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FLOW MEASUREMENT About This Chapter We are learning this chapter 4 hours for 8 marks. Flow measurement is important in hydraulic, chemical and petrochemical industry. We are learning instruments to measure flow of liquid, slurry and corrosive fluids. The important instruments are variable area meter, variable head meter, rotameter, electromagnetic flow meter and ultrasonic flow meter.

5.1 MEASUREMENT OF FLOW Measurement of flow is i m p o r t a n t f o r l a b o r a t o r y w o r k a s w e l l a s f o r i n d u s t r i a l a p p l i c a t i o n s . T h e s e l e c t i o n of p a r t i c u l a r f l ow m e a s u r i n g e q u i p m e n t d e p e n d s o n t he n a t u r e of f l o w . F l o w m e a s u r i n g d e v i c e s a r e g e n e r a l l y of t w o t y p e s n a m e l y q u a n t i ty meter a n d r a t e m e t e r.

5.2 DIFFERENT TYPES OF FLOW F l u i d f l ow is c l a s s i f i e d a s :

1. Steady and Unsteady Flow : S t e a d y f l ow is d e f i n e d a s ' t h e f l ow in w h i c h the f l u i d c h a r a c t e r i s t i c like v e l o c i t y, p r e s s u r e , d e n s i t y e t c . a t a p o i n t d o e s not c h a n g e w i t h t i m e ' . T h u s , in s t e a d y f l ow w e can s ay *o, Yo> z o xo , yo> z 0 xo, yo,

W h e r e ( x 0 , yo, z0) is f i x e d p o i n t in f l u i d flow.

U n s t e a d y f l ow is ' t h a t t y p e of f l ow in w h i c h v e l o c i t y , p r e s s u r e a n d d e n s i t y at a point c h a n g e s w i t h r e s p e c t t o t i m e ' . T h u s , f o r u n s t e a d y f l ow : ^ v-Xo,z yo*, Zo° ' ® xx v z - ° ' ^ U z / ° 0 , yo, Zo xo, yo, zo

2. Uniform and Non-uniform Flow : U n i f o rm f l ow is t y p e of f l ow in w h i c h 'the v e l o c i t y at a n y g i v e n t i m e d o e s n o t c h a n g e w i t h r e s p e c t t o s p a c e (i.e. along l e n g t h of d i r e c t i o n of f l o w ) ' . F o r u n i f o rm

flow, Measurement and Control 5-2 Flow Measu

Where, dV = Change of velocity ds = Length of flow in direction S. Non-uniform flow is type of flow in which the velocity at any given time cha with respect to space. For non-uniforrh flow, Laminar flow is type of flow in which the fluid particles move along well del paths or stream line and all the stream lines are straight and parallel. Thus particle moves in laminas or layers gliding smoothly over the-adjacent layer, type of flow is also called as "stream line flow or viscous flow'.

Turbulent flow is that type of flow in which fluid particle move in zigzag way. to the movement of particle in zigzag way, the eddy are formed which responsible for high energy loss. For a pipe flow, non-dimensional number knowd 'Reynold number" determines the turbulent flow.

VD , Reynold number = — Where, D = Diameter of pipe.

V = Mean velocity of flow. v = Kinematic viscosity of fluid.

If Reynolds number is < 2000, the flow is called laminar flow.

If Reynolds number is > 4000, flow is called as turbulent flow.

If Reynolds number lies between 2000 and 4000, the flow may be laminaij turbulent.

4. Compressible and Incompressible Flow : Compressible flow is that type of flow in which the density of the fluid chat. from point to point or in other words density is not constant for the fluid, compressible flow, p * constant.

Incompressible flow is that type of flow in which the density is constant for fluid flow. For incompressible flow, p â&#x20AC;˘= constant.

5. Rotational and Irrotational Flow : Rotational flow is the type of flow in which the fluid particles while moving stream lines also rotate about their own axis. And if fluid particles while flowi stream line do not rotate about their own axis, flow is called 'irrotational flow'. , 6. One, Two and Three Dimensional Flow : 4 One dimensional flow is type of flow in which the flow parameter such as vel is a function of time and one space co-ordinate only, say x. For a steady dimensional flow, velocity is a function of one space co-ordinate only. For dimensional flow, u = f (x), v = 0 and w = 0.

