Tru Calibration; Draught Calibration (Single Point Calibration) - JRC JLN-900 Installation And Instruction Manual

Speed depending calibration - multiple point calibration factors compensating differences in
indicated speed over the whole speed range. Factors can be preset for up to ten different
speeds. Speeds between the calibrated points are interpolated automatically.
Note: The minimum necessary calibrations to be set are the TRU-calibration plus at least one of the
draught calibrations. If so, after setting the TRU-calibration, choose e.g. FULL LOAD and
compensate for the normal cruising speed used at normal load conditions (se below)
Draught Calibration and Speed Depending Calibration shall normally not be used at the same time.
4.2.1

TRU Calibration

This type of calibration is compensating small differences in characteristics of individual transducers.
The transducer has been independently calibrated in factory and its compensator factor must be
programmed in-to the speed log it is connected to. The factor is engraved on the transducer housing as
TC±000. It is also labelled at the termination end of the TRU-cable. Example: TC+123 means
calibration factor +1,23% and TC-101 means calibration factor –1,01%.
Note: The TRU Calibration shall be set to its value before any other calibration is performed.
4.2.2

Draught calibration (Single point calibration)

This type of calibration is meant for merchant vessels where the water flow around the hull is
changing due to different draughts and trims. Three calibrations can be performed for cruising speeds
relevant to each of three decided draught/load conditions (FULL LOAD or BALLAST 1 or
BALLAST 2). These calibrations are compensating with the same calibration percentage, respectively,
over the whole speed register.
Changing between the three draught/load calibration types can then be done later in menu C1 by
pressing plus (+) or minus (-). However the change must be saved by pressing the ENTER button.
Before the calibration runs, make sure:
that correct TRU-calibration factor is set (see above)
that the desired draught condition is chosen and set to 0% (Menus C1/C1.1)
the Multi Point Calibration is DISABLED (Menu C3)
The system is calibrated by sailing a true, known distance in calm waters. To eliminate variations
caused by tide, current and wind, the ship should run the same route in both directions. For each
separate run, carefully observe beginning and end of the true sailed distance and corresponding
measured distance on log display. Then, for each speed, do following calculation to find the
calibration factor CF:
CF=[(expected dist.1 + expected dist.2 ) / (measured dist.1 + measured dist.2)] x100-100
Normally the true distance is based on optical observations, but it is also possible to use other
reference systems like DGPS or similar. If the calibration factor is based on a very long (several hours
or days) comparison with GPS observations, make sure that it has not been affected by wind and
current.
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