CATHODE RAY OSCILLOSCOPE- CRO

Introduction
An oscilloscope (sometimes abbreviated as “scope”) is a voltage sensing electronic instrument that
is used to visualize certain voltage waveforms. An oscilloscope can display the variation of a
voltage waveform in time on the oscilloscope’s screen.CRO depends on the movement of an
electron beam which is bombarded on a screen coated with fluorescent material, to produce a
visible spot. If the electron beam is deflected on both X and Y axix, a two dimensional display is
produced. The beam is deflected at a constant rate relative to time along the X axix and is deflected
along the Y axix in response to an input voltage. This produces a time dependent variation of input
voltage.

Types of Oscilloscopes Analog and Digital: An analog oscilloscope works by directly applying a
voltage being measured to an electron beam moving across the oscilloscope screen. The voltage
deflects the beam up and down proportionally, tracing the waveform on the screen. This gives an
immediate picture of the waveform. In contrast, a digital oscilloscope samples the waveform and
uses an analog-to-digital converter (or ADC) to convert the voltage being measured into digital
information. It then uses this digital information to reconstruct the waveform on the screen.

Block Diagram and Operation of General Purpose CRO

A general-purpose oscilloscope consists of the following parts:

1. Cathode ray tube

2. Vertical amplifier

3. Delay line

4. Time base generator

5. Horizontal amplifier

6. Trigger circuit

7. Power supply

Cathode Ray Tube - It is the heart of the oscilloscope. When the electrons emitted by the electron
gun strikes the phosphor screen, a visual signal is displayed on the CRT.The CRT is a vacuum tube in
which a beam of electrons is accelerated and deflected under the influence of electric or magnetic
fields. The electron beam is produced by an assembly called an electron gun located in the neck of the
tube. These electrons, if left undisturbed, travel in a straight-line path until they strike the front of the
CRT, the “screen,’’ which is coated with a material that emits visible light when bombarded with
electrons.
CRTs constituted of three basic systems
a) Electron gun assembly
b) Electrical deflection system consisting of X – X (horizontal) and Y – Y (vertical) plates
c) Phosphor coated screen to visually depict the electrical waveform.

Electron gun assembly consists of indirectly heated cathode with its heater, the control grid and
Pre accelerating, focusing and accelerating anodes. Intensity of electron beam is controlled by
the control grid ie no of electrons emitted from the cathode. The electrons emitted from the
cathode and passing through the control grid are accelerated by the pre accelerating and
the accelerating anodes. The electron beam is focused by the focusing anode. CRO uses a
electrostatic method of focusing.
Electron beam from electron gun passes through two types of deflection plates. One pair of
plates is mounted horizontally and produces an electric field in the vertical plane. This pair
produces a vertical deflection of electron beam and hence called vertical deflection plates or Y
plates. Similarly the other pair of plates is mounted vertically and produces a horizontal
deflection, hence called horizontal deflection plates or X plates.
Front of the CRT is called face plate. The inside surface of face plate is coated with phosphor.
The electron beam on striking the phosphor gives off light. The bombarding electrons striking
the screen release secondary electrons. These secondary electrons are collected by an aqueous
solution of graphite called aquadag on sides of CRT.

Vertical Amplifier - The input signals are amplified by the vertical amplifier. Usually, the
vertical amplifier is a wide band amplifier, which passes the entire band of frequencies.

Delay Line - As the name suggests, this circuit is used to delay the signal for a period of time
in the vertical section of CRT. Horizontal deflection circuit consists of trigger circuit, time base
generator and horizontal amplifier. The delay for these sections is compensated in vertical
deflection system by the delay line. when the delay line is not used, the initial part of input
signal is lost and only a part of signal is displayed. Thus sweep at horizontal plates starts with
signal input reaching the vertical plate.

