How to measure Flow?

Flow Strainers, Impellers, Magnetic sensing pickups. 

Turbine Meter

• good for telemetry
• errors may be caused 'coz of excessive frictional torque.

Electromagnetic flowmeter

• suitable for slurry, sludges etc
• Adv: size adaptability,No pressure drop
• Disad: for heavy slurry, high operating cost.

Hot Wire Anemometers

• more flow > more cooling
• voltage variation
• measured by POT

Using Thermister

• can measure velocity of air

Ultrasonic flow meter

• used mainly for liquid
• useful for bidec tional flow
• Adv:linear relationship
• Adv:insensitive to variation in viscocity, density and temp
• Disadv: complexity
• Disadv: relative high cost

Why energy meter reads energy while wattmeter does not. An energy meter has a registration constant of 100rev/kwh if the meter is connected to a load

An energy meter is fitted with some type of registration mechanism whereby all the instantaneous reading of power are summed over a definite period of time.ENERGY = POWER X TIME So, an energy meter can read energy.Whereas, a wattmeter indicates the value of power at a particular instant when it is read and hence it can not read energy.
Registration constant= 100 rev/Kwh I = 20 A, V = 230 V, cos φ = 0.8
Energy in 5 hours = VI Cos φ x 5 / 1000
= 20 x 230 x 0.8 x 5 / 1000
= 18.4 Kwh
The number of revolution of the meter should make it is correct
= 18.4 x 100
= 1840 revolution.
Number of revolution actually made
= 1800 revolution
(% error = 1840 - 1800 x 100 )/ 1840
= 0.0217%
It would be better from the customer point of view % age error is very less.

A Note on PMMC Instruments

1. DEFLECTING TORQUE:- The deflecting torque is produced by making use of one of the magnetic, chemical, electrostatic and electromagnetic induction effects of current or voltage and causes the moving system of the instrument to move from its zero position when the instrument is connected in an electrical circuit to measure the electrical quantity. The method of producing this torque depend upon the type of instrument. In attracting the type of instrument, this torque to equal to Td = 1/2 I2 dL/dθWhereas in Pmmc instruments Td = Bilur Where B - magnetic density i - current flowing l - length of coil u - number of turn r - radius of coil 2. CONTROLLING TORQUE:- The magnitude of the movement to the moving system would be somewhat indefinite under the influence of deflecting torque unless some controlling torque exist. This torque opposes the deflecting torque and increases with increase in deflection of the moving system without controlling system the irrespective magnitude of current and moreover, once deflected it would not return to its zero position on removing the current. In attraction type instrument it is produced by spring control and in PMMC too it would be produced by spring control. 3. DAMPING TORQUE:- This torque is also necessary to avoid oscillation of the moving system about it's final deflected position owing to the inertia of the moving parts and to bring the moving system to rest in it's final deflected position quickly.

An energy meter revolves 10 revolutions of disc for unit of energy. Find the number of revolutions made by it during an hour when connected across whe

Energy consumed in one hour = VI cos φ / 1000
= 210 x 20 x 0.8 / 1000
= 3.360 kwh.

The number of revolution the meter should make it is correct
= 3.360 x registration const in revolution per kwh
= 3.360 x 100
= 336Number of revolution actually made
= 350
% error = (350-336) x 100 / 350
% error = 0.1466 %

Moving Iron Type Inbstruments

These instruments are widely used in laboratories and switch board at commercial frequencies because these are cheaper in cost, robust in construction and can be manufactured with required accuracy. These are generally of two types:- 1. The attraction type.2. The repulsion type.
The attraction type instrument operate on the principle of attraction of a single piece of soft iron into a magnetic field and repulsion type instrument operate on the principle of repulsion of two adjacent iron pieces magnified by the same magnetic field. Repulsion type instrument are more sensitive than attraction type instrument as in repulsion type instrument large separating torque is developed by having two iron element positional class together inside the field coil where the magnetizing effect is maximum.
In both type of these instruments, the current under measurement is passed through a coil of wire. This current carrying coil set up the necessary field depending on the magnitude of the current to be measured. The coil may be of a few turns of very heavy conductor or of many turns of fine wire. The instrument to be used as an ammeter is provided with a coil of few turns of thick wire in order to have low resistance and carry large current and that to be used as a voltameter is provided with a coil of large number of turns of wire in order to have high resistance and draw as small current as possible.
EXPRESSION FOR TORQUE
Let L be the self inductance corresponding to a total angular deflection of q radians and change in inductance be dL correponding to small change in deflection angel dq due to small change in current.
The change in energy of magnetic field,
dw = Td dθ
Since change in energy dE = workdone, dw
Td dθ = ½ I2dL
Td = ½ I2dL/dθ
where I is in amperes, L is in Henry and θ is in Radians.
Thus toruqe is proportional to the square of the instrument current and to the rate of change of inductance with deflection.

Frequencies

Frequencies increase from VG to HP to TA.

Vibration Galvanometers (VG):- 5 - 1000 Hz
Head Phones ( HP) :- 200 - 2 KHz
Tuneable Amplifier Detecter( TA) :- 10 - 100 KHz output of this can be fed to pointer type instrument.