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The sounding tape and reel-type level sensor is sometimes called a fishing-reel level sensor. The name of this sensor comes from the fishing term sounding.
In modern level sensors a weight is attached to a line and the line is lowered from the top of a tank. The weight will cause the line to have tension as long as it's suspended in air. When the weight comes into contact with the level of the material it's measuring, the amount of tension will be reduced and a sensor can detect this change. When the tension changes, a mechanism stops the line from lowering and begins to retrieve the line. A spool is used to reel the line in and the number of rotations the spool uses to reel the line completely to the top is measured and converted to a level measurement. Some of these sensors use a magnet in the spool and the number of times the magnet passes a detector is counted. In other applications a rotary potentiometer is connected to the spool and the number of rotations is converted directly into a change of resistance. In both types of sensors, the transducer signal is converted to a usable 4-20 mA output signal. If the spool uses a magnet and counter, the counter is reset when the tension changes and indicates the weight at its lowest point. When the reel begins to pull the line in, the counter will be ready to determine the amount of line that was lowered. Figs. 1, 2, and 3 show example applications using the sounding-type level sensor.
Above: ill. 1: (a) An application of a sounding-type
level sensor that uses a weight and line to measure the level of granular
material in a tank. (b) An application of a sounding-type level sensor
that uses a weight and line to measure the level of liquid in a tank.
The weight in this application will float on the liquid when it reaches
its surface.
Above: ill. 2: Example of a sounding-type level
indicator shown mounted in place to indicate the level of a tank.
Above: ill. 3: (a) An example of the spool mechanism
used to measure the amount of line that has been lowered to determine
the level of material. The output of this amplifier is 4-20 mA. (b) An
example of the output of the level sensor sent to a display. (c) An example
of a level sensor connected to a floating roof for a tank, and the output
signal is sent to a transmitter and a display.
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