Unit-II
Measurement of Acceleration, vibration, density
Part –A
- Define acceleration?
The ratio in which the velocity changes with respect to time, measured in m/s2
A=dv/dt,
- What is seismic Instrument?
A system which consist of a mass- spring and damper combination housed in a chamber is called seismic instrument.
- 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
- What are the methods of vibration measurement?
(i)methods based on sensor
(ii)mechanical measurement
(iii)electrical measurement
(iv)optical measurement
- What are the types of accelerometer?
(i)LVDT Accelerometer
(ii)Seismic accelerometer
(iii)Piezo-electric accelerometer
(iv)Strain gauge accelerometer
(v)Variable reluctance accelerometer
- Give different modes of Seismic instruments?
(i)Displacement mode: with large Mass and soft Spring
(ii)Acceleration mode: with soft mass and stiff spring
- 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,..
- 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
- What are the choices of vibrational pick up?
(i)mechanical impendence pick-up
(ii)sensitivity
(iii)frequency range
(iv)phase characteristics
- 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.
- Give some units of density?
Kg/m3, Kg/liter, g/cm3, ounce/inch3, pounds/inch3, pounds /gallon, pounds/bushel, slug/feel3
- 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
- What is baume Scale?
The Baume scale is actually two scale, one liquids heavier than water and other liquids lighter that water, ◦Be
- 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
- what is specific viscosity?
It is the ratio of absolute viscosity of the fluid to absolute viscosity of the standard fliud at constant temperature.
- 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
- 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.
- Define Dynamic viscosity.
It is the ratio of Shear stress to velocity gradient
μ= τ/(dv/dy)
- 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
- 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
- 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.
- 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.
- 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.
- 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
- What is vibrational pick-up? How it is calibrated?
- 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.
- 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
- 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.