Where u, v, and w are velocity component in x, y and z direction respectively.

Measurement and Control 5-3 Flow Measurement

Two dimensional flow is the flow in which the velocity is a function of time and rectangular space co-ordinate say x and y. For steady two dimensional flow the velocity is a function of two space co-ordinate only. The variation of velocity in third direction is negligible. Thus, for two dimensional flow, u = fi (x, y), v = f2 (x, y) and w = 0 Three dimensional flow is the flow in which the velocity is a function of time and three mutually perpendicular directions. But for a steady three dimensional flow the fluid parameters are functions of three space co-ordinates (x,- y and z) only. Thus, for three dimensional flow, u = fi (x, y, z ), v = f2 (x, y, z) and w = f3 (x, y, z). ,

5.3 FLOW MEASUREMENT TECHNIQUE Many accurate and reliable methods are available for measurement of flow, some of which js applicable to liquids, some only to vapours and gases and some of them are applicable to both. Fluids measured may be clear or opaque, clean or dirty, wet or dry, erosive or corrosive. Fluid streams may be multiphase, vapours or slurries.

The flow of fluid may be laminar or turbulent. Fluid may have variation in temperature and pressure. The various methods used for flow measurement are : 1. Inferential Type Flow Meter : In this method of measurement, the flow is not directly measured but it is inferred from measurements of other parameters related to flow. The other parameters may be pressure, temperature, force, displacement, velocity etc. The

information sensed by primary sensor is converted into velocity. To determine the volumetric flow rate, velocity is correlated to area of cross-section with respect to time. Various inferential methods of flow measurement are : (a) Variable head or Differential method : In this type of flow meter, rate of flow is inferred from difference in pressure across obstruction or constriction.

Various variable head meters are : (i) Orifice plate. (ii) Venturimeter. (iii) Pitot tube. (iv) Flow nozzle. (b) Rotameter or Variable area meter : In this type of meter, the flow rate is inferred from displacement of object that results from a balance of weight force against a velocity force. (c) Magnetic meter : In this type, of flow meter, the flow rate is inferred from velocity of flow utilizing Faradays law of electromagnetic induction. (d) Turbine meter : In this type of meter, the velocity of flow is inferred from rotational speed of rotor. The rotor speed is proportional to velocity of flow through meter.

Measurement and Control 5-4 Flow Measurement

(e) T h e r m a l f l ow m e t e r : In t h i s t y p e of f l ow m e t e r , t h e f l ow rate is i n f e r r ed f r om r i s e in t e m p e r a t u r e of a s t r e a m s u b j e c t e d to h e a t f l u x. (f) Swirl m e t e r s : In t h i s t y p e of m e t e r , t h e f l ow rate is i n f e r r e d f r om p r e s s u re a n d t e m p e r a t u r e o s c i l l a t i o n s.

2. Positive D i s p l a c e m e n t Method : P o s i t i v e d i s p l a c e m e n t m e t e r s a r e v o l u m e t y p e m e t e r s , w h i c h c a p t u r e a n d release a f i x e d v o l u m e of l i q u i d in its s t r o k e . T h e s e m e t e r s c o u n t s t h e n u m b e r of s t r o k e s orj c y c l e s of o p e r a t i o n and gives the v o l u m e h a n d l e d . The d e v i c e s used may be v o l u m e t r i c p u m p s d e l i v e r i n g a f i x e d v o l u m e per p i s t o n , v a n e or d i a p h r a g m s t r o k e orj m o v e m e n t . T h e s e m e t e r s g i v e v o l u m e but if d e n s i t y r e m a i n s c o n s t a n t , - i t a l s o g i v e^ m a s s f l ow r a t e . V a r i o u s p o s i t i v e d i s p l a c e m e n t d e v i c e s a r e : (a) R e c i p r o c a t i n g p i s t o n. (b) S e m i - r o t a r y p i s t o n. (c) N u t a t i n g d i s c . .

