Metrology unit-v (7-10-2015)

METROLOGY AND MEASUREMENTSOBJECTIVE

To Provide knowledge on various metrological equipments.To utilize various techniques used to measure the dimension of the components.Knowledge to adopt correct procedure to measure the dimension of the components.To study about advances in metrology.

( UNIT-5) SYLLABUS MEASUREMENT OF POWER, FLOW AND

TEMPERATURE Force, torque, power - mechanical , Pneumatic, Hydraulic and Electrical type. Flow measurement: Venturimeter, Orifice meter, rotameter, pitot tube – Temperature: bimetallic strip, thermocouples, electrical resistance thermometer – Reliability and Calibration – Readability and Reliability.

M E A S U R E M E N T O F F O R C E

Compiled by P.Sathish kumar - UIT

Measurement of force is done by Direct methods: Involves direct comparison with a known gravitational force on a standard mass, e.g., by a balanceIndirect methods: It involves measurement of effect of force on a body, such as acceleration of a body of known mass subjected to force.

F O R C E M E A S U R E M E N T- D I R E C T M E T H O D

PEND

ULUM

SCA

LE


F O R C E M E A S U R E M E N T- D I R E C T M E T H O D

Spring balance used for direct measurement of force by using coil spring deflection.

It consists of spring located at one end and the load applied at other end.

The displacement at the free end due to the applied force is shown by a pointer moving on a scale.The deflection of the spring when a force is applied at the free end is given by the equation


INDIRECT METHODFORCE MEASUREMENT BY ACCELEROMETERAcceleration is defined as the rate of

change of velocity; and velocity is defined as the rate of change of position. Velocity is commonly called speed, so the definition of acceleration could also be the rate of change of speed. Accelerometers are sensors that convert the motion that represents the aspect of acceleration into an electrical signal. These devices are typically used to measure vibration on machines and structures and other related acceleration detectors and motion detectors for mines, highways, and bridges that are located in areas subject to earthquakes.


The most recent application for accelerometers is to measure vibration on large machines with pumps or motors with bearings that tend to wear out. When these devices wear out unexpectedly, they cause downtime that is very expensive. If the vibration is monitored, the amount of wear on bearings and gears can be calculated and the parts can be changed during scheduled maintenance so that failure will not cause downtime.

INDIRECT METHODFORCE MEASUREMENT BY ACCELEROMETER


INDIRECT METHODFORCE MEASUREMENT BY ACCELEROMETER

A force will make a body to accelerate. By measuring the acceleration, the force may be determined, from the equation F=ma. To measure acceleration, accelerometers are used. Compiled by P.Sathish kumar - UIT

INDIRECT METHODFORCE MEASUREMENT BY HYDRAULIC LOAD CELL


INDIRECT METHODFORCE MEASUREMENT BY HYDRAULIC LOAD CELLBasic Principle of Hydraulic Load cell

When a force is applied on a liquid medium contained in a confined space, the pressure of the liquid increases. This increase in pressure of the liquid is proportional to the applied force. Hence a measure of the increase in pressure of the liquid becomes a measure of the applied force when calibrated. Compiled by P.Sathish kumar - UIT

INDIRECT METHODFORCE MEASUREMENT BY HYDRAULIC LOAD CELLOperation of Hydraulic Load Cell

The force to be measured is applied to the piston.The applied force moves the piston downwards and deflects the diaphragm and this deflection of the diaphragm increases the pressure in the liquid medium (oil).This increase in pressure of the liquid medium is proportional to the applied force. The increase in pressure is measured by the pressure gauge which is connected to the liquid medium.The pressure is calibrated in force units and hence the indication in the pressure gauge becomes a measure of force applied on the piston.Compiled by P.Sathish kumar - UIT


INDIRECT METHODFORCE MEASUREMENT BY PNEUMATIC LOAD CELL

INDIRECT METHODFORCE MEASUREMENT BY PNEUMATIC LOAD CELLOperation of Pneumatic Load cell