Time Base (Sweep) Generator - Time base circuit uses a uni-junction transistor, which is used
to produce the sweep. The saw tooth voltage produced by the time base circuit is required to
deflect the beam in the horizontal section. The spot is deflected by the saw tooth voltage at a
constant time dependent rate. The sweep must be linear and spot must move in one direction
only. Also sweep voltage must drop suddenly after reaching its max value. So saw tooth
waveform is selected as sweep voltage.

Horizontal Amplifier - The saw tooth voltage produced by the time base circuit is amplified by
the horizontal amplifier before it is applied to horizontal deflection plates.

Trigger Circuit - The signals which are used to activate the trigger circuit are converted to
trigger pulses for the precision sweep operation whose amplitude is uniform. Hence input signal
and the sweep frequency can be synchronized. This is used to convert the incoming signal into
trigger pulses so that input signal and saw tooth sweep frequency can be synchronized.

Power supply - The voltages required by CRT, horizontal amplifier, and vertical amplifier are
provided by the power supply block. It is classified into two types -

(1) Negative high voltage supply

(2) Positive low voltage supply

 The voltage of negative high voltage supply is from -1000V to -1500V. The range of
positive voltage supply is from 300V to 400V.

Functioning

 The cathode ray is a beam of electrons which are emitted by the heated cathode (negative
electrode) and accelerated toward the fluorescent screen. The assembly of the cathode,
intensity grid, focus grid, and accelerating anode (positive electrode) is called an electron
gun.

 Its purpose is to generate the electron beam and control its intensity and focus. Between the
electron gun and the fluorescent screen are two pair of metal plates - one oriented to provide
horizontal deflection of the beam and one pair oriented to give vertical deflection to the
beam. These plates are thus referred to as the horizontal and vertical deflection plates.

 The combination of these two deflections allows the beam to reach any portion of the
fluorescent screen. Wherever the electron beam hits the screen, the phosphor is excited and
light is emitted from that point. This conversion of electron energy into light allows us to
write with points or lines of light on an otherwise darkened screen.

 The linear deflection or sweep of the beam horizontally is accomplished by use of a sweep
generator that is incorporated in the oscilloscope circuitry.
 In the most common use of the oscilloscope, the signal to be studied is first amplified and
then applied to the vertical (deflection) plates to deflect the beam vertically, and at the same
time, a voltage that increases linearly with time is applied to the horizontal (deflection)
plates, thus causing the beam to be deflected horizontally at a uniform rate.

 The signal applied to the vertical plates is thus displayed on the screen as a function of time.
The horizontal axis serves as a uniform time scale.

Role of sweep in CRO

The sweep, also known as saw tooth pulse, is required to deflect the beam in the horizontal
section.

To obtain steady traces on the tube face, frequency of vertical input signal must be equal to
or an exact multiple of the sweep generator signal frequency. Thus, with such a matching of
synchronization of the two deflections, the pattern on the tube face repeats itself and hence
appears to remain stationary

The persistence of vision of the human eye and of the glow of the fluorescent screen aids in
producing a stationary pattern.

In simple words, the sweep is the horizontal speed of the cathode ray tube’s spot which is
used to create a trace. This ensures that the signal being tested is locked on the screen and
does not drift.

Synchronization of waveforms can be accomplished in two different ways: Free running

sweep and triggered sweep.

Free running sweep: If the frequency of vertical input signal is equal to or an exact multiple
of the sweep generator signal frequency free running sweep is used. When both signals at
the same frequency an internal synchronizing pulse will lock the sweep generator with
vertical input. It is also called recurrent sweep. Here the saw tooth sweep is repeated again
and again and voltage rises to max and suddenly falls to min. The electron beam moves
slowly from left to right, retraces rapidly to the left and the pattern is repeated. Limitation is
when input signal frequency is varied rapidly like voice or music signal this type cannot be
used.

Triggered sweep: In triggered sweep the trigger circuit receives an input from vertical
amplifier. When vertical input signal reaches a certain level the trigger circuit provides a
trigger pulse to the sweep generator, ensuring synchronization between sweep generator
output and the vertical input signal that triggers it.

