YOKOGAWA EML500 Series, User Manual, Page: 52 / 85

6-9
IM80B80T10E 21th Edition : Jul.1,2007-00
< 6.INSTALLATION >
6.5 Milepost Measurement (Maintenance Mode)
In actual navigation, the flow of water detected by the sensor in the measurement rod depends on
the installation position of the sensor, type of ship, length of the protruding sensor, draft, and the
ship's speed. Therefore, to obtain more accurate data, it is necessary to compare the indicated
speed with the actual one, and correct any discrepancy between them.
The actual speed can be measured when the ship is sailing on calm water over a certain distance.
In this case, however, the tide and wind can affect the speed . To eliminate these factors, the ship
makes a round trip between mileposts. For accurate measurement, preliminary sailing is also nec-
essary to minimize any variations in the speed, and the ship must take the correct course between
mileposts.
In milepost calibration, the ship makes a round trip (2 one-way trips ) or a round trip plus another
on-way trip (3 one-way trips) to obtain a set of data which serves as the unit of measurement. The
mean values of the actual speed and log speed are calculated based on these milepost measure-
ment data.
(1) One round trip
Measurement for a round trip is recommended if the influence from the tide and wind is constant
with respect to the time.
Mean log speed /mean actual speed : VL
VL= VL1+VL2
2
IN VL1 OUT
OUT VL2 IN
The trip out
The trip back
Milepost Milepost
Round-trip measurement
(2) One and a half round trips
Measurement for three one-way trips is recommended if the influence from the tide and wind is
changing along with the time.
Mean Log speed / mean actual speed : VL
VL=
VL1 + 2
×
VL2 + VL3
4
IN VL1 OUT
OUT VL2 IN
IN VL3 OUT
The trip out
The trip back
The second
trip out
Milepost Milepost
One-and-a-harf-round-trip measurement