FATHOMETERS
All fathometer systems, whether they are indicating or recording, work on the principle of producing short pulses of sound vibrations. These pulses are transmitted vertically to the ocean floor and when they are received, the difference in time between transmission and reception is calculated. Based on the concept that sound travels at a near constant 4,800 feet per second (1,500 meters per second), the fathometer measures the time the signal takes to return and then determines the approximate depth.
Sound vibrations travel in a beam pattern that has an angle of about 12 to 25 degrees in width, perpendicular to the vessel bottom. This beam pattern travels downward with the wave front striking the ocean bottom where the signals may be reflected directly back (specular reflection) or at various angles (diffuse reflection). The bottom surface is usually not perfectly flat and signal return may vary due to the contour of the bottom. It is important to note that the fathometer is dependent on both the diffuse and specular reflection of a signal in order to function. This is also good when a vessel is rolling and the signals are returning at different angles at different times.
Like radio waves, density differences in the ocean cause the fathometer signal to be absorbed, scattered, or reflected. This causes attenuation of the sound vibrations. The effect is not only on the initial emitted signal but also on the returning echo as well. The fathometer compensates for this attenuated signal by utilizing a swept gain, which essentially increases the amplification of the signal at a rate dependent upon the amount of time the signal takes to return. The longer a signal travels, the more it is absorbed. This absorption can be decreased by reducing the frequency of the signal. Fathometer signal frequencies vary from about 55 kHz for shallow depths to about 10 kHz for great depths. The signals are above the range of audible sound; therefore, they are considered ultrasonic. This is done specifically to lock out any noise generated by the vessel. Most fathometers have a range of about 1.5 feet to about 4,500 feet depending on depth. They emit vibrations between 10 and 600 pulses per minute.
Fathometer systems include an indicator unit, which contains an oscillator to create the electrical signal, and a transducer, which converts the electrical signal to ultrasonic vibrations when transmitting. The transducer also converts the returning echoes back into an electrical signal, which then passes to an amplifier that boosts a signal to a usable level where it is read on the indicator or on a recording graph. The indicator unit is essentially the the controlling element of the system, coordinating the sending and receiving of signals, timing pulses, and adjusting frequencies.
There are two types of transducers used in commercial application. The first is the electrostrictive type which converts the electrical signal to sound vibrations by passing the current through two plates which form a sandwich with a nonconductive material. The plates vibrate due to the magnetic field induced. The second type oftransducer is the magnetostrictive type transducer which uses a form of electromagnet creating vibrations in a diaphragm, thus producing the fathometer signal. Most transducers are installed in special hull openings and are in direct contact with the water. The deck officer should be aware of the location of the transducer in order to prevent damage to the unit when dry-docking or to provide protection when doing any hull blasting or hull painting.
There are two types of fathometers: the indicating fathometer provides some form of visual presentation to show depth; the recording fathometer uses a graph leaving a permanent record. Most fathometers are capable of indicating depth in fathoms, feet, or meters. The vessel officer should keep in mind that this indication is depth under the keel of his vessel, not overall depth of the water. Indicating fathometers use various methods of presenting a display including a rotating neon light, digital readout, or cathode-ray tube presentation. Recording fathometers utilize a special graph paper moving over the top of a metal desk. When an electrical signal is sent for transmission a stylus conducts an electric current through the paper to the metal desk. The electrical charge burns a nonconductive coating off the special paper leaving a mark on the graph. When the signal returns, the stylus once again passes an electrical charge through the paper to the metal desk leaving a second mark on the graph. Depending on the way the signal is heard upon its return, the presentation can show the trained eye the type of bottom the vessel is encountering. Clear dark lines indicate a hard bottom. A wider, less distinct line is an indication of a soft bottom. It is essential that the user set the fathometer on the right depth scale based on information taken from a navigational chart. This will ensure that the signal timing and frequency are properly set for the appropriate depth, allowing the system to function efficiently. The depth indication left on the graph paper, which usually displays several depth scales available on the particular unit, is a profile of the seabed along the course ofthe ship. Incorrect scale settings can result in additional tracings on the graph due to over-amplified signals at shallow depths or false soundings as a result of signals returning later than anticipated. Most fathometers also have a manual gain. If turned up too high the unit will pick up noise caused by vibration created by the vessel moving through the water.
Different temperatures and varying water densities due to salinity content can also affect the proper functioning of the fathometer system. Colder, denser water will permit the signal to travel faster, thus indicating a shallower depth than is actually present. Warmer water will slow the passage of the signal and may compromise your safe limits by indicating a greater depth than may be actually present. The user should keep these factors in mind when his vessel encounters extremes in either case.
Colder or denser layers may also return the fathometer signal before it reaches bottom. A dual trace can appear on the fathometer graph. This does not usually compromise the vessel's safety because it indicates a shallower depth than is actually present. Every fathometer whether indicating or recording has its own particular method of operation. The operator needs to consult the manual for the specific unit to determine proper operation.