1 5.4 VARIABLE HEAP FLOW METERS OR PIFFEI B a s i c W o r k i n g P r i n c i p le V a r i a b l e head flow meters operate on the principle that 'a restrictior ( o b s t r u c t i o n ) in t h e line of f l o w i n g f l u i d , ' i n t r o d u c e d by t h e o r i f i c e p l a t e o r ventur t u b e or e l b o w , p r o d u c e s a d i f f e r e n t i a l p r e s s u r e a c r o s s t h e r e s t r i c t i o n e l e m e n t , whic is p r o p o r t i o n a l to the flow rate'. T h e d i f f e r e n c e in p r e s s u r e b e f o r e and after th o b s t r u c t i o n is m e a s u r e d by m e a n s of d i f f e r e n t i a l p r e s s u r e s e n s o r.

T h e e q u a t i o n c o n v e n i e n t f o r p r a c t i c a l u s e is d e v e l o p e d w i t h f o l l o w i n g s t e p s.

1. Write g e n e r a l e n e r g y e q u a t i o n a n d m o d i f y it f o r a s s u m e d c o n d i t i o n s.

2. S o l v e t h e e q u a t i o n for d o w n s t r e a m v e l o c i t y.

3. Get f l ow r a t e by p r o d u c t of a r e a a n d v e l o c i t y.

E n e r g y at a n y s e c t i o n of f l u i d c o n s i s t s of i n t e r n a l e n e r g y (U), f l ow w o r k (pv p/p), k i n e t i c e n e r g y (^j and p o t e n t i a l e n e r g y ( g z ) . For f l u i d f l o w , heat (Q) and wc (W) c a n a l s o be c o n s i d e r e d.

C o n s i d e r i n g t h a t e n e r g y c a n n o t be c r e a t e d nor d e s t r o y e d , w e c a n w r i t e :

l

Pi + 2 + g Z l + Q U 2+ ^ + y + g z 2+ W . For m e a s u r e m e n t of flow, flow is a s s u m e d as s t e a d y flow with follow! โ ขassumptions: 1. The f l u i d f l ow is a d i a b a t i c , i.e. no t r a n s f e r of h e a t a n d w o r k ( Q = 0) 2. No m e c h a n i c a l w o r k is d o n e by f l u i d (W = 0 ).

3. Flow is f r i c t i o n l e s s ( U i = U2 ).

4. Flow is h o r i z o n t a l (z1 = z2) 5. Mass f l ow r a t e is c o n s t a n t ( m i = m2 ).

1

Ul+

Measurement and Control 5-5 Flow Measurement

Considering above assumptions, steady flow energy equation can be written as,

Pi 2

4

The abov e equat ion f o rms thPe2 bas2is of operat ion of var iable head flow measur ing device. By replacing density by specific weight, we get,

2g fit Wi w2

The above equation is known as Bernoulli's equation.

The basic equations derived for Venturimeter from Bernoulli's equation : Q=AxV Q = Cd 2 92

V2ih Where, Where, Q K- c K-s/2gh aia?

2

a2 V = Velocity of flowing fluid. ยง4 = Area of inlet. a 2 = Area of throat.

Q = Volume flow rate A = Cross-sectional area through which fluid is passing h = Differential pressure head across restriction g = Acceleration due to gravity Advantages of Differential Flow Meters : 1. For large lines it's cost is low.

2. It has wide application than any type of meter.

3. Accuracy is more about ± 1A to ± 2 %.

4. It can be easily removed without shutting down the process.

5. It is adaptable to any pipe size and flow rate.

Disadvantages or Limitations of Differential Flow Meter : 1. There i s r e l a t i v e l y high permanent pressure loss in it.

2. It is difficult to use for slurry services. • » 3. It do not have linear characteristics.

4. Connecting pipes sometimes present a problem - condensation etc.

5. Low flow rates are not easily measured. such as freezing, Measurement and Control 5-6 Flow Measuremerc

6. Accuracy is dependent on many fluid characteristics such as temperature, pressure, specific gravity, compressibility etc.

7. It is difficult to measure pulsating flow with this type of meter.

Various variable head flow meters are : (a) Orifice plate. (b) Venturimeter. (c) Pitottube. (d) Flow nozzle. (a)Orifice Plate : Orifice plates are simplest and cheapest form of primary elements and are used more frequently than other elements for measurement of flow. The orifice plate consists of a thin circular metal plate with hole in it. A n orifice plate is held in pipeline between two flanges. The differential pressure across it is measured to get flow rate.

