Calibration of Level Sensors in Cargo Tanks

Accurate measurement of liquid cargo is critical for the commercial operation and safety of any tanker or gas carrier. The Custody Transfer System (CTS) relies heavily on the precision of cargo tank level sensors. Overfill incidents can lead to catastrophic environmental damage and vessel loss. This technical instruction details the methodologies for calibrating and verifying the accuracy of level sensors in cargo tanks.

Types of Level Sensors on Modern Tankers

Several technologies are employed for level gauging, each requiring specific calibration approaches:

• Radar Level Gauges (FMCW): The industry standard for LNG and LPG. They emit microwave signals down a stilling well and measure the time of flight of the reflection. They are highly accurate and non-intrusive but require complex software calibration to account for the dielectric constant of the vapor phase.
• Float-Type Gauges: Traditional mechanical systems using a float connected to a magnetic tape. Prone to mechanical wear and require regular physical verification of the tape and float integrity.
• Capacitance Probes: Often used for high-level and overfill alarms rather than primary continuous gauging. They measure changes in electrical capacitance as the liquid covers the probe.

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The Calibration Procedure

Calibration is not merely pressing a “zero” button. It involves verifying the sensor against a known reference point (datum) and ensuring accuracy across the entire measuring range.

1. Establishing the Datum: Every tank has a defined reference height (from the tank bottom to the gauging point). This must be verified against the ship’s approved calibration tables.
2. Zero and Span Checks: For radar gauges, this often involves software verification using fixed reference pins built into the stilling well (e.g., AutroCAL systems). The software compares the measured distance to these pins against their known physical locations to adjust the velocity factor.
3. Physical Verification (Dip Tape): The most reliable cross-check is taking a manual sounding using a calibrated, Class-approved UTI (Ullage, Temperature, Interface) tape. The UTI reading is compared against the remote gauge reading, taking into account vessel trim, list, and temperature corrections.

Simulation and Class Documentation

To test the high-level (e.g., 95%) and overfill (e.g., 98%) alarms, physical filling is impractical. Instead, engineers use specialized calibrators or software simulation to inject a signal mimicking a rising liquid level. This verifies that the entire alarm loop, from the sensor to the Cargo Control Room (CCR) panel and emergency shutdown (ESD) system, functions correctly.

Every calibration and alarm test must be meticulously documented. Classification societies (e.g., DNV, ABS) and SIRE inspectors require valid calibration certificates for the primary gauges and the reference UTI tapes, ensuring compliance with the IGC Code and company SMS.