The IMO requirement on Tanker Safety and Pollution Prevention, Annex I - Regulation 13B (requirement for C.O.W.) and its Resolution A.446 (XI), 6.6(b) (use and control of Inert Gas) states "during Crude Oil Washing, the pressure of the atmosphere at the tank is no longer positive, the washing must be stopped until satisfactory conditions are restored."
The C.O.W. Operation / Control for this requirement can be judged by fitting portable gas pressure meter temporarily on Tank Gauging Station (Vapor Locks) and watch condition of inert gas pressure on each tank.

MMC Inert Gas Pressure Meter Model. "GVP-75NF" consists of a pressure meter (Low measuring meter) on a gas seal attachment, this arrangement can be fitted on MMC Tank Gauging Station (Vapor Locks) Model. "ANK-52" (Self-Closing Valve) or MMC Hand Open/Close Ball Type Valve in order to prevent leakage of inