IN CASE GAUGING OF CARGO OIL VOLUME IN TANK, IT CAN BE KNOWN TO GAUGE OIL LEVEL WITH TANK TABLE. HOWEVER, OIL LEVEL IN TANK SHALL BE CHANGED BY TEMPERATURE, ALSO ESPECIALLY, CRUDE OIL CONTAINS LOTS OF WATER, IT IS TO BE SEPARATED FROM OIL NATURALLY AND ACCUMULATED AT TANK BOTTOM.
THEREFORE, ACCORDING TO GAUGING (DIRECT READ-OUT) OF OIL LEVEL(ULLAGE), TEMPERATURE, AND OIL-WATER INTERFACE CONTINUOUSLY BY ONE PORTABLE GAUGING DEVICE, OIL VOLUME ONLY IN TANK CAN BE KNOWN ACCURATELY.
PORTABLE TYPE TANK GAUGING INSTRUMENT HAS MANY ADVANTAGES COMPARED WITH FIXED TYPE TANK GAUGING SYSTEM AND IS BENEFICIAL TO TANK GAUGING JOB AND CARGO SURVEY (DIRECT MEASURING) ON DECK OF TANKSHIP. IT SEEMS THAT RECENTLY PORTABLE TANK GAUGING DEVICE WITH "CLOSED" CONSTRUCTION IS PARTICULARLY REQUIRED BY SAFETY CONTROL OF GAUGING WORKS AT PORT SIDE.

IN CASE GAUGING BY PORTABLE EQUIPMENT IS SATISFIED AS "CLOSED GAUGING", BY PASSING THROUGH TANK GAUGING STATION (VAPOR LOCKS) INSTALLED ON DECK OF TANKSHIP.

IMO ADOPTION 1981 AMENDMENTS TO THE INTERNATIONAL CONVENTION FOR THE SAFETY OF LIFE AT SEA (SOLAS) 1974, REGULATION 60 AND ITS PARAGRAPH 7 REQUIRES CARGO TANK PROTECTION BY FIXED INERT GAS SYSTEM IN ADDITION TO CLOSED ULLAGE SYSTEM FOR TANKERS OF 20,000 TONS DEADWEIGHT AND UPWARDS.

ISGOTT : 7.2 MEASURING AND SAMPLING (7.2.1 General) STATES "Depending on the toxicity and/or volatility of the cargo, it may be necessary to prevent or minimize the release of vapour from the cargo tank headspace during measurement and sampling operations. Wherever possible, this should be achieved by use of closed gauging and sampling equipment. Equipment required for the measurement of ullage and temperature within cargo tanks may be either fixed (permanently installed) or portable and samples will normally be drawn using portable equipment. Closed gauging or sampling will be undertaken using the fixed gauging system or by using portable equipment passed through a vapour lock. Such equipment will enable ullages, temperatures, water cuts and interface measurements to be obtained with a minimum of cargo vapours being released. This portable equipment, passed through vapour locks, is sometimes referred to as 'restricted gauging equipment'. "

[FOR REFERENCE: THE U.S.C.G. REPORTED "THE COAST GUARD MARINE SAFETY CENTER (MSC) CONCERNING USE OF A RESTRICTED DEVICE (A DEVICE WHICH PERMITS ONLY A SMALL QUANTITY OF CARGO VAPORS TO ENTER THE ATMOSPHERE) CAN BE USED TO COMPLY WITH 45 CFR PART 39.30-1(i)."]

THE BULK CHEMICAL CODE FOR CHEMICAL CARRIER CLASSIFIES CONSTRUCTION OF TANK GAUGING SYSTEM AS "CLOSED," "RESTRICTED," OR "OPENED." IN CASE "CLOSED GAUGING" IS REQUIRED FOR CHEMICAL CARGO LIQUID ACCORDING TO BULK CHEMICAL CODE, THE USE OF "CLOSED" (GAS-TIGHT) TYPE PORTABLE GAUGING DEVICE IS ACCEPTED FOR BCH CODE CHEMICAL CARRIER BY IMO, MSC CIRC.551.

FOR IBC CODE CHEMICAL CARRIER, FIXED TYPE TANK GAUGING SYSTEM IS APPROVED AS "CLOSED SYSTEM", AND THE USE OF PORTABLE GAUGING DEVICE [ "CLOSED" (GAS-TIGHT) ] IS ACCEPTED AS SECONDARY FOR FIXED TYPE TANK GAUGING DEVICE AND/OR FOR SURVEY OF CARGO [DIRECT READ-OUT MEASURING].

THE IMO MARPOL 1973/78 REQUIREMENT IS THAT SLOP TANK(S) MUST BE PROVIDED TO RETAIN THE SLOPS GENERATED BY TANK WASHING, OIL RESIDUES AND DIRTY BALLAST RESIDUES IN ORDER TO PRESERVE THE SEA FROM POLLUTION BY OIL.
FURTHERMORE, IMO MARPOL REGULATION 15, (3) (b) STATES "EFFECTIVE OIL/WATER INTERFACE DETECTORS APPROVED BY THE ADMINISTRATION SHALL BE PROVIDED FOR A RAPID AND ACCURATE DETERMINATION OF OIL/WATER INTERFACE IN SLOP TANKS AND SHALL BE AVAILABLE FOR USE IN OTHER TANKS WHERE THE SEPARATION OF OIL AND WATER IS EFFECTIVE AND FROM WHICH IT IS INTENDED TO DISCHARGE EFFLUENT DIRECT TO THE SEA."