Electrical Deflection System

After materializing out of the electron gun assembly the sharply focused electron beam with
ample linear acceleration is made to pass through pairs of vertical (Y) and horizontal plates
(X). Initially only one set each of the Y and X plate pairs were present in the CROs however
with the advent of dual-trace CROs two sets of Y plates (Y1 - Y1 and Y2 – Y2) and one set
of X plates (X1 – X1) came into existence. In the dual-trace CRO the two input signals that
are to be analyzed (determination of amplitude, frequency etc.) are fed directly to the (Y1 -
Y1 and Y2 – Y2) plates respectively. The pair of plates that make the electron beam traverse
in the horizontal direction only are referred to as X plates (time). While the pair that makes
the electron beam move in vertical direction only are called Y plates.

Horizontal Deflection System (HDS)

Typically in any CRO trigger circuit, time-base generator and horizontal amplifier together
constitute the Horizontal Deflection System (HDS). The main function of the HDS is to
deflect horizontal portion of the trace at a constant rate as a function of time and which is a
function of the input signal to be tested.
Block Diagram of the HDS
 Brief description of the components of HDS:
a) Trigger Circuit: The trigger circuit ensures that generation of the horizontal time-base
(sweep) starts at the same point of the vertical input signal. It is used to convert the
incoming signal into trigger pulses so that input signal and sweep frequency can be
synchronized.
b) Time-base: The time-base (sweep) generator controls the rate with which the electron
beam is scanned across the face of the screen of the CRT. The same is adjusted from the
front panel of the CRO. It is used to generate the saw tooth voltage required to deflect the
beam in horizontal section.
c) Horizontal amplifier: This is used to amplify the saw tooth voltage before it is applied to
horizontal deflection plates.

Vertical Deflection System (VDS)

The main function of the Vertical Deflection System (VDS) of a CRO to suitably amplify
the signal input to it such that its basic characteristics do not undergo any change or
modification. Further, VDS converts the input signal to a proper level such that it can then

drive the vertical plates, devoid of any distortion whatsoever

Vertical amplifier consists of two stages input amplifier and main amplifier. The first stage is input
amplifier with a FET source follower with high input impedance which isolates amplifier from
attenuator by impedance matching. The phase inverter provides two anti phase output signals which
are required to operate the push pull output amplifier. Input attenuator is used to attenuate any noise
present in the input signal
X-Y MODE
In XY Mode of operation of CRO sweep generator is switched off and two different input signals are
simultaneously applied to both horizontal and vertical deflection plates. XY mode is obtained by pressing XY
switch on CRO front panel. When CRO is operated in XY mode and two sinusoidal signals are simultaneously
applied to both plates, the resultant pattern obtained on screen is called Lissajous Pattern.

LISSAJOUS PATTERN
When two sinusoidal voltages are simultaneously applied to horizontal and vertical plates Lissajous Patterns
are obtained

When two sinusoidal voltages of equal frequency which are in phase with each other (phase shift = 0)are
applied to horizontal and vertical plates the pattern appearing on the screen is a straight line with a slope of 45
degree with x axix.
When two sinusoidal voltages of equal frequency but with phase shift= 90 degree are applied to horizontal
and vertical plates the pattern appearing on the screen is a circle.
When two sinusoidal voltages of equal frequency but with phase shift not equal to 0 or 90 degree are
applied to horizontal and vertical plates the pattern appearing on the screen is an ellipse.

Conclusions
When two sinusoidal signals of same frequency are applied
1. A straight line results when two voltages are equal and are either in phase(1 st and 3rd quadrant) or 180 degree
out of phase with each other ( 2 nd and 4th quadrant). The angle formed with horizontal is 45 degree when
magnitude voltages are equal, greater than 45 degree if vertical voltage is greater and less than 45 degree if
horizontal voltage is greater.
2. A circle is formed when magnitudes of two voltages are equal and phase difference is 90 degree.
3. If two voltages are not equal an ellipse is formed. If two voltages are equal but phase difference is not equal
to 0 or 90 degree an ellipse is formed.
4. If the major axis of the ellipse lies in the 1 st and 3rd quadrant ie positive slope phase angle is between 0 and
90 degree. If the major axix of the ellipse lies in the 2 nd and 4th quadrant ie negative slope phase angle is
between 90 and 180 degree.