There are four types of orifice plates listed below : (i) Concentric type. (ii) Eccentric type. (iii) Segmental type. (iv) Quadrant edge type. , (i) Concentric Orifice Plate : It is the most widely used orifice plate, which is made of stainless steel and its thickness (1/8 to % inch) varies depending on pipe size and flow velocity. It has a circular hole in middle and is installed in pipeline with hole concentric to pipe. It is made from other materials such as nickel, phosphor, bronze etc. to withstand the corrosive effects of t he fluid.

Fig. 5.1 : Concentric orifice {7/y ficce/zfrrc 0r/Wce P/ate ; ft /s s/mz/ar to concentric type, oat /t tras o. hole, which is a circle, the diameter of which is 98% of the pipe diameter. Location offset hole prevents damaging of solid material or foreign particles and makes useful for measuring fluids containing solids.

Fig. 5.2 : Eccentric orifice Measurement and Control 5-7 Flow Measurement

(Hi) Segmental Orifice Plate : Segmental plate orifice is placed at bottom of pipe. The opening is a segment of a circle, which is 0.98 of the pipe diameter. It is nstalled with the dam horizontal and with the curved section of the opening

coincident with the lower surface of pipe. (iv) Quadrant Edge Orifice Plate : This type of orifice plate is used for flows such as heavy crude oil, syrups and slurries and viscous flow. It is constructed in such a way that the edge is rounded to form a quarter-circle. The plate has a concentric opening with a rounded upstream edge rather than the sharp, square Advantages of Orifice Plate : 1. Low cost.

2. Ease in installation and repair.

3. Can be used in wide range of pipe sizes (3.175 to 1828 mm), 4. Requires less space as compared with venturimeter.

5. Available in many materials.

Disadvantages of Orifice Plate : 1. Cause relatively high permanent pressure loss.

2. Tend to clog, thus reducing use in slurry service.

3. Have square root characteristics.

4. Accuracy depends on care during installations.

5. Changing characteristics because of erosion, corrosion and scaling.

6. Coefficient of discharge has low value, (b) Venturimeter : A venturimeter is used where permanent pressure loss is of prime importance and where maximum accuracy is desired of high viscous fluids.

Fig. 5.3 : Segmental orifice

edge normally used.

Fig. 5.4 : Quadrant edge orifice Measurement and Control 5-8 Flow Measu

Convergent core

\ Divergent cone

TX Fig. 5.5 : Venturimeter Venturimeterconsistsof: 1. S t r a i g h t inlet s e c t i o n : I t is s a m e a s t h e d i a m e t e r of t h e p i p e a n d i n w h i g h p r e s s u r e t a p is l o c a t e d . For s a t i s f a c t o r y o p e r a t i o n a s t r a i g ht l e n g t h equal to. 5 t o 10 t i m e s t h e d i a m e t e r of i n l e t s e c t i o n is u s e d be fitting venturimeter.

2. A c o n v e r g i n g c o n i c a l s e c t i o n : In c o n v e r g i n g p o r t i o n , c r o s s - s e c t i o n a l a d e c r e a s e s a n d v e l o c i t y of f l ow i n c r e a s e s . T h e c o n v e r g i n g p o r t i o n is provi w i t h i n c l u d e d a n g l e 21°. T h e p r e s s u r e h e a d d e c r e a s e s in t h i s p o r t i on 3. Throat 3. T h r o a tI l:^ I.t^ ^is^ cMylji^nd^?rocfa.TJ~^sne'cimci6TMn• uV^^^^^^^^:^a.?r e a . A t t h i ,5.a_•^'• "-"is s e c t i o n , v e l o c i t y, m a x i m u m a n d p r e s s u r e is m i n i m u m . T h e t h r o a t d i a m e t e r is 1/4 t o 1/2 of i p i p e d i a m e t e r . L e n g t h of t h r o a t is e q u a l to i t s d i a m e t e r . P r e s s u r e tap providedatthroat.I 4. D i v e r g i n g or r e c o v e r y cone : In t h i s s e c t i o n , v e l o c i t y d e c r e a s es

d e c r e a s e d v e l o c i t y is r e c o v e r e d as a p r e s s u r e head. To get m a x im r e c o v e r y of e n e r g y , d i f f u s e r s e c t i o n is p r o v i d e d w i t h a n g l e 5째 t o 7째. T h e ang! i s kept l ow s o t h a t t h e f l u i d f l ow has l e a s t t e n d e n c y to s e p a r a t e out fr b o u n d a r y of s e c t i o n . I n o r d e r t o r e d u c e c o s t a n d l e n g t h of s e c t i o n , a n g l e m be i n c r e a s e d t o 14째.

