Echo Sounder is a piece of equipment fitted on board to measure the depth of sea-bed.
SOLAS Chp V, Regulation 19.2.3 states that:
All ships of 300 GRT and above and passenger ships irrespective of size shall, in addition to meeting the requirements of paragraph 2.2, be fitted with an echo sounding device, or other electronic means, to measure and display the available depth of water;
- Short pulses of sound vibrations are transmitted from the bottom of the ship to the sea bed.
- These sound waves are reflected back by the sea bed and the time taken from the transmission to reception of the reflected sound wave is measured.
- Since the speed of sound waves in water is about 1500 meters per second, the depth of the seabed is calculated which will be half of the distance traveled by the sound waves.
- The received echo is then converted into an electrical signal by the receiving transducer and then the signal is amplified and it richest the stylus, which one out the coating of Aluminum powder and produces the black mark on the paper indicating the depth of the sea bed.
Principle used in working of Echo Sounder
- In Echo Sounder stylus is mounted on a circular belt driven by means of a stylus motor which moves at a certain speed and transmission takes place when the stylish passes the zero mark
- A magnet fixed on the stylus belt to rigged to the transmitter to transmit a pulse every rotation of the belt. When the stylus is at Zero mark on the paper scale the transmission of the acoustic waves is synchronized with the stylus at Zero mark.
- The acoustic waves are reflected from the seabed and echoes are received by the transducer and after passing through various processes current is supplied to the stylus which one the black mark on the paper indicating the depth of the sea bed.
- This cycle is repeated every rotation so the paper is pulled across the display the profile of the seabed is obtained.
- For example, the lowest range scale is 0 to 50 meters, the transmission will take place when the stylus will reach Zero mark.
- When the higher range is selected (0 to 100 meters) in order to cater to this range scale the speed of stylus motor will reduce.
- In this process, this scale magnification is lost and as we switch over to the highest range the scale becomes more and more congested.
- To overcome this problem some of echo sounder machine works on phasing technique.
- In this mode, 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 meters then, the second range will be 50 – 100 instead of 0 to 100.
- Various sensors are positioned around the stylus.
Errors of Echosounder
- Velocity of propagation in water:
The velocity changes with temperature salinity and pressure. A thermistor is used to compensate for the error of echo sounder due to temperature and error due to pressure is not so significant.
- Stylus Speed Error:
Due to the fluctuation in voltage supplied to the stylus motor, it will cause an error in related depth.
- Pythagoras Error:
Error found due to two transducers being used one for transmission and seconders for the reception.
This error generates when the transducers are two meters apart or more.
This error is calculated with the help of Pythagoras principle
- Multiple Echoes:
The Echo may be reflected number of times from the bottom of sea bed hands providing the multiple death marks on the paper.
- Thermal and Density layers:
The density of the water changes with the temperature and salinity, which all tend to form different layers. The sound waves may reflect from these layers.
- Zero line adjustment:
It is only an error that can be corrected on board. Zero is not activated properly, it will give an error of death.
- Cross Noise Error:
If the sensitivity of the amplifier is high the sound wave from water is also, acoustic waves so it will show another line or dots and the acoustic wave line is different. It is diminished by gain control.
When the sound wave is reflected from the air bubbles it will appear as a dot, this is known as aeration.
This aeration could be due to heavy weather, astern propulsion, or pitching in light condition.
Rudder hard over causes drastic aeration of course