PHASE MEASUREMENT FROM LISSAJOUS PATTERN

For phase measurement a known sinusoidal input signal is applied to horizontal plate and another
sinusoidal input signal of same frequency but with a phase shift is applied to vertical plate and lissajous
pattern is obtained.
1. If lissajous pattern obtained is a straight line with positive slope with angle 45 degree, <45 degree or >
45 degree. Then phase shift is 0.
2. If lissajous pattern obtained is a circle. Then phase shift is 90.
 3. If lissajous pattern obtained is an ellipse. Then phase shift is between 0 and 90 degree which can be
obtained as
Y1 X1
Sin ᶲ = =
Y2 X2 where ᶲ is phase difference

FREQUENCY MEASUREMENT FROM LISSAJOUS PATTERN

For frequency measurement a sinusoidal input signal with known frequency is applied to horizontal
plate and another sinusoidal input signal of unknown frequency is applied to vertical plate and lissajous
pattern is obtained. The known frequency is adjusted until a stationary lissajous pattern is obtained on
screen(when both frequencies are same or integral multiple).
Then the ratio of vertical(fv) and horizontal(fh) frequencies is obtained as
fv no of horizontal loops no of horizontal tangencies
= =
fh no of vertical loops no of vertical tangencies

fv=2 fhfv=3 fh fv=4 fh

 Applications of CRO

(1) Study of waveform – In this case the signal (sine, cosine, rectangular, square, triangular or
sawtooth) is fed into the vertical plates of the CRO by means of a shielded cable with BNC
connector
(2) Measurement of Voltage, Frequency, Time-period and phase.
(3) Other physical quantities like current, strain, acceleration, pressure are converted to voltage with
transducers and then measured on CRO screen.
(4) It can be used in any field where a parameter can be converted into a proportional voltage for
observation. Eg meteorology, biology and medicine.

DIGITAL STORAGE OSCILLOSCOPE

DSO stores a digital copy of the waveform to be displayed in the digital memory which can be analysed further by
using DSP techniques. In DSO signals are received, stored and then displayed.
Input signal is applied to pre amplifier and attenuator to amplify input signal and attenuate noise. Input
signal is sampled using a sample and hold circuit. Sampling frequency is controlled by the control logic. Sampled
signal is digitized by the ADC. Normally Flash ADC is used being the fastest. Sampled and digitized input is stored
in digital memory whose operation is controlled by the control logic. Data stored in memory can be read out and
used for various DSP techniques. Output of memory is converted back to analog by DAC. This analog signal is
amplified by the vertical amplifier and fed to vertical deflection plates of CRT.
Signal from the pre amplifier and attenuator is given to trigger circuits, which generates the trigger for time
base generation and synchronization. Control logic is a microprocessor which generates horizontal sweep using an
inbuilt binary digital counter. The output of the counter is converted to analog by DAC and amplified by HA and fed
to HDP of CRT. The clock speed of binary counter is controlled by the control logic. Once the counter reaches its
maximum count output is reset to binary zero and the process is repeated by creating a ramp signal(sawtooth) for
time base.
The maximum frequency measured by a DSO (band width) depends upon sampling rate of sample and hold
circuit and speed of ADC.

