Radar Level Gauge

Electronics unit

Converts the raw echo into a level value, applies temperature and density compensation, and outputs to the local display and to the cargo control room. On tankers this feeds the loading computer directly.

Reference and calibration point

A fixed reference target or a manual dip point lets the gauge be checked against a physical measurement, which is the only way to catch electronic drift between class surveys.

Selection and sizing

The choice is driven by the tank's height range, the cargo's dielectric properties and the vapour condition expected above it, since inert gas and hydrocarbon vapour both change the signal's travel time slightly and a good installation compensates for this. Custody transfer duty calls for the tightest accuracy class the system offers; general operational gauging can use a coarser one.

• Frequency band matched to tank height and required resolution, since higher frequency narrows the beam and improves accuracy over long ranges
• Process connection rating matched to the tank's design pressure and the cargo's vapour pressure
• Antenna material compatible with the cargo, since chemical carriers rotate through very different products
• Accuracy class stated against a recognised custody transfer standard if the reading is used for commercial quantity

Regulations and class

The IGC Code requires gas carriers to have a level gauging system independent of the high-level alarm, and both are surveyed separately. Chemical and oil tankers under MARPOL are increasingly required to use closed gauging so cargo vapour is not released to check the level, which radar naturally supports since the tank is never opened. Class societies survey the gauging system's function and calibration at intervals set in the ship's survey scheme, and a custody transfer installation is also checked against national metrology or OIML-referenced accuracy requirements where the cargo is sold on the reading.

Typical faults

What to look for in a supplier

• Type approval for the specific hazardous area zone and gas group the tank requires
• A stated accuracy class with supporting test data, not just a manufacturer's claim
• Compensation software that accounts for the actual vapour composition the ship carries, not only air
• Service support able to recalibrate or replace the electronics unit without gas-freeing the tank where possible

Cross-check the radar reading against a manual dip at the start of every cargo the ship has not carried recently - a change in dielectric constant between products is the most common reason a previously accurate gauge starts to drift.