ECHO SOUNDER

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—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 1500 m/sec, the depth of the sea bed is calculated which will be half the distance travelled by the sound waves.

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—The received echoes are converted into electrictal signal by the receiving transducer and after passing through the different stages of the receiver, the current is supplied to stylus which burns out the coating of the thin layer of aluminium powder and produces the black mark on the paper indicating the depth of seabed. 

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—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. 

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—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.

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—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.  

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—Pulse Length

—The pulse length is the duration between the leading edge and the trailing edge.The pulse length determine the minimum distance that can be measured by the echo sounder.The minimum measurable distance will be equal to the half of the pulse length.for the shallow water short pulse is used while for the deeper water long pulse is used.

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—Pulse repetition frequency

—This is the nos of pulse transmitted per second.This determines the maximum range that can be measured by the echo sounder.The PRF is normally automatically selected and changes as the range scale is changed.for lower range,High PRF is used whereas for the higher range ,low PRF is used.

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—TRANSDUCER

—Electrostrictive transducer

—This type makes use of the special properties of crystals (e.g. crystals of barium-titanate and lead zirconate). If an alternating voltage is applied to the opposite faces of a flat piece of one of the above materials, the crystal will expand and contract, and hence vibrate creating sound waves for  as long as the vibrations continue. The process is reversible, i.e. when varying pressure from a returning echo, is applied to the opposite faces, an alternating voltage is generated across the faces and the same can be further amplified and used to activate an indicator.

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—Magnetostrictive transducer

—In this type, the use is made of the magneto-striction effect which is a phenomenon whereby magnetization of ferromagnetic materials produce a small change in their dimensions, and conversely the application of mechanical stresses such as weak pressure vibrations, as from an echo to them, produce magnetic changes in them; e.g. a nickel bar when placed in the direction of or strength of the magnetic field. If the nickel bar is placed in a coil with an alternating current flowing through it (a solenoid), the varying current and magnetic field will cause the ends of the bar to vibrate and hence create a sound wave. This is what happens when the transducer is transmitting.

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—Echo sounder

—SITING OF TRANSDUCER

—Factors affecting the siting of transducer:

AIR BUBBLE & CROSS NOISE: The transducer should be installed in a position where there is very less chance of formation of the air bubbles.Air bubbles will act as large reflectors of transmitted energy if lot of air bubbles are formed close the transducer.This will also create the cross noise.

  There are various locations on the ship where formation of air bubble is less e.g.

a) On large ,fast,deep draft ships-1/8 to ¼ L of the ship from forward

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—On medium speed ships- forward most portion of the ship.

—On slow cargo ships-1/4 L from aft

—On oil tanker –normally forward end of the E/Room bulkhead.

—Ranging

—In echo sounder the stylus is moving with certain constant speed and transmission takes place when the stylus passes the zero marks.When the higher range is selected the speed of the stylus is reduced as stylus has to paper for the longer duration.This system is called the ranging.

—PHASING

—In phasing the speed of the stylus motor remains constant.In stead of changing the speed of the stylus,the transmission point is advanced.

—The sensors are positioned around the stylus belt.The magnet generates the pulse when it passes the sensors which in turns activate the transmitter.

—PHASING

—ERRORS OF ECHO SOUNDER

—1.Velocity of propagation in water:

      The velocity taken for the calculation of the is 15oom/sec.The velocity of the sound wave is changing due to the change of the salinity and temperature of the sea water. As velocity is varying hence depth recorded will be erroneous.

2. 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 wave to travel twice the range selected.

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 but due to fluctuation in voltage supplied to stylus motor ,will cause error in the recorded depth.

3. PYTHAGORAS ERROR:

    This error is found when two transducer are used one for transmission and one for reception.This error is calculated using the Pythagoras principle.

4.Multiple ECHO:The echo may be reflected  no of times from the bottom of the sea bed,hence providing the multiple depth marks on paper.

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—5.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 .

  6.Zero line adjustment error:

    If the zero is not adjusted properly,it will give error in reading

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—CROSS NOISE:

    If sensitivity of the amplifier is high,just after zero marking a narrow line alongwith 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.

 

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—AERATION:

  When the sound wave is reflected from the reflected from the air bubbles,it will appear as dots,this is known as aeration.

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