This error is found when two transducers are used, one for transmission 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.
The echo may be reflected no. of times from the bottom of the sea bed, hence providing the multiple depth marks on paper.
Velocity of propagation in water
- The velocity changes with temperature salinity & pressure.
- The velocity of the acoustic wave assumed at the temperature of 16°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 (Echo Sounding Correction Tables) 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.
Cross Noise Error
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.
When the sound wave is reflected from the reflected from the air bubbles, it will appear as dots, this is known as aeration.
- Aeration can be due to pockets of bubble due to heavy weather.
- Rudder hard over causing drastic alteration of course.
- Pitching in light condition.
- Whilst astern propulsion. (Switch over to forward transducer if available.)
Zero-Line Adjustment Error
If the zero is not adjusted properly, it will give error in reading.