P r e s s u r e taps are l o c a t e d to m e a s u r e d i f f e r e n t i a l p r e s s u r e s at inlet a t h r o a t , w h i c h is u s e d t o d e t e r m i n e a f l ow r a t e of f l o w i n g f l u i d.

F l o w v e l o c i t y t h r o u g h V e n t u r i m e t e r i s g i v e n by, Where, V= V

ai a2 9 h

VaT 2 a2 V e l o c i t y of f l o w i n g f l u i d.

A r e a of i n i e t.

A r e a of t h r o a t.

Accelerationduetogravity.

D i f f e r e n c e of p r e s s u r e h e a d b e t w e e n i n l e t a n d t h r o a t.

Measurement and Control 5-9 Flow Measurement

Advantages of Venturimeter : 1. Causes low permanent pressure loss.

2. Widely used for high flow rates.

3. Available in very large pipe size.

4. More accurate over wide ranges than orifice plate.

5. It has high coefficient of discharge.

Limitations of Venturimeter : i : High cost.

2. More difficult to install due to its longer length construction.

3. Generally not useful below 76.2 mm pipe size.

Difference between Orifice Plate and Venturimeter : Orifice Plate Venturimeter 1. It causes high pressure loss.

2. Non-linear characteristics.

3. Accuracy is less.

4. Its construction is simple., 5. It can be used for pipe size from 4 mm to 1800 mm. , 6. Low cost.

1. Low permanent pressure loss.

2. It gives'linear characteristics.

3. Accuracy is more.

4. It is complicated in construction.

5. It can be used for larger pipes but cannot be used below 76.2 mm.

6. High cost. (c) Pitot Tube : Pilot tube »-

—_N ~FI«i\i/

— r^

Fig. 5.6 : Pitot tube

Pitot tube is primary obstruction type element used for fluid velocity measurement. The measurement of velocity can be converted into discharge. In its amplest form it consists of a glass tube bent at right angles near one end having a small opening. The bent end is placed in the flowing fluid having motion. When liquid Measurement and Control 5-10 Flow Meas

enters this bent tube, its kinetic energy is converted into pressure head, due to liquid rise in tube to height h.

V2 h = which is velocity head. The flow can be calculated by inferring vel Advantages of Pitot Tube : 1. There is negligible loss of pressure head.

2. It is cheaper than venturimeter or orifice meter.

3. It c an be inserted through small hole into pipe.

Limitations of Pitot Tube : 1. It can not be used for liquids that contain suspended solids like slurry.

2. Its sensitivity decreases with decrease in velocity of fluid.

3. It gives non-linear relationship of pressure and velocity.

4. It is unsuitable for d i r ty and stick fluid, (d) Flow Nozzle : Flow nozzle is the compromise between orifice plate and venturi meter, nozzle is similar to venturimeter but without diverging cone. ^\\\\\\\\\\\\\\\\\\^)//////////////////////7 Fluid flow

ZZZZZZZZZZZ22ZZZZZZ Nozzle Fig. 5.7 : Flow nozzle

In general engineering practice, it-is used to create the j e t of high velocity! when it is used at t he end of pipe as metering device it is called as xflow nozzle". its simple form it consists of a cone of gradually decreasing diameter with s entry. Flew nozzles are generally made of gunmetal, stainless steel or monel m Advantages of Flow Nozzle : • 1. i t is cheaper than venturimeter.

2. It c an be installed in existing main pipe easily.

3. As compared to venturimeter it is short in length.

4. It is useful for flow, which contains solid suspended particles.

5. Permanent pressure loss is less than that of orifice plate.

6. It has high coefficient of discharge than orifice meter.

7. Can be used for f l ow Of high velocities, as t h e y are more rugged.

Measurement and Control 5-11 Flow Measurement

Disadvantages of Flow Nozzle : 1. Pressure recovery is poor.