MODES OF OPERATION

1. Roll mode
2. Store mode
3. Hold or save mode

Roll mode similar to normal CRO operation, input is applied and trace is displayed on screen.
Store mode or refresh mode is used when sampling rate of waveform is too high and waveform is repetitive in
nature.
Hold or save mode displays the saved data read from memory.
 Phasor Measurement Unit ( PMU)

• One of the most important measurement devices in power systems is phasor measurement unit
(PMU).
• The PMU is a power system device, capable of measuring the magnitude and phase angle of
synchronized voltage and current phasor in a power system using a common time source for
synchronization.
• Synchronizing provided by GPS and allows real time measurement of multiple remote points on
grid.
• A typical commercial PMU can report measurements with high resolution in the order of 30 to 60
measurements in one second. This helps engineers in analyzing the dynamic events in the grid
which is not possible with traditional SCADA measurements that generate one measurement every
2 or 4 seconds.
The synchronized time is given by GPS. Without GPS providing synchronized time, it is hard to
monitor whole grid at the same time. Voltage and current recordings from different substations can
be directly displayed on the same time axis and in the same phasor diagram.
The micro processor calculates positive sequence estimates, frequency and rate of change of
frequency of all the current and voltage signals using DFT technique and is included in the output
of PMU.
MODEM is a device that modulates an analog carrier signal and encodes digital information from
the signal and can also demodulate the signal to decode the transmitted information from signal.
The objective of modem is to produce a signal that can be transmitted and decoded to make a
replica of the original digital data.

APPLICATIONS OF PMU
1) Frequency stability monitoring
2) Voltage stability monitoring
3) Power system real time monitoring
4) Advanced network protection
5) Fault recording
6) Disturbance recording
7) Fault location

TRANSDUCERS
 • A Transducer is a device usually electrical, electronic, electromagnetic or photovoltaic that
converts one form of energy into another form. Usually, a signal in one form of energy is converted
to a signal in another form by a Transducer.
• Electrical transducers are defined as the transducers which convert one form of energy or physical
quantity (velocity, pressure, heat, displacement) to electrical signal such as voltage or current for
measurement purposes.

Basic requirements of a Transducer

• The basic requirements of a transducer are
 Ruggedness: It should be capable of withstanding overload and some safety arrangement should be
provided for overload protection.
 Linearity: Its input-output characteristics should be linear and it should produce these
characteristics in symmetrical way
 Repeatability: It should reproduce same output signal when the same input signal is applied again
and again under fixed environmental conditions like temperature, pressure, humidity etc.
 Sensitivity: electrical output per unit change in physical parameter.
 High reliability and stability
 High output signal quantity

Classification of Transducers
1)Active and passive transducers
2) Analog and digital transducers
3)On the basis of transduction principle used
4)Primary and secondary transducers
5)Transducers and Inverse transducers

1)Active and passive transducers

• Active transducers are defined as those transducers which do not require any external or auxiliary
power source for their operation of conversion of physical quantity into electrical signal .They are
self generating transducers
Eg: Piezoelectric transducer which converts pressure into emf. Photovoltaic transducers coverts
light into voltage.
• Passive transducers are defined as those transducers which require an external or auxiliary power
source for their operation of conversion of physical quantity into electrical signal. They are
externally powered transducers
Eg: Linear potentiometer, LVDT, Strain gauge etc.
2) Analog and digital transducers
• An analog transducer converts the input physical quantity into analog output which is a continuous
function of time. Eg:LVDT, Strain gauge, Thermocouple etc
• A digital transducer converts the input physical quantity into digital form ie; in the form of pulses
having logic 0 and logic 1 levels. Eg; a rotary encoder.
3) On the basis of transduction principle used
• In this type of classification, the transducers are classified on the basis of principle of transduction
viz. resistive, inductive, capacitive
Eg: Thermistor,variable capacitance pressure guage,LVDT etc.

4) Primary and secondary transducers: It is a combination of 2 or more transducers for measurements

 • Fig. shows a method for pressure measurement using a Linear Variable Differential
Transformer(LVDT) where LVDT does not accept pressure signal for converting it into electrical
voltage.
• Here first pressure is converted into linear displacement using Bourden tube.
• Then the linear displacement is converted into voltage using LVDT.
• Here the Bourden tube acts as a primary transducer and LVDT acts as a secondary transducer.
• If a particular physical quantity is not suitable for the main transducer for obtaining electrical
signal, it is sensed by the first stage of the transducer called primary transducer.
• The secondary transducer converts the output of primary transducer into electrical signal.

