Note

Do not use a 50-kHz transducer for record surveys.

Recording units are generally tuned to a specific transducer element and cable. The unit should be test-operated far in advance of the job when switching transducer types or frequencies in order to determine the need for internal adjustments. Transducers of the same style are interchangeable.

Transducer Mounting. The transducer should be firmly mounted as suggested in the O&M manual. The transducer head should be at least a foot below the surface to avoid it becoming airborne if the boat pitches. The following conditions must exist with the boat on an even keel or the same list that will exist during the survey:

(1) Transducer mast vertical to prevent reading a slant distance for depth

(2) Transducer head tilted backward about 1 degree from vertical to maintain smooth water flow over the transducer face without cavitation

(3) Transducer mast mounted for a fixed depth

Recorder. The recorder must be protected from water spray. The unit may be enclosed in a plastic bag with the bag opening facing aft. The operator can read into the bag to open the recorder door and write on the paper while still protecting the unit.

Fathometer Operations Tips

Stylus Problems. Stylus wear, breakage, or loss of spring tension can affect both the black line trace darkness and the location where the trace is burned into the paper.

Stylus Belt Problems. A loose set screw on either of the stylus drive belt wheels will result in the belt slipping off. A tired, weak, or worn stylus belt sponge will result in the stylus wire not tracking properly. If the sponge is too loose, the stylus wire will either fall out of its retaining clips or will alternately track and then underride the support plate for the electrical contact wire. This last condition will tend to shear the top end of the stylus and will result in double-struck, unintelligible marks. Field repairs to the sponge can be performed by gluing a small replacement sponge to the belt with contact cement. A soft synthetic sponge or a piece of yellow foam earplug will work satisfactorily.

Paper Creep. A misaligned takeup platen will cause the paper to creep up or down on the recorder table. The symptoms are as follows:

(1) Drifting zero mark at draft mark.

(2) Paper buckling on the top or bottom edge as it runs against the plastic alignment guides.

(3) Stylus marking the top edge of the paper above the zero mark (this is caused by the stylus physically marring the tops of the folds of the buckling pressure-sensitive paper). Correct for paper creep by adjusting the slot screw holding the axle arm for the paper takeup reel.

Reconstructing Readings for a Drifting Zero Due to Paper Creep. It may be discovered after recording a number of runs that the paper has crept up and down on the recording due to an improperly aligned takeup platen. A misaligned trace can be corrected mathematically. If the error was the same for the whole trace, add or subtract the correction as appropriate. For a gradually increasing or decreasing error, divide the trace into equal sections about 6 inches long and use an average correction for that 6-inch strip by interpolation. Determine different corrections for each 6-inch section.

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