2. It can not be used where low pressure head is available.

3. It can not be used for larger pipe size, 4. More expensive and difficult to install as compared to orifice plate.

Fig. 5.8 : Rotameter Rotameter is variable area meter in which area is varied by a float in a tapered kibe. In variable area meter, the size of area is adjusted by the amount necessary •r>en flow changes. The float of the rotameter adjusts the size of area by rising and tofling in the tapered tube. Depending on the rate of flow, the float takes a position m the tube that increases or decreases the size of the area and thus keeps the inferential pressure constant; therefore rotameter is called as 'variable area flow meter'.

• The rotameter consists of a transparent glass tube, placed vertically»such that the •rger end is at top. This assembly is enclosed in a safety transparent shield for rotection. Flow inlet is provided at bottom of glass tube while flow outlet is at the top of^tube. The glass tube is graduated with a linear scale. When there is no flow, fee float rests at the bottom of the glass as maximum diameter of float is less than me m i n i m um bore of glass tube.

Measurement and Control 5-12 Flow Meas

When float is at equilibrium position forces acting on float are: 1. Downward gravitational force.

2. Upward buoyant force.

3. Pressure force.

4. Viscous drag.

The basic requirement of float is, it should be heavier than the fluid to be measured. The general material used for float is stainless steel, gunmetal, Monel or brass. The shape of float is selected such that it creates the minimum obstruction for fluid flow.

Advantages of Rotameter: 1. Flow rate is linear function of area.

2. It is low cost direct indicating instrument.

3. Pressure loss is constant and very small.

4. It can handle corrosive fluid.

5. The scale of instrument is linear.

6. It can be compensated for change in density and viscosity of fluid.

7. It can be used for metering liquid, gases and vapours.

Limitations of Rotameter: 1. It must be installed in vertical position only.

2. For higher pressure and temperature it is expensive.

3. It cannot be used for flow measurement of slurry.

TURBINE METER

Turbine flow meter is suitable for measurement of flow in tubes and pipes. The rotor is placed in path of moving stream directly. The rotor spins freely at the rate proportional to flow velocity. A permanent magnet is sealed inside the rotor body is polarized at 90 degree to the axis of rotation. As rotor rotates, along with it magnet also rotates and produces rotating magnetic field. This produces an A.C. voltage pulse in the pick-up coil located external to the meter housing. The frequency of this voltage is, directly proportional to the rate of flow. These voltage pulses are counted by means of electronic digital counter to give total flow.

Alternatively, the frequency is converted into voltage and is feed to an analog/digital voltmeter to give the rate of flow.

In turbine flow meter, it is possible to get measurement of total flow as well as rate of flow.

Advantages of Turbine Flow Meter: 1. Very good accuracy Âą 1 % over design range.

2. It has low pressure drop.

3. Analog as well as digital readings can be obtained.

4. It gives fast response.

Disadvantages of Turbine Flow Meter: 1. Bearing maintenance is the problem.

2. Accuracy drops at low flow rate.

3. It is costly.

4. It has very limited applications in slurry flow measurement.

5. It requires inline mounting.

HOT WIRE ANEMOMETERS: Hot wire anemometer is used for the measurement of flow when flow is fluctuating and unsteady. A sensor of 5 micron diameter platinum tungsten wire is welded between two prongs of the probe and heated electrically to form part of Wheatstone circuit. When the probe is introduced in a fluid stream, it tends to get cooled by the velocity of fluid and there is decrease in its resistance. The rate of cooling of sensor probe wire depends upon: (a) Shape, size and properties of hot wire. (b) Temperature difference between hot wire and fluid.

(c) Properties of flowing fluid. (d) Velocity of flowing stream.

From all above properties the only fluid velocity is the variable and rest of the properties remain fairly constant. Thus, instrument response is direct measure of flow velocity.

The heat lost in sensor changes the temperature of wire as well as subsequent change in resistance of wire. Therefore there are two methods of measuring fluid (i) Constant current type. (ii) Constant temperature type.

(i) Constant current type: In constant current mode, the heating current i.e. voltage across the bridge is kept constant. Initially circuit is adjusted such that the galvanometer reads zero current when probe wire lies in stationary air. When air flows, the hot wire cools and changes its resistance. The change in resistance of wire shows the deflection in galvanometer. The galvanometer deflection is calibrated to get flow velocity. (ii) Constant temperature type: In constant temperature mode, the operating resistance of wire and hence the temperature of wire is maintained constant. The hot wire will have tendency to cool when it comes in contact with moving air, the external voltage is applied to keep its temperature constant. The bridge voltage is varied to bring the galvanometer needle to zero; the reading of voltmeter is recorded and correlated with fluid velocity.

