Echo Sounder – Let’s find out its depth..
As known to every Seafarer ,Echo Sounder is generally used to measure the depth of the seabed or detecting objects in water by measuring the time taken for sound echoes to return to the listener.
Let’s go into the depth of it only..
Beginning with it’s History to be known:
The first patent for an echo sounding device was granted in 1907. The Fathometer, a registered trademark often loosely applied to all depth-sounding gear, was developed (1914) as a result of research by the Canadian engineer R. A. Fessenden in the application of echo-sounding principles to iceberg detection. Application of echo-sounding principles to submarine detection during World War II resulted in the development of equipment to sound all ocean depths. In 1954 an advanced, highly accurate echo sounder called the precision depth recorder (PDR) was developed. By the early 1960s, the U.S. Navy used the new technique of Sonar Array Survey System (SASS).
Principle as described:
▪️Echo sounder measures the depth of water by measuring the time for a pulse of energy to travel to the sea bed and back and work on the principle of reflection of acoustic energy.
▪️Short pulse of sound energy is
transmitted vertically down from the ship.
▪️This pulse having been reflected from the sea bottom returns to the ship in the form of an echo.
▪️Travel time (t) taken for its return,
depends upon the depth of water (d), and on the velocity of sound (v) through the water.
▪️Time taken (t ) = (2 x d) / v or
d =(v x t)/2.
▪️Performance of a sonar system
depends on the accuracy with which the velocity of source propagation is measured.
The velocity of propagation is a function of temperature of water, pressure and salinity. The velocity is usually taken as 1500 meters/second for sea water at 13°C and atmospheric pressure.
An echo-sounder mainly consists of four units.
(i) Driving unit
(iii) Receiving unit
(iv) Recording unit.
consists of an electric motor with a governor ‘G’. A 12 volts battery supplies current to the motor. Governor helps in maintaining constant speed of the motor under normal voltage. The motor is coupled to a gear box.
The gear box has arrangement for maintaining two speeds. A drum is
mounted on a shaft. The shaft is rotated by the motor through the gear box. The drum is made up of insulated discs. A stylus is mounted on the first disc of the drum.
The first disc is continuous and it receives the echo impulse in the electrical form. From amplifier through the brush and is passed on to the stylus. Remaining two discs of the drum make the contact once in each revolution and as a result an electrical impulse is sent to the transducer. Recording unit consists
of a box. (It is also called a tank) in which a recording paper roll is wound over the rollers. One of the rollers is driven by a shaft from the motor. Thus the recording paper is drawn down at a regular speed (say 2.5 to 5 cm/minute).
The paper moves over the face of the box or the tank. The paper is chemically treated with a solution of potassium iodide and starch. The stylus moves in anticlockwise direction over the paper. When a small electrical impulse passes through the stylus it produces a brownish- purple stain at the point of contact.
consists of a transmitter or a transducer. The 12 volt battery supplies current to a rotary converter C. It transforms the battery voltage of 12volts to 400 volts DC This voltage is used in operating the transmitter. In the
transmitter a sound impulse is produced by a blow from a spring driven electromagnetically controlled hammer on a steel diaphragm. The diaphragm is in contact with the water.
consists of a receiver which arrests the reflected sound echo. A corresponding electrical impulse goes to the stylus via an amplifier. Current for operating the valve filaments of the amplifier is also obtained from the same battery. High tension supply to the amplifier IS given from the tappings of the rotary converter.
JUMPING TO THE WORKING OF ECHO SOUNDER:
▪️The recorder sends a signal to the pulse generator which triggers a pulse and simultaneously starts the recorder trace.
▪️The rate may be between 5-600 pulses a minute depending upon the depth to be measured.
▪️Lesser the depth scale, faster the Pulse Repetition Frequency.
▪️The pulse from the Pulse Generator activated the transmitter which sends a powerful electric pulse to the Transducer.
▪️The transducer converts the electric energy into sound energy producing a pulse.
▪️The sound energy travels to the bottom, hits the bottom and gets reflected back to the transducer.
▪️The transducer converts the received sound energy of the echo into an electric pulse and sends it to the receiver.
▪️In the receiver the received pulse is processed and then amplified and sent to the Recorder.
▪️At the recorder, the stylus which has been at a speed proportional to the speed of sound, creates a physical mark on a paper or a visual blip or a digital signal for the recorder.
Errors of the Echo Sounder
Velocity increases as the salinity, pressure and temperature increases.Of these three, salinity setting is the most important followed by the temperature.If the velocity reading is less than what it should be, the E/S will under read which though not desirable in not dangerous.So if you forget to correct the settings when the ship enters the Red Sea from the Gulf of Aden, the E/S will under read the depth.
▪️Stylus speed error:
The stylus is rotating with a certain constant speed and the speed of the stylus that the time is taken for the stylus to travel from top to bottom is exactly equal to that for an acoustic pulse to travel twice the distance of the range selected. Due to the fluctuation in the voltage, the speed of the stylus motor changes hence the depth recorded Will be inaccurate. It should be checked periodically and adjusted as per the instruction is given in the manual.
The echo may be reflected a number of times between the keel and the seabed, thereby giving multiple depth marks on the record, in such case the first echo is the correct depth.
This error is found when two transducers are used one for transmission and other for reception.
▪️Thermal and Density layer.
The density of the water varies with temperature and salinity, which will tend to form different layers. It is possible for echoes to return from the surface of these layers and a faint line appears between zero and actual depth.
▪️Zero line adjustment error :
If the zero adjustment is not correct, the depth recorded will not be correct.