5) Transducers and Inverse transducers

• Transducers convert non-electrical quantity into electrical quantity whereas inverse transducers
convert electrical quantity into non electrical quantity. For example, microphone and loudspeaker.

Linear Variable Differential Transformer (LVDT) (displacement transducer)

• The Linear Variable Differential Transformer converts the linear displacement into an electrical
signal. Widely used inductive transducer
• It works on the principle of mutual induction, i.e., the flux of the primary winding is induced to the
secondary winding. So it is called as a transformer.
The output of the transformer is the difference of the secondary voltages, and hence it is called a
differential transformer

Construction of LVDT
• The basic construction of the LVDT is shown below in the figure.
• The P is the primary winding of the LVDT and S1 and S2 are the secondary winding of the
transformer wound on the cylindrical former.
• The secondary winding has an equal number of turns, and it is placed identically on both the side
of the primary winding.
• A sinusoidal voltage is used to excite the primary
• The output voltage of the secondary winding S1 is ES1 and that of the S2 is ES2. The secondary
voltage signal is converted into an electrical signal by connecting the secondary winding in series
 opposition as shown in the figure below.
• The output voltage of the transducer is determined by subtracting the voltage of the secondary
windings.
• E0 = ES1 – ES2

The displacement which is to be measured is attached to the arm of the iron core.

• The output voltage of the secondary winding is equal when the core is in the middle position or
(null position). ES1 = ES2 E0 =0
• When the soft core moved towards left the flux linked in S1 is more as compared to S2. The output
voltage of the winding S1 is more than the S2 and it is in phase with the primary voltage. ES1 > ES2
E0 = ES1 – ES2
• Similarly, when the soft iron core move towards right the magnitude of the flux linked S2 is more
than S1. ES2 > ES1 The output voltage is 180 degree out of phase with the primary winding voltage.
• Amount of voltage change is proportional to the amount of movement of core.
• Hence we have an indication of linear motion.
• Bt noticing whether output voltage is increased or decreased we can determine the direction of
motion.

• The change in output voltage is directly proportional to the displacement of the core. Any
displacement will increase the flux of one of the secondary winding and on the other hand, reduces
the other.
• The curve between the output voltage and displacement is shown in the figure. The curve is linear
for small displacement and beyond this range, it starts to deviate from the straight line.
• The output voltage is proportional to displacement for about 5mm from the null position.

Advantages
• High Range – The LVDT has the very wide range for measurement of displacement. Their
displacement range is from 1.25 mm to 250 mm.
• High output and High Sensitivity – The LVDT gives a high output and also there is no need for
amplification. The sensitivity of the transducer is also very high.
• Rugged – It can tolerate the high degree of shock and variation.
• Low Hysteresis – The LVDT has low hysteresis because of which their repeatability is excellent.
• Low Power Consumption – The LVDT consumes power less than 1W power.
• No friction -No wear and tear and hence long life.
Disadvantages
• Large displacement is required for getting the considerable differential output.
• The LVDT transformer is very sensitive to the stray magnetic field.
• The performance of the transducer is affected by the vibrations.
• The dynamic response is slow.
• The performance of the LVDT is affected by the temperature.
Applications
• It is used for measuring the displacement having a range from few mm to cm. The LVDT directly
converts the displacement into an electrical signal.
• It is also used as the secondary transducer. The LVDT is used as a device for measuring the force,
weight and pressure. Some of the LVDT used for measuring the load and pressure.
• The LVDTs are mostly used in servo mechanisms and other industrial applications.

Measurement of Flow
Electromagnetic flow meter
• Electromagnetic flow meter are particularly suitable for the flow measurement of slurries, sludge
and any electrically conducting liquids.
• A constructional diagram of electromagnetic flow meter is shown in above figure.