The force to be measured is applied to the top side of the diaphragm. Due to this force, the diaphragm deflects and causes the flapper to shut-off the nozzle opening. Now an air supply is provided at the bottom of the diaphragm. As the flapper closes the nozzle opening, a back pressure results underneath the diagram applied force when calibrated.Compiled by P.Sathish kumar - UIT

INDIRECT METHODFORCE MEASUREMENT BY PNEUMATIC LOAD CELLOperation of Pneumatic Load cell

This back pressure acts on the diaphragm producing an upward force. Air pressure is regulated until the diaphragm returns to the pre-loaded position which is indicated by air which comes out of the nozzle. At this stage, the corresponding pressure indicated by the pressure gauge becomes a measure of the applied force when calibrated.



FORCE MEASUREMENT BY INDIRECT METHODSTRAIN GAUGE LOAD CELL


INDIRECT METHODFORCE MEASUREMENT BY STRAIN GAUGEA load cell is a device that is used to convert a force into electrical signal. Strain gauge load cells are the most common types of load cells. Load cells are used for quick and precise measurements. Compared with other sensors, load cells are relatively more affordable and have a longer life span.The principle of operation is that the resistance of the electrical conductor changes when its length changes due to stress.


INDIRECT METHODFORCE MEASUREMENT BY STRAIN GAUGECu Ni alloy is commonly used in strain gauge construction . The change in resistance of the strain gauge can be utilized to measure strain accurately when connected to an appropriate measuring circuit. A load cell usually consists of four strain gauges in a Wheatstone bridge configuration. The electrical signal output is typically very small in the order of a few millivolts. It is amplified by an instrumentation amplifier before sending it to the measurement system. The output can be Digital or Analog (0-5V) depending on the application.

T O R Q U E M E A S U R E M E N T


Torque : It is a measure of the ability of a force to cause an object to turn or rotate.

Equal to the product of the force vector and the radius vector from the axis of rotation to the point of application of the force.

A shaft rotating with angular velocity and carrying power will undergo a torque T, where

P = T

T O R Q U E


T O R Q U E R E A C T I O N M E T H O D


Any system involving torque transmission through a shaft of the transmitted torque can be measured by cradling (concept of mounting sources or sinks (power absorber) in bearings is called cradling) then measuring the reaction force F and the arm length L.

Torque is then calculated as F x L

Pendulum scales are commonly used for measuring the reaction force.

T O R Q U E R E A C T I O N M E T H O D



P R O N E Y B R A K E f o r t o r q u e m e a s u r e m e n t


It is one of the earliest methods of measuring the torque in a rotating shaft.In this system, all the power produced is absorbed by friction in a brake.A rope or belt brake is wrapped around a flywheel carried by the shaft.The rope passes once around the flywheel and is attached to a mass, m at the bottom. The other end of the rope is connected to a spring balance which measures the tension in the rope, Fs.

P R O N E Y B R A K E


The force in the lower end of the rope arises from the weight, and is mg.If the spring balance reading is Fs, the difference in tension between the ends of the rope is (mg – Fs)

P R O N E Y B R A K E




Strain gauge transducer converts torque into electrical signal.

The strain gauge is bonded to a beam or structural member that deforms when a torque or force is applied.

Deflection induces a stress that changes its resistance.

A Wheatstone bridge converts the resistance change into a calibrated output signal.

The sensor's output is a function of force and distance.

Two common ways to obtain torque measurements are by strain-gauging the shaft and by using in-line torque cells. Compiled by P.Sathish kumar - UIT

T O R Q U E M E A S U R E M E N T U S I N G S T R A I N G A U G E S


DY N A M O M E T E RA dynamometer is a machine used to measure torque and rotational speed from which power produced by a motor can be measured.


DY N A M O M E T E R.The main components of a dynamometer are:Base ,PedestalsCollection BoxRotor (Shaft) – The rotating elementStator (Housing) – Encases and supports the rotor. Also assists in cooling.Lubrication System (Pump, Tubing etc.) – Provides lubrication and cooling for rotor bearing assemblies.Load Cell – Provides a calibrated output that is usually measured as torque.

