A difference between a radar ullage reading and a manual hand dip does not immediately mean that the radar gauge is inaccurate. On a legacy SAAB TankRadar STaR installation, the two readings may refer to different positions on the tank. The displayed radar value may also include trim and list corrections.
Before changing calibration or replacing equipment, establish what each number represents.
Start with the reference point
Ullage is a distance measured from a defined reference point down to the liquid surface. A hand dip taken at a separate ullage plug and a radar reading referenced to the gauge position are not automatically comparable, particularly when the vessel has trim or list.
The STaR service documentation describes installations in which the vertical distance between separate reference points is stored in the system database. The workstation and Supply and Communication Unit can use this configuration when presenting the corrected ullage.
Record the physical hand-dip location and identify the reference point used by the radar display before assessing a discrepancy. Check the vessel’s drawings, calibration records and installed setup; do not assume that another STaR vessel uses the same arrangement.
Check whether trim and list correction is active
Depending on its configuration, a STaR system can present an ullage corrected to a separate ullage plug or to a tank reference associated with the centre of gravity used for volume calculations. A hand dip taken at a plug should therefore be compared with the corresponding radar value and correction mode.
The comparison changes with the setup:
- Same ullage-plug reference: compare the manual reading with the corresponding radar ullage under the correction conditions specified for that installation.
- Separate plug with radar corrected to that plug: compare the hand dip at the separate plug with the radar value corrected to that plug.
- Radar corrected to a volume reference: use the approved tank tables to derive volumes from the hand dip and the corrected radar value, then compare the volumes.
Comparing two displayed millimetre values without confirming their reference points can create an apparent error where none exists.
Take readings under comparable conditions
Record both readings as close together in time as practicable. Note the tank, cargo condition, vessel trim and list, hand-dip point, radar correction mode and whether loading or discharge is in progress.
A moving liquid surface makes a single comparison less informative. The legacy manual notes that turbulence, waves, foam and strong trim or list can weaken the radar echo; exceptional operating conditions can lead to loss of echo. If the radar status is abnormal, diagnose that condition before using its value as a calibration reference.
Manual measurement and any access to tank fittings must follow the vessel’s cargo and safety procedures.
Investigate the pattern before adjusting the gauge
A useful troubleshooting question is whether the difference is consistent.
If the offset changes with trim or list, review the reference-point and correction configuration. If it remains similar under stable conditions, check the documented reference heights, manual measuring method and historical comparison records. If the radar value jumps or its status changes, investigate echo quality and the relevant gauge or communication fault.
None of these patterns alone identifies a failed component. Preserve the as-found configuration and readings so a later adjustment can be justified and verified.
What to include in a comparison report
A service record should identify:
- Tank and gauge identification.
- Date, time and cargo-operation status.
- Hand-dip location and its defined reference point.
- Hand-dip and radar readings, with units.
- Trim, list and selected correction mode.
- Tank-table edition if volumes were compared.
- Radar status or active alarms.
- Difference observed and any repeated readings.
- As-found findings, action taken and as-left result.
This record makes the result more useful than a statement that “radar and hand dip differ.” It also helps the next engineer repeat the comparison on the same basis.
For wider fault-finding context, see our SAAB TankRadar service guide and STaR Ground Failure troubleshooting article.
Note on legacy installations
This guide draws on the sixth-edition SAAB TankRadar STaR service manual from November 2005, especially its Comparing Ullages section. Gauge type, reference geometry, correction settings and software can differ between vessels and later product generations. Use the installed system’s current documentation and approved tank tables for any onboard assessment. Reform Marine is an independent service company; product names identify installed equipment and do not imply manufacturer authorization.