MODULE V

FLOW MEASUREMENT

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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

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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

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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

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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

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Works on the principle of turbine

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Rotor velocity is proportional to fluid velocity .

Turbine Flow meter

• • • Consists of turbine wheel(rotor) which is mounted 90 0 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 outside of the meter housing .

Speed is monitored using Magnetic pick up coil ,which is filled to the

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.

ADVANTAGES

Turbine Flow meter

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

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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

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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

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This type of flowmeter measures the difference in transit times between two ultrasonic pulses transmitted upstream t 21 and downstream t 12 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

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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

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• • 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

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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

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A dynamometer is used for measuring the torque and rotational speed from which power output of an engine can be calculated

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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)

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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.

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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

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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

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• •

Current Measurement

Hall effect sensor Ammeter

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Force/Strain and Pressure Measurement

Force/Strain Strain Gauge Piezo electric Transducer Load cell

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Pressure Measurement Ionization Transducer Microphone Manometer

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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

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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<

p1-p2 = h (dm-d1)

U tube manometer

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Advantages of U-tube Manometer:

Simple in construction/Low cost Very accurate and sensitive It can be used to measure other process variables .

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Disadvantages of U-tube Manometer:

Fragile in construction.

Very sensitive to temperature changes.

Error can happen while measuring the h.

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Characteristics of liquid used in U-tube Manometer:

Viscosity should be low.

Low surface tension is required.

The liquid should stick on the walls

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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

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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

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p1-p2 = dm.h.Cosb (1+a2/a1)

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Strain Measurement

Piezo electric Transducer Strain Guage

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Resistance Measurement Resistance Transducer Wheastons bridge Wein Bridge

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Capacitance Measurement

Capacitive Transducer Schering bridge

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Inductance Measurement Inductance Transducer LVDT Hays Bridge

Voltage Measurement

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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.

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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.