In an echo sounder the stylus is rotating with certain constant speed and transmission takes place when the stylus passes the zero mark.
When higher range scale is selected, the transmission will still take place when the stylus comes to zero. But the stylus speed is reduced because the stylus has to remain on the paper for longer period of time since the echos are returning from greater depths.
This system is known as “ RANGING “.
The range scales are generally provided as :
0 – 100 mtrs
0 – 200 mtrs
0 – 300 mtrs
0 – 400 mtrs
and so on.
Since the same length of paper now covers a larger depth the graduations become closer and it becomes difficult to read the depth accurately.
PHASING arrangement is used to avoid this.
In Phasing arrangement, the speed the stylus motor is kept constant, but the transmission point is advanced.
As shown in Fig, four sensors are positioned around the stylus belt and stylus is rotating at a constant speed.
A magnet mounted on the belt generates the pulse when it passes the sensor, which in turn activates the transmitter.
When the minimum range ( 0 – 100 mtrs ) is selected, the sensor 1 is used and the delay circuit arrangement is such that the transmission occurs exactly when the stylus passes the zero mark.
The bottom scale of the paper will depend on the speed of the stylus motor and it is set to 100 mtr.
On selecting the higher range ( 100 – 200 meters ), the sensor 2 is used for activating the transmitter and the zero of the scale so shifted that the top of the paper corresponds to 100 meters.
Since the speed of the stylus motor remains same, the bottom graduation corresponds to 200 mtrs.
Similarly, for measuring higher ranges different sensors are selected and the transmission time will be advanced accordingly.
It is always advisable to start the echo sounder at minimum range scale when phasing facility is provided, so that shallow depths are not missed.
Thus the danger of missing shallow waters can be avoided.