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depths less than 1.5 m are to be measured then a shorter pulse length is
required. One sounder has a pulse length of 0.3 m, which gives a
theoretical minimum sounding of 0.225 m and a practical minimum of
0.45 m.
If a very deep measurement is to be made then more energy is required.
This could possibly be achieved by increasing the amplitude of the pulse,
but this is usually limited by the output of the active element in the
transmitter and therefore it is injected directly into the water. Sounders,
which have to cover very shallow and very deep sounding on different
ranges, will usually be designed so that the pulse length can be changed
as the range is changed.
PULSE LENGTH

Shallow 0.3 milisecond Up to 200 or 400 meters

l to 5 milisecond 2000 meters or more
The commonest form of echo sounder has a display, which records the
depth on electro-sensitive paper. It may take the form of rotating arm
moving anti-clockwise across the paper, which is marked by the stylus at
the end of the arm when a DC pulse is applied on receipt of the echo.
Another type has a moving belt to which the stylus is attached and which
is made to move across the paper from top to bottom at a constant rate,
which is decided by the depth scale displayed. The paper is marked in
the same way, and the indicated depth is measured from the top of the
paper by a suitable vertical scale at the side.
Another type of display more suitable for shallow depths consists of a
disc or arm carrying a neon lamp at its extreme edge, which is spun
around at constant speed. A scale is fitted around the edge of the area
covered by the spinning neon which is made to flash at zero on
transmission and again on receipt of the echo at the point in its revolution
appropriate for the depth measured. The overall recording accuracy
claimed for one echo sounder is close to +/- 2% of the actual depth.

Recording Paper may be of two kinds, moist and dry. The moist paper is
impregnated with a solution of potassium iodide and starch. When a
direct current is passed through it from the stylus to the metal plate at the
back, it releases iodine and causes a brown stain to appear. The stylus
is tipped with iridium. This action only takes place when the paper is damp
- it becomes an insulator when dry.

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