Anemometer - Hot film type A modification of hot wire anemometer is the hot film anemometer. It consists of resistance element, which is a thin film of platinum deposited on a glass base. The film takes the place of hot wire and the circuit is similar to hot wire anemometer. The film transducer has great mechanical strength and is used for measurement of very high temperature by constructing them with internal cooling water passage. Hot film transducer is extremely sensitive to fluctuations in the liquid flow velocity and has been used for measurement involving frequencies as high as 50 KHz. Sophisticated instrumentation systems is required for such measurement.

ELECTROMAGNETIC FLOW METER: The magnetic flow meter is traditionally the first type of flow meter considered for highly corrosive application and for applications involving measurement of slurries. These flow meters utilizes the principle of Faraday's Law of electromagnetic induction for flow measurement which states that, 'Whenever conductor moves through magnetic field of given field strength, a voltage is induced in conductor which is proportional to relative velocity between the conductor and magnetic field'.

This concept is used in electric generators.

In case of magnetic flow meter, electrically conductive flowing liquid works as conductor. Therefore, voltage induced in it is given by,

Where, E = Induced voltage in volts C = dimensional constant B = Magnetic field strength L = length of conductor in m V = velocity if conductor in m/s

It consist of electrically insulated or non-conducting pipe such as fiberglass, with pair of electrodes mounted opposite to each other and flushes with inside wall of its pipe. Magnetic coil is wounded around pipe so that magnetic field generated, in plane is mutually perpendicular to the axis of flow meter body and to the plane of electrodes.

The basic operating principle of magnetic flow meter in which a flowing liquid acts as a conductor having length V is a distance between two electrodes and equal to the diameter of pipe. When the liquid passes through the pipe section it cuts the magnetic flux set-up by magnetic coils, then according to Faraday's Law of electromagnetic induction, the emf is induced in it, which is measured between electrodes mounted in pipe walls. The magnitude of emf. induced is directly proportional to the velocity of fluid in pipe.

Advantages of Electromagnetic Flow Meter: 1. It can handle slurries and greasy materials.

2. It can handle corrosive fluids.

3. It has very low pressure drop.

4. It is totally obstruction less.

5. Available in several construction materials.

6. Available in large pipe size and capacities. . 7. Measurement unaffected by density, pressure, temperature etc.

8. Capable of handling extremely low flow rates or very high flow rates.

9. Voltage output of meter is proportional to average velocity and it does depend on whether flow is laminar or turbulent.

Disadvantages of Electromagnetic Flow Meter: 1. Relatively expensive.

2. Works only with fluids, which are electrical conductors.

3. Relatively heavy, especially in large size.

4. Must be explosion-proof when installed in electrical hazard area.

5. Must be full at all times.

ULTRASONIC FLOW METER

Ultrasonic flow meters are based on the apparent change in the velocity of propagation of sound waves in a fluid with the change in the velocity of flow of the fluid. The basic principle of operation of simple ultrasonic flow meter is shown Fig. In simple ultrasonic flow meter, two transducers are inserted into a pipe line at a resistance 'L' apart. The upstream transducer is energized from an electronic oscillator so that it transmits ultrasonic waves in the direction normal to the downstream transducer (which is a receiver).

Let, Vs = Velocity of sound in fluid.

V = Velocity of fluid.

L = Distance between transmitter and receiver, t1 = Transit time along flow. t2 = Transit time against the flow.

Thus, the output signal proportional to t1 - t2 is linear in V for constant Vs. However, the calibration constant is strongly dependent on Vs which depends on the temperature and pressure of the flow.

Advantages of Ultrasonic Flow Meter: 1. It offers negligible obstruction to flow, which is to be measured.

2. It has Âą2 % accuracy of full scale value.

3. It has linear relationship with velocity and output.

4. The output is electrical signal and can be displayed in analog or digital form.

5. The output is insensitive to variation in viscosity, density and temperature.

6. It can be used for bi-directional flow, Disadvantages of Ultrasonic Flow Meter: 1. It is costly; therefore its use is limited to industrial applications only.