detectors are wires into an electrical
Wheatstone bridge and the value is noted now the bridge will be in balanced condition.
When the4 sample gas is passed through S, it splits and mixes with the
reference gas. If the density of sample gas is more amount of sample gas will pass
through D2 causing rise in temperature, and if the density of sample gas is lighter than
the reference gas more amount of sample gas will flow through D1. in both the case the
bridge will be unbalanced and the output of the Wheatstone bridge gives the value of
density of sample gas
8. Explain float type and ultrasonic type density meter.
Float type densitometer
(i)
based on Archimedes law
(ii)
weight of given volume of liquid is proportional to density
(iii)
the buoyancy force is proportional to density of liquid
the container is filled with liquid whose density to be measured. The float fully
submerged inside the liquid. The float is mechanically connected with a force sensing
element. When the displacer fully submerged in the liquid it is buoyed up with a force
equal to weight of the liquid displaced. This force is measured using force sensing
element which is proportional to density.
Ultrasonic densitometer
The change in sound velocity in the fluid is proportional to density of the
liquid.
The transducer includes a sound source and microphone. The waves propagation is
measured by sensing the phase difference between the transmitted and received signals.
Indirectly the sound velocity can be determined by measuring the resonant frequency of
the cavity that is excited by a microphone sound, source feedback amplifier circuit.
9. How saybolt viscometer is used for measuring density
Viscosity is the property of a liquid to resist shearing of the layers. It consist of
a tank containing the test liquid. A caplliary tube is attached to bottom of the tube.
The tank is surrounded by a constant temperature bath. A 60ml collecting flask is
placed below the capillary tube.
When the temperature of the capillary and the bath is same the capillary tube
is opened and the time taken to collect 60 ml of the sample liquid is noted, which is
proportional to the viscosity of the liquid at constant temperature T
V=[(πgR4t)/8Q(l+λ)][h-kVm2/g]
The empirical law is given by
V=At+B/t
10. Explain about rotameter type viscometer
It consists of a variable area or tapered tube mounted in a bypass
line.It consists of one meter pump that draws some of the liquid from the main process
stream and pumps it to the tapered section.In tapered section there are 2 floats. One is
flow rate setting float & another one is viscosity measuring float. Using LVDT the output
is taken in electrical form.
Liquid whose viscosity to be measured is pumped out by a
constant flow pump. The flow rate float is set & it is controlled. At a constant flow rate
the float will move up or down depending upon the fluid viscosity.