Q) Explain the basic principle of an echo sounder with the help of a block
diagram. (May-18, March-17, Aug-16, July-16, March-16) OR
Q) Describe the main components of an Echo-sounder with the help of a
simple block diagram and state the function of each component? (July-19,
July-17, March-16) OR
Q) Draw & explain the working of an Echo Sounder. (Jan-20, Sept-19, Jan-19)
Ans:- Basic Principle:-
Short pulses of sound vibrations are transmitted from the bottom of the ship to
the seabed. These sound waves are reflected back by the seabed and the time taken
from transmission to reception of the reflected sound waves is measured. Since the
speed of sound in water is about 1500 m/sec, the depth of the sea bed is calculated
which will be half the distance travelled by the sound waves.
The received echoes are converted into electrical signal by the receiving
transducer and after passing through to stylus which burns out the coating of the thin
layer of aluminum powder and produces the black mark on the paper indicating the
depth of seabed.
COMPONENTS:-
Basically an echo sounder has following components:
Transducer – to generate the sound vibrations and also receive the reflected sound
vibration.
Pulse generator – to produce electrical oscillations for the transmitting transducer.
Amplifier – to amplify the weak electrical oscillations that has been generated by
the receiving transducer on reception of the reflected sound vibration.
Recorder - for measuring and indicating depth.
CONTROLS:-
An echo sounder will normally have the following controls:
Range Switch – to select the range between which the depth is be checked e.g. 0-
50 m, 1 – 100 m, 100 – 200 m etc. Always check the lowest range first before
shifting to a higher range.
Unit selector switch – to select the unit feet, fathoms or meter as required.
Gain switch – to be adjusted such that the clearest echo line is recorded on the
paper.
Paper speed control – to select the speed of the paper – usually two speeds
available.
Zero Adjustment or Draught setting control – the echo sounder will normally
display the depth below the keel. This switch can be used to feed the ship‟s
draught such that the echo sounder will display the total sea depth. This switch is
also used to adjust the start of the transmission of the sound pulse to be in line
with the zero of the scale in use.
Fix or event marker - this button is used to draw a line on the paper as a mark to
indicate certain time e.g. passing a navigational mark, when a position is plotted
on the chart etc.
Transducer changeover switch – in case vessel has more than one switch e.g.
forward and aft transducer.
Dimmer – to illuminate the display as required.
Q) Explain in brief the working of Echo Sounder. (Nov-17, July-16)
Ans:- Working:
The acoustic pulses of very short duration are transmitted vertically at the rate of
5 to 600 pulses per minute having a beam width of 12 to 25°.
These pulses strike the seabed and get reflected back towards the receiving
transducer as echoes.
These received echoes are converted into electrical signals by the receiving
transducer and after passing through the different stages of the receiver, the
current is supplied to the stylus which bums out the coating of the thin layer of
aluminium powder and produces a black mark on the paper indicating the depth of
the seabed.
Principle used in the working of an Echo Sounder:-
There are two techniques:-
Ranging
Phasing
Ranging:-
In echo sounder the stylus is mounted on circular belt driven by means of a stylus
motor which moves at certain speed and transmission takes place when the stylus
passes the zero marks.
A magnet fixed on the stylus belt triggers the transmitter to transmit a pulse every
rotation of belt when stylus is at zero mark on the paper scale, the transmission of
the acoustic waves from the transducer is synchronized with the stylus at the zeromark.
The acoustic waves are reflected from the seabed and echoes are received by the
transducer and after passing through various stages eventually the current is
supplied to stylus which burns out the coating of the thin layer of aluminum
powder and produces the black mark on the paper indicating the depth of seabed.
This cycle is repeated for every rotation so as the paper is pulled across the
display, the profile of seabed is obtained.
Suppose the lowest range scale is 0 to 50 M, the transmission will take place when
stylus reaches at the zero mark.
When the higher range is selected say 0 to 100 M, in order to cater for this range
scale, the speed of the stylus motor is reduced, in this process the scale
magnification is lost and as we switch over to higher ranges the scale becomes
more & more congested.
