Tank gauging turns the level in a tank into a number the crew can act on, and the alarm on that number is what prevents an overfill. That makes the gauging system a cargo safety system rather than an instrumentation convenience, particularly on chemical and product tankers where the consequences of an overfill are a pollution and a fire risk. The measuring principle, the calibration and the alarm settings all have to agree with the tank and the cargo.
Key facts and acceptance criteria
The table below lists the values that decide whether the system passes at survey and at maintenance. They come from the maker documentation and the applicable rules, not from general practice, and they are the numbers a surveyor checks.
| Parameter | Requirement | Reference |
|---|---|---|
| Gauging accuracy | Within the maker and approval requirement for the tank type | Maker and approval documentation |
| High level alarm | Set at the level required by the cargo and tank rules | SOLAS II-1/18 and class requirements |
| Overfill alarm | Independent of the high level alarm, and annunciated | SOLAS II-1/18 |
| Remote indication | Level available at the required control positions | SOLAS II-1/31 and maker procedure |
| Sounding and ullage | A means of independent level verification remains available | Vessel procedure |
Regulatory and technical basis
SOLAS Chapter II-1 regulations 18 on tankers and cargo systems, MARPOL Annex II for oil and noxious liquid cargoes, the maker technical manual, and the class survey plan. Tank gauging and alarm arrangements on tankers are examined at the annual survey, and the gauging calibration is a frequent subject of inspection findings.
What the service assessment covers
Seanav Marine provides independent service assessment. The following items define a starting scope for a competent service assessment. The actual test method and acceptance criteria come from the installed equipment documentation and the agreed job scope.
- Level transmitter, radar and servo gauging calibration and verification
- High level and overfill alarm setting check, proving and annunciation test
- Remote indication and repeater testing at the required stations
- Density and temperature compensation check against the cargo declaration
- As-found calibration record, alarm schedule and defect report
How the work is planned and executed
- Tank and cargo review. Establish the tank type, the cargo declaration, the required alarm levels and the approved gauging arrangement, and obtain the maker data.
- Calibration check. Verify the measuring chain against a known level, check the density and temperature compensation, and record the as-found errors.
- Alarm proving. Prove the high level and the overfill alarm independently, confirm the annunciation at each required station and confirm the alarm schedule.
- Report. Issue the calibration results, the alarm proving record, the as-found and as-left values and the outstanding defects for the class and the cargo system record.
Failures that must be distinguished before testing
Do not widen a setpoint or suppress an indication without an engineering assessment. The following failure modes have to be separated from a genuine plant fault before a conclusion is drawn, because they are common, they are visible, and they are usually the cheaper explanation.
- Gauging systems left in a manual mode during cargo operations, so the alarm is not active
- Density compensation set for the wrong cargo, so a level reading in metres is read as the wrong volume
- Alarm levels re-set locally by the crew without the cargo and class approval that the rules require
Equipment and OEM compatibility
Replacement parts and calibration methods must come from the original manufacturer. Seanav Marine works with Honeywell and Endress+Hauser tank gauging, Veeder and Franklin fuel gauging, Kongsberg and ABB cargo systems, and maker-specific servo and radar gauging units. and sources parts through authorised distributor channels.
Frequently asked questions
Why are tank alarm levels so tightly controlled?
Because the high level and overfill alarms are the last line of defence before a tank is overfilled. On a tanker the required levels are set by the cargo rules and the class, and changing them without that authority invalidates the arrangement.
What is the difference between high level and overfill alarms?
They are separate functions with separate setpoints and separate annunciation. An overfill alarm normally has a higher setpoint and a more conspicuous indication, and it must not share the measuring chain in a way that lets one failure disable both.
How is gauging accuracy checked on board?
By comparing the indicated level with an independent means of verification, normally by sounding or ullage, at several levels. A consistent offset or a non-linear error points to a calibration or density compensation problem.
Can a tank gauge be serviced during a port call?
Measurement and alarm function testing is normally possible with the cargo system isolated as the procedure requires. Work that affects an approved cargo arrangement needs the maker or approval-body involvement, and it should be arranged before the ship arrives.
Reference framework
These sources explain the relevant system or regulatory context. They do not replace the installed equipment manual or prove the applicability of a particular rule to every vessel.
service@seanav.org · +971 58 568 1197 · Contact the operations desk
About this guide
Published by Seanav Marine. Editorial and source review: . This general service guide is based on the references shown; it is not a signed vessel-specific engineering procedure. Equipment manuals and the responsible technical personnel determine the job requirements.
For technical assessment or a factual correction, contact Seanav Marine with the equipment identity and supporting documents.