MODULE V. FLOW MEASUREMENT Magnetic Flow Meters Based on Faradays Laws of Electromagnetic Induction Whenever A conductor moves through a magnetic field.

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 Disadvantages

• Handle slurries ,greassy materials, corrosive fluids

• Supports low flows and very high flow rate

• Obstruction less and low pressure drop.

• Bidirectional meter

• 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 meterADVANTAGES

DISADVANTAGES

•High Accuracy•Suitable for Extreme

Temperatures and Pressures

•Can Be Used On Gas or Liquid

•Only For Low Viscosities•Moving Parts•Sensitive to Flow Profile

Ultrasonic flow metervarious 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

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Temperature Measurements• Types of temperature sensors are:

• Thermocouples, • Resistive temperature devices (RTDs)• Thermistors,

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Temperature Measurements

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Comparison of Temperature sensors

Current Measurement• Hall effect sensor • Ammeter

Force/Strain and Pressure Measurement• Force/Strain• Strain Gauge• Piezo electric Transducer• Load cell

• 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 manometerAdvantages 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

• p1-p2 = dm.h

• 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.Cosb (1+a2/a1)

Inclined tube Manometer

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 0-0.3v, 0-3v, 0-30v, 0-300v.

• A digital voltmeter typically consists of an analog to digital converter (A/D) with a digital display.