To overcome this problem some of echo sounding machines work on phasing
technique.
Phasing:-
In phasing the speed of the stylus motor remains constant.
Instead of changing the speed of the stylus, the transmission point is advanced.
If the first range is 0 to 50 M the second range will be 50 to 100 M (instead of 0 to
100 M).
Various sensors are positioned around the stylus belt, the magnet generates the
pulse when it passes the sensors which in turns activates the transmitter.
In the below diagram, when we select the lowest range i.e. 0 to 50 M, the magnet
mounted on the stylus belt will activate sensor no. 1, transmission takes place
when the stylus exactly passes over the zero mark, when we switch over to higher
range, say 50 to 100 M, the magnet mounted on the stylus belt will activate
sensor no.2 and transmission will take place early, at the time of the transmission,
the stylus will not be passing over 50 M mark on display unit, in other words there
will be delay introduced by delay unit no.2 & the stylus will reach the 50 M on
display unit after delay of 0.067 seconds. (50 x 2 / 1500, where 50 correspond to
the range, multiplied by 2 because double of distance is covered by acoustic waves
& the echoes and 1500 is the speed of acoustic waves).
Likewise, when we switch over to higher range say, 100 to 150 M, magnet
mounted on the stylus belt will activate sensor no. 3 & more delay will be
introduced for the stylus to pass over the 100 M.
Caution when using phasing technique: - We must always start sounding at lowest
range and check for echoes, adjust the gain control if required and then only switch
over to higher range.
Q) Describe the various errors associated with an echo sounder. (Nov-18,
May-18, Nov-17, March-17, Aug-16, March-16)
Ans:- ERRORS OF ECHO SOUNDER:-
Velocity of propagation in water:- The velocity changes with temperature salinity &
pressure. The velocity of the acoustic wave assumed at the temperature of 16
degree C & Salinity of 3.4% is 1505 m/sec, but generally it is taken as 1500 m/sec
for calculations. As velocity is varying hence depth recorded will be erroneous.
Depth indicated in Fresh water can be about 3% higher than the actual depth. NP
139 can be referred in order to obtain the corrections. To compensate the error
due to temperature variation, a component called “thermistor” may be mounted
near the transducer & change in velocity of the acoustic wave through water from
the standard value due to the change in sea water temperature is accounted for.
Error due to pressure is not so significant.
Stylus speed error:- The speed of the stylus is such that the time taken by the
stylus to travel from top to bottom on chart is same as the time taken by sound
waves to travel twice the range selected, but due to fluctuation in voltage supplied
to stylus motor, will cause error in the recorded depth.
Pythagoras error:- This error is found when two transducers are used, one fortransmission and the other one for reception. This error is calculated using the
Pythagoras principle. This error becomes prominent whenever distance between
two transducer is more than 2 mtrs, manual should be referred in order to use the
table for corrections.
Multiple echoes:- The echo may be reflected no. of times from the bottom of the
sea bed, hence providing the multiple depth marks on paper.
The thermal and density layers:- The density of the water varies with temperature
and salinity, which all tends to form different layers. The sound wave may be
reflected from these layers.
Zero line adjustment error:- If the zero is not adjusted properly, it will give error
in reading.
Cross noise:- If sensitivity of the amplifier is high, just after zero marking a
narrow line along with the several irregular dots and dashes appear and this is
called cross noise. The main reasons for the cross noise are aeration and picking
up the transmitted pulse. If intensity of cross noise is high, it will completely mask
the shallow water depths. This is controlled by swept gain control circuit.
Aeration:- When the sound wave is reflected from the reflected from the air
bubbles, it will appear as dots, this is known as aeration.
o Aeration can be due to pockets of bubble due to heavy weather.
o Rudder hard over causing drastic alteration of course.
o Pitching in light condition.
o Whilst astern propulsion. (Switch over to forward transducer if available.)
Q) With respect to Echo sounder, explain the effect of density, temperature
and the depth indicated. (Jan-18, May-17)
Ans:- The thermal and density layers:- The density of the water varies with
temperature and salinity, which all tends to form different layers. The sound wave
may be reflected from these layers.
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