Measurement of thermocouple emf using potentiometer & milivoltmeter. Milivoltmeter consists of a PMMC type meter. The coil gets magnetised when the

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Theodora Harvey
5 years ago
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2 Measurement of thermocouple emf using potentiometer & milivoltmeter. Milivoltmeter consists of a PMMC type meter. The coil gets magnetised when the voltage is applied across the coil. The coil rotates until magnetic force of it is balanced bt magnet. A spiral hair spring keeps check on thr which electrical connections of TC are also made to coil. I = E / R where I is current flowing thr circuit.

3 E is thermocouple emf & R resistance of total circuit. Total resistance includes Internal resistances (moving coil & internal circuit of Milivoltmeter) external resistance (resistance of thermocouple & lead wires. Attempts are made to keep total resistance constant so that I is directly proportional to E.

4 Operate potentiometer on standard cell, obtain null position of galvanometer. The poten. Circuit is standardized & known current flows thr the circuit. Now connect potentiometer to TC output. The galvanometer shows deflection. Now move slider on slide wire to bring the galvanometer to zero again. This position of slider is proportional to emf.

5 Laminar /Turbulent Rotational / Ir rotational Steady state / Transient Uniform & Nonuniform Compressible / In compressible One / Two / Three Dimensional

6 A. Rate meters (Inferential) Do not measure flow directly but measures other quantities like pressure, temperature, position etc. The rate meters include Constant area variable head meters ; Venturi, flow nozzle, orifice, pitot tubes Constant head variable area meters rotameters Variable velocity meters current meters, anemometers.

7 Variable area variable head Weirs & flumes In addition to this Ultrasonic flow meters, electromagnetic flow meters etc. B. Quantity meters Weight meters - Weigher & traps, Volumetric meters Rotating impellers, nutating disk type meters.

8 Assumptions 1. The fluid flow is adiabatic 2. W = 0 3. Frictionless process du = 0 4. Measuring device is horizontal z1 = z2 5. Steady flow of fluid. P1 / σ1 + V 1 2 / 2 = p2 / σ1 +V 2 2 / 2

9 Cylindrical entrance section - shall be preceded by 5 to 10 pipe diameters & free from fittings, misalignments, other sources of turbulence. Converging section Angle o. V increases & static pressure decreases. Throat length = diameter. Diameter = ½ or ¼ of inlet diameter. V is maximum & P is minimum here.

10 Diverging section Recovery of static pressure. Overall loss is minimum. The angle is 5 o to 7 o. The angle is kept less to avoid separation of fluid. So with small angle length & cost increases. So when pressure gain is not much important angle is as high as 14 o. The pressure taps are made at throat & at enterance. Small venturi brass / bronze. Large CI. Very large dia. (6m) - concrete

11 Venturi sizes 100 mm to 813 mm Cd = Accuracy - +/ to 3 % No sharp corners affecting accuracy. Not useful below 76.2 mm size.

12 High pressure recovery possible. Due to smooth surface less wear & abrasion. Less chances of clogging by sediments. High Cd Applications large flow of water, process fluids, wastes, gases, suspended solids. Long length space required more Quite expensive in installation / replacements Difficult to construct to get reproducibility.

13 Nozzles sre used for creation of jets & streams for all purposes as well as for metering. It consists of a smooth, gradual contraction thr throat followed by free, uncontrolled expansion back to the original pipe flow area. Nozzle has a pressure loss of 70 to 80 %. Gun metal, SS, monel metal. Pipe wall taps at One pipe diameter upstream *& half pipe diameter down stream good results

14 Cheaper than venturi & ease of installation. High Cd than orifice (0.99). Good for fluid containing solid that settle. More rugged & resistant to erosion than orific Poor pressure recovery Expensive than orifice meter. Limited to moderate sizes (< 120 cm) Suitable for high velocity water / steam where orifice can cause high pressure drops.

15 Consists of thin circular metal plate with a hole it. The plate is held between two flanges called orifice flanges. The minimum area called vena contracta is one where pressure is minimum. Down stream pressure tap shall be locate carefully as this vena contracta depends on no. of factors like Re, roughness, comprssibility, pipe diameter etc.

16 Three types taps are provided 1. Flange taps 25 cm from each face of orifice plate. 2. Vena contracta (downstream tap 0.3 to 0.8 D from orifice) 3. Radius taps One pipe dia. Up & half pipe dia down. Orifice plates are of three types 1. Concentric - widely used. Thickness to mm as per flow velocity & pipe size.

Circular hole at the center of pipe. Other materials used are Ni, Monel, Phospher bronze. Plate thickness not more than D(pipe) / 50 or d (orifice) / 18. 2.

17 Circular hole at the center of pipe. Other materials used are Ni, Monel, Phospher bronze. Plate thickness not more than D(pipe) / 50 or d (orifice) / Eccentric Suitable when liquid is containing solid particles or dissolved gases. 3. Segmental same services as eccentric. 4. Quadrant edge Suitable for Re 1,00,000.

18 Plate thickness just sufficient to withstand buckling due to fluid forces. Circular holes have sharp edges. Wear & abrasion of these edges can affect accuracy. So not used for fluids containing abrasive materials. Or replacement is suggested to maintain accuracy.

19 Low initial cost / ease of installation. Characteristics well known & predictable. Long years in use. Suitable for pipes 1.25 cm to 150 cm. Poor pressure recovery pressure loss 40 to 90 % of differential pressure. Low Cd value Not suitable for slurries, tends to clog. Careful installation necessary for good accuracy.

Experiment No.3: Flow through orifice meter. Background and Theory

Experiment No.3: Flow through orifice meter Background and Theory Flow meters are used in the industry to measure the volumetric flow rate of fluids. Differential pressure type flow meters (Head flow meters)