• This type of flow meter works on the principle of Faraday’s law of electromagnetic induction.
• It consist of a pair of insulating electrodes buried flush in opposites sides of non conducting, non
magnetic pipe carrying liquid whose flow is to be measured.
• It is surrounded by electromagnet which produces magnetic field.
• The fluid passes through this pipe. This is anlagous to a conductor moving across a magnetic field.
Works on faradays law of electromagnetic induction.
• As the fluid passes through the pipe it also passes through magnetic field produced by magnet,thus
an emf is produced which is proportional to the velocity of flow.
• Induced voltage detected by pair of electrodes mounted on pipe wall.
Induced voltage is given by, e = B l v
Where,
• B = flux density in wb/m2.
• l = length of conductor.
• e= emf generated.
• v = velocity of conductor(flow) in m/s
• Assuming a constant B the magnitude of the voltage will be directly proportional to velocity.
• Q= V*A V- velocity A-area of flowmeter Q= e A / B l . For given flowmeters A, B l constant.
• Q= K e. induced voltage proportional to flow rate.
• The pipe used must be non magnetic to allow the field to penetrate the fluid and usually is non
 conductive such as plastic or glass so that it doesn’t provide a short circuit path between positive
and negative induced potential at the fluid surface.

• Advantages
• A magnetic or electromagnetic flow meter can be installed in a comparatively simple fashion.
• An existing pipe network can be converted into a measurement system.
• These flow meters can track both forward and reverse flow.
• It has a linear relationship between input and output.
• Disadvantages
• The magnetic flow meter has to be installed in vertical position.
• The instrument becomes expensive if heavy slurries are handled.
• The conductivity of the liquid being metered should not be less than 10 micro ohm/m.

Ultrasonic flowmeters
• Ultrasonic flowmeters use sound waves to determine the velocity of a fluid flowing in a pipe.
• There are two types of Ultrasonic flow meters depending upon the operating principle.
1) which operates on the transit-time differential method technology
2) Doppler effect
1) Ultrasonic meters that operates on the transit-time differential method technology
• A sound wave travelling in the direction of flow of the fluid is propagated at a faster rate than one
travelling against the flow .
• Transit time ultrasonic flowmeters send and receive ultrasonic waves between transducers in both
the upstream and downstream directions in the pipe.
• At no flow conditions, it takes the same time to travel upstream and downstream between the
transducers.
• Under flowing conditions, the upstream wave will take less time than the downstream wave.
• When the fluid moves faster, the difference between the upstream and downstream times increases.
• The transmitter processes upstream and downstream times to determine the flow rate.
• The difference in time travelled by the two ultrasonic waves is directly proportional to the mean
flow velocity .

• tdown= L / (c+ v cosq)

• f1 = (c+ v cosq) / L
• tup= L / (c- v cosq)
• f2= (c- v cosq) / L
• ∆f = (2v cosq) / L
• L-distance between transmitter and receiver
• c- velocity of sound propagation in medium
• v- linear velocity of flow

2) Ultrasonic flowmeters using Doppler effect

• This type of Ultrasonic flowmeters are ideal for wastewater applications or any dirty liquid which
is conductive or water based.
• The basic principle of operation employs the frequency shift (Doppler Effect) of an ultrasonic
signal when it is reflected by suspended particles or gas bubbles (discontinuities) in motion.
• Doppler effect is change in frequency or wavelength of a wave in relation to an observer who is
moving relative to the wave source.
• This metering technique utilizes the physical phenomenon of a sound wave that changes frequency
when it is reflected by moving discontinuities in a flowing liquid.
• Ultrasonic sound is transmitted into a pipe with flowing liquids, and the discontinuities reflect the
ultrasonic wave with a slightly different frequency that is directly proportional to the rate of flow
of the liquid.

Advantages
• Ultrasonic meters are made up of no moving parts.
• They experience no pressure loss.
• Ultrasonic flow meters are consistently more accurate and reliable than a lot of other metering
systems.
Disadvantages
• Transit time meters require clean liquids
• Doppler meters only for slight contamination or few gas bubbles
• Gas bubbles cause errors