A drive motor (the unit to be tested) is physically connected to the dynamometer rotor shaft using adaptors. A flow of coolant then begins to run thru passages inside of the dynamometer to absorb heat that will be generated from the “braking” action of the dynamometer. The drive motor begins to rotate, and in turn rotates the dynamometer rotor shaft. When the drive motor and rotor shaft reach the desired speed a “brake” is applied to the rotating shaft housed inside the dynamometer. Application of the “brake” creates a counter rotational force causing the load cell to deflect. The movement of the load cell is measured and is converted to torque. The drive motor power output is then calculated by using the following formula:


D C DY N A M O M E T E R f o r p o w e r m e a s u r e m e n t

• The armature of the dynamometer is rotated by coupling it to the shaft of the motor under test. The rotation of the armature coils in a uniform magnetic field results in an induced emf in them (generator action). If the armature circuit is completed by connecting the armature coils to a resistive load, a current will flow in the armature winding. The magnitude of the current depends upon the induced emf and the load resistance. The current-carrying conductors in a uniform magnetic field now experience a force acting on them (motor action). The direction of the force is such that it tends to resist the rotation of the armature. Because the stator, which houses the field windings, is free to rotate, it is pulled around equally by the motor action. The only restraining force acting on the stator is provided by the spring, very much like the prony brake test. By controlling the flux in the motor, we can control the speed of the motor and make it run at any desired speed.

• A 5-hp motor rated at 1200 rpm is tested on a dynamometer whose torque arm is 40 cm in length.


D . C . DY N A M O M E T E R


E D DY C U R R E N T DY N A M O M E T E R

• an internal combustion engine connected to an eddy-current dynamometer. The engine is considered a nonlinear angular velocity source (ϖE) modulated by the throttle setting θ(t). The main energy storage is associated with the rotating inertia JE, lumped at the output of the engine shaft. The torque transmission shaft has compliance and energy dissipation, and is modeled with a rotational spring KS and rotational damper BS. The shaft inertia is neglected.



• The dynamometer consists of a toothed rotor JR that rotates (ϖR) in the magnetic field created by passing current (t) through the stator windings. A voltage is induced in the conductive rotor rotating in the stator magnetic field (Faraday’s Law). This induced voltage creates eddy currents in the rotor that generate a magnetic field (Ampere’s Law), which opposes the stator magnetic field (Lenz’s Law).

• The stator inertia JS, mounted in trunnion bearings, is free to rotate, but is restrained by a torque arm to measure the torque developed.Compiled by P.Sathish kumar - UIT



P R E S S U R E M E A S U R E M E N T

• Bourdon Tubes are known for its very high range of differential pressure measurement in the range of almost 100,000 psi (700 MPa). It is an elastic type pressure transducer.

• the pressure input is given to a socket which is soldered to the tube at the base. The other end or free end of the device is sealed by a tip. This tip is connected to a segmental lever through an adjustable length link. The lever length may also be adjustable. The segmental lever is suitably pivoted and the spindle holds the pointer as shown in the figure. A hair spring is sometimes used to fasten the spindle of the frame of the instrument to provide necessary tension for proper meshing of the gear teeth and thereby freeing the system from the backlash


B O U R D O N T U B E P R E SS U R E G A U G E

http://www.instrumentationtoday.com/pressure-transducer/2011/09/

• As the fluid pressure enters the bourdon tube, it tries to be reformed and because of a free tip available, this action causes the tip to travel in free space and the tube unwinds. The simultaneous actions of bending and tension due to the internal pressure make a non-linear movement of the free tip. This travel is suitable guided and amplified for the measurement of the internal pressure.


B O U R D O N T U B E P R E SS U R E G A U G E

M E A S U R E M E N T O F F LO W


Measurement of FLOW is done by Mechanical flow meters: Flow is measured by placing an obstruction in the flow pipe, which results in development of torque on vanes due to pressure difference.Electrical flow meters: The electromagnetic field will be produced in a coil corresponding to the amount of fluid flowing in the pipe. But in recent days, flow sensors are found in many consumer products or electro-mechanical transducers are used to find out the flow rate of pipe fluid.

M E A S U R E M E N T O F F LO W


Flow measurement can be done by two kinds of instruments,

Quantity meters: In these, the total quantity of fluid which flows in a given time is measured and an average flow rate is obtained by dividing the total quantity by time.Flow meters: In these, the actual flow rate is measured.


T Y P E S O F F L O W M E T E R S


ORIFICE METER


ORIFICE METERADVANTAGES OF ORIFICE METER

DIS-ADVANTAGES OF ORIFICE METER


VENTURIMETER


VENTURIMETER


V A R I A B L E A R E A M E T E R S - R O TA M E T E R


P I T O T T U B E

P I T O T T U B E


Advantages:Cost effective measurementNo moving partsSimple to use and installLow pressure drop

Disadvantages: Foreign material in a fluid can easily clog pitot tubes and disrupt normal readings, which leads to disastrous results in aircrafts.

Application:Pitot tube is employed in a variety of flow measurement applications like air speed in racing cars and Air Force fighter jets.

In industries, pitot tubes are invariably put into use for measurement of Air flow in pipes, ducts, and stacks, and Liquid flow in pipes, weirs, and open channels


Principle:The basic principle involved in thermistor is, when it is subjected to temperature change, the resistance of the thermistor changes. This change in resistance will be the increase in temperature.

T E M P E R AT U R E M E A S U R E M E N T( E L E C T R I C A L T H E R M A L R E S I S TA N C E )


( E L E C T R I C A L T H E R M A L R E S I S TA N C E )

Advantages:Very high accuracyIt can be manufactured for very small sizesIt is used to measure very high temperature.It has the ability to withstand mechanical and electrical stresses.

Disadvantages: Self heating may occurHighly non-linear behavior over its range of operation.

Application:It is used for varying temperatures.It is used in time delay circuits.Thermistors are used for temperature

compensation.It is used to measure pressure and flow of liquids.


B I M E T A L L I C S T R I P F O R T E M P E R AT U R E M E A S U R E M E N T

A bimetallic strip is used to convert a temperature change into mechanical displacement. It consists of two strips of different metals which expand at different rates as they are heated, usually steel and copper, or in some cases steel and brass. 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

https://en.wikipedia.org/wiki/Steel

https://en.wikipedia.org/wiki/Copper

https://en.wikipedia.org/wiki/Brass

https://en.wikipedia.org/wiki/Rivet

https://en.wikipedia.org/wiki/Brazing

https://en.wikipedia.org/wiki/Welding

https://en.wikipedia.org/wiki/Coefficient_of_thermal_expansion


B I M E TA L L I C S T R I P T E M P E R AT U R E M E A S U R E M E N T

A bimetallic coil from a thermometer reacts to the heat from a lighter, by uncoiling and then coiling back up when the lighter is removed.

T H E R M OC O U P L E


T H E R M O C O U P L E


A Thermocouple is a sensor used to measure temperature. Thermocouples consist of two wire legs made from different metals. The wires legs are welded together at one end, creating a junction. This junction is where the temperature is measured.

A thermocouple is a device consisting of two dissimilar conductors or semiconductors that contact each other at one or more points. A thermocouple produces a voltage when the temperature of one of the contact points differs from the temperature of another, in a process known as the thermoelectric effect or seeback effect

T H E R M O C O U P L E


A Thermocouple is a sensor used to measure temperature. Thermocouples consist of two wire legs made from different metals. The wires legs are welded together at one end, creating a junction. This junction is where the temperature is measured.

A thermocouple is a device consisting of two dissimilar conductors or semiconductors that contact each other at one or more points. A thermocouple produces a voltage when the temperature of one of the contact points differs from the temperature of another, in a process known as the thermoelectric effect or seeback effect

Engineering

Metrology unit-v (7-10-2015)