SOLAS (International Convention for Safety of Life at Sea), Chapter II-2, Part B, Regulation 4, 5.5.1.1 states "For tankers of 20,000 tonnes deadweight and upwards, the protection of the cargo tanks shall be achieved by a fixed inert gas system in accordance with the requirements of the Fire Safety Systems Code".

(and) 5.7.1 / 5.7.2.1 [ 2000 - resolution MSC.99(73) ]　states "Tankers shall be equipped with at least one portable instrument for measuring flammable vapour concentrations, together with a sufficient set of spares. Suitable means shall be provided for the calibration of such instruments. / Suitable portable instruments for measuring oxygen and flammable vapour concent-rations shall be provided."
(and) MARPOL (International Convention for the Pre-vention of Pollution From Ships, 1973 as modified by the Protocol of 1978), resolution A.446(XI) (amended by A.897 (21)) [ Specifications For The Design, Operation And Control Of Crude Oil Washing Systems], 6.6 [ Use and control of inert gas ] states "before each tank is crude oil washed, the oxygen level shall be determined at a point 1 metre from the deck and at the middle region of the ullage space and neither of these determinations shall exceed 8 per cent by volume.

"MMC" Gas Sampling (Sealing) Attachment is prepared for Portable Oxygen meter / Gas detector which will be supplied by shipowner or manufacturer.
Portable Oxygen meter / Gas detector with this attachment operated on MMC Tank Gauging Station (Vapor Locks) installed on tank top complies with the above requirement [suitable portable instruments].

* Reference
THE U.S. COAST GUARD MARINE SAFETY CENTER (MSC) CONCERNING USE OF OXYGEN SAMPLER AS A RESTRICTED DEVICE (A DEVICE WHICH PERMITS ONLY A SMALL QUANTITY OF CARGO VAPORS TO ENTER THE ATMOSPHERE) CAN BE USED ON BOARD TO COMPLY WITH 46 CFR PART 39.30-1(i).