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MODULE V
FLOW MEASUREMENT
Magnetic Flow Meters
• Based on Faradays Laws of Electromagnetic
Induction
• Whenever A conductor moves through a magnetic
field of given field strength, a voltage is induced on
the conductor proportional to the relative velocity
between the conductor and the magnetic field
• They offer true noninvasive measurements.
• They can measure reverse flows and are insensitive to
viscosity, density, and flow disturbances
• Electromagnetic flowmeters can rapidly respond to flow
changes and they are linear devices for a wide range of
measurements
Magnetic Flow Meters
• This law states that if a conductor of length l (m) is moving with a
velocity v (m/s–1), perpendicular to a magnetic field of flux density B
(Tesla), then the induced voltage e across the ends of conductor can
be expressed by:
•
•
E= BDV and Q=AV
here, Q= volumetric flow rate, A= cross sectional area of flow
meter,v= fluid velocity.
• V= E/BL and Q= EA/BL,
• Therefore Induced voltage is directly proportional and linear to
volumetric flow rate.
Magnetic Flow Meters
Magnetic Flow Meters
• Consists of electrically insulated or non conducting pipe (fibre
glass),with a pair of electrodes mounted opposite to each other and a
flush with the inside walls of the pipe . A magnetic coil wound around
the pipe so that magnetic field is generated in a plane mutually
perpendicular to the axis of the flow meter body and to the plane of
the electrodes. Electrically conductive flowing liquids works as the
conductor.
• here, Length = distance between the electrodes= Pipe Diammeter
• As the liquid passes through the pipe section , it passes through the
magnetic field set up by magnetic coils , thus inducing a voltage in
the liquid which is detected by a pair of electrodes mounted on the
pipe wall.
• Amplitude of the induced voltage is proportional to the fluid
velocity.
Magnetic Flow Meters
Advantages
• Handle slurries ,greassy
materials, corrosive fluids
• Supports low flows and very
high flow rate
• Obstruction less and low
pressure drop.
• Bidirectional meter
Disadvantages
• Works only on conductive
fluids.
• Must be full at alltimes
• Expensive
Turbine Flow meter
• Works on the principle of turbine.
• Rotor velocity is proportional to fluid velocity.
Turbine Flow meter
• Consists of turbine wheel(rotor) which is mounted 900 to the axis of
the flowing liquid. It is free to rotate about its axis. When the flowing
liquid strikes the turbine blades, causes it to rotate . Speed is
monitored using Magnetic pick up coil ,which is filled to the
outside of the meter housing Rotor velocity is proportional to
fluid velocity and hence to the volumetric flow rate..
• Magnetic pick up coil consists of a permanent magnet with coil
windings which is mounted in close proximity to the rotor, but
internal to the fluid channel.
• As the rotor blade passes the magnetic pick up coil , it generate
a voltage pulse which is a measure of flow rate.
• The total number of pulses gives a measure of the total pulse.
•
K=T f/Q
here K-pulses per volume unit, t- time constant in minutes ,
f-frequency in hertz and Q- volumetric flow rate.
Turbine Flow meter
ADVANTAGES
DISADVANTAGES
•High Accuracy
•Only For Low Viscosities
•Suitable for Extreme
Temperatures and
Pressures
•Moving Parts
•Can Be Used On Gas or
Liquid
•Sensitive to Flow Profile
Ultrasonic flow meter
various types of ultrasonic flow meters
• (1) Transit time (most widely used type): This type of
ultrasonic flowmeter makes use of the difference in the
time for a sonic pulse to travel a fixed distance. First against
the flow and then in the direction of flow. sensitive to
suspended solids or air bubbles in the fluid.
(2) Doppler: This type is more popular and less expensive, but
is not considered as accurate as the transit time flow meter.
• It makes use of the Doppler frequency shift caused by
sound reflected or scattered from suspensions in the flow
path and is therefore more complementary than
competitive to transit time flow meters.
Principle of transit time flowmeters.
Transit Time Flowmeter
• Principle of Operation
• The acoustic method of discharge measurement is based
on the fact that the propagation velocity of an acoustic
wave and the flow velocity are summed vectorially.
• This type of flowmeter measures the difference in
transit times between two ultrasonic pulses transmitted
upstream t21 and downstream t12 across the flow.
• If there are no transverse flow components in the
conduit, these two transmit times of acoustic pulses are
given by:
Since the transducers are generally used both as transmitters and receivers, the
difference in travel time can be determined with the same pair of transducers.
Thus, the mean axial velocity along the path is given by:
pH MEASUREMENT
pH Meter
• pH is a measure of acidity/Alkalinity of an aqueous a solution
• It is measured on a scale of 0 to 14.
• If pH is less than 7,solution is acidic. A neutral solution has pH of 7
and alkaline solutions have pH greater than 7.
• The pH value of a substance is directly related to the ratio of the
hydrogen ion and hydroxyl ion concentrations.
• If the H+ concentration is higher than OH- the material is acidic.
• If the OH- concentration is higher than H+ the material is basic
• Measured by concentration hydrogen ion in a solution. Ie,
pH= -log(concentration of H+).
pH Meter
• pH is measured by immersing a special electrode and
reference electrode into the solution .
• When an electrode is immersed in a solution , potential
arises at the electrode known as electrode potential
which depends on the ion concentrations of the
electrolyte in the solution.
• A pH meter consists of pH measuring electrode, reference
electrode and high input impedance meter. pH measuring
electrode is hydrogen ion sensitive glass hub. The
reference electrode output does not vary with the activity
of hydrogen ion. Reference electrode gives a constant value
against we measure the potential of the pH electrode .
pH Meter
• The potential difference between the two electrode
is measured. The meter converts this reading into
pH value by using Nernst Equation.
• E=Eo+RT log {a}/nF
• where Eo –constant depends on the reference
electrode. R- universal gas constant , T-absolute
Temperature, N –charge of ion, F-Faradays
constant, a –activity of the ion
Digital pH Meter
pH meter circuit schematic
TORQUE MEASUREMENT
Dynamometer
• A dynamometer is used for measuring the torque and
rotational speed from which power output of an engine
can be calculated.
• A dynamometer can also be used to determine the torque
and power required to operate a driven machine such as a
pump.
• Several types are
• Dry friction dynamometers (Mechanical breaking
device)
• Hydraulic dynamometers (Hydraulic pumps)
• Eddy current dynamometers (Electromagnetic load
device)
Eddy current dynamometers
• Basic Operation
• The engine spins a disk in the dynamometer.
• Electrical current passes through coils surrounding the disk,
and induce a magnetic resistance to the motion of the disk.
• Varying the current varies the load on the engine.
• The dynamometer applies a resistance to the rotation of
the engine.
Eddy current dynamometers
• Several components are typically packaged together in a
dynamometer: the shaft with bearings, the resistance
surface, the resistance mechanism in a “free” rotating
housing, a strain gage, and a speed sensor
Eddy current dynamometers
• The force signal (F) from the strain gage may be
converted into a torque (T) by multiplying by the
distance from the center of the shaft to the pivot
point of the strain gage (R): T = R x F
• If the units are in Newton-meters and shaft speed
(S) is measured in radians per second, then the
shaft power or break power (P) of the engine can
be calculated by multiplying the speed and the
torque: P = T x S
Temperature Measurements
• Types of temperature sensors are:
• Thermocouples,
• Resistive temperature devices (RTDs)
• Thermistors,
23 May 2017
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Temperature Measurements
23 May 2017
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Comparison of Temperature sensors
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Current Measurement
• Hall effect sensor
• Ammeter
Force/Strain and Pressure Measurement
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Force/Strain
Strain Gauge
Piezo electric Transducer
Load cell
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Pressure Measurement
Ionization Transducer
Microphone
Manometer
Manometer
• The manometer is used to measure the pressure ,by
the balancing gravity force and acceleration due to
gravity
• Mainly is used for low differential pressure
measurement
• There are basically two types of manometers.
• U-Tube Manometer
• Well Type Manometer
• Variations of the above basic types are Enlarged-Leg
Type Manometer, Inclined Tube Manometer and
Ring-Balance Type Manometer
U tube manometer
• The unknown pressure is applied in the one arm of
the tube and manometeric liquid filled in the tube
moves in the tube or rises to the constant region
and then the movement is stopped
• If ‘dm‘is the manometric fluid density, ‘d1’ is the
density of the fluid over the manometer, ‘P2’ is the
atmospheric pressure and ‘P1’ is the gas pressure,
and also if d1<<dm, then the differential pressure
can be obtained by the relation
• p1-p2 = h (dm-d1)
U tube manometer
Advantages of U-tube Manometer:
• Simple in construction/Low cost
• Very accurate and sensitive
• It can be used to measure other process variables.
Disadvantages of U-tube Manometer:
• Fragile in construction.
• Very sensitive to temperature changes.
• Error can happen while measuring the h.
Characteristics of liquid used in U-tube Manometer:
• Viscosity should be low.
• Low surface tension is required.
• The liquid should stick on the walls
Well Type Manometer
• The main difference between a U-tube manometer and a well type
manometer is that U-tube is substituted by a large well and instead of
measuring a differential height, a single height in the remaining
column is measured
• If a1 and a2 are the areas of the well and the capillary, and if (h1-h2)
is the difference in height in the well due to the pressure difference
(p1-p2)
• p1-p2 = dm.h (1+a2/a1)
Enlarged-Leg Manometer
Inclined tube Manometer
• The inclined tube manometer is
an enlarged leg manometer
with its measuring leg inclined
to the vertical axis by an angle
b. The factor cosb expands the
scale of the instrument
• p1-p2 = dm.h
• p1-p2 = dm.h.Cosb (1+a2/a1)
Strain Measurement
• Piezo electric Transducer
• Strain Guage
• Resistance Measurement
• Resistance Transducer
• Wheastons bridge
• Wein Bridge
Capacitance Measurement
• Capacitive Transducer
• Schering bridge
• Inductance Measurement
• Inductance Transducer
• LVDT
• Hays Bridge
Voltage Measurement
Voltmeter
• Analog voltmeters move a pointer across a scale in
proportion to the voltage of the circuit; digital voltmeters
give a numerical display of voltage by use of an analog to
digital converter.
• A voltmeter is an instrument used for measuring
electrical potential difference between two points in an
electric circuit.
Voltmeter
• A digital voltmeter, or DVM, is used to take highly
accurate voltage measurements.
• Digital Voltmeter is an instrument which use to
measured the voltage & display the measured
voltage using LCDs or LEDs to display the result in
a floating point format.
• Digital voltmeters usually have scales that are 00.3v, 0-3v, 0-30v, 0-300v.
• A digital voltmeter typically consists of an analog
to digital converter (A/D) with a digital display.