Eagle Electronics SeaFinder 250 DF - Manual (Page 7)

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NOTICE!
The storage and operation temperature range for your unit is from
-4 degrees to +167 degrees Fahrenheit (-20 degrees to +75 degrees
Celsius). Extended storage or operation in temperatures higher or
lower than specified will damage the liquid crystal display in your
unit. This type of damage is not covered by the warranty. For more
information, contact the factory's Customer Service Department;
phone numbers are inside the manual's back cover.
How Sonar Works
Sonar has been around since the 1940s, so if you already know how it
works, skip ahead to the next segment. But, if you have never owned a
sonar fish finder, this segment will explain the under water basics.
Sonar is an abbreviation for SOund NAvigation and Ranging, a tech-
nology developed during World War II for tracking enemy submarines.
A sonar consists of a transmitter, transducer, receiver and display. In
simple terms, here's how it finds the bottom and the fish:
The transmitter emits an electrical impulse which the transducer con-
verts into a sound wave and sends into the water. The sound frequency
can not be heard by humans or fish. The sound wave strikes an object
(fish, structure, bottom) and bounces back to the transducer which con-
verts the sound into an electrical signal.
The receiver amplifies this return signal, or echo, and sends it to the
display where an image of the object appears on the scrolling sonar
chart. The sonar's microprocessor calculates the time lapse between the
transmitted signal and echo return to determine the distance to the
object. The whole process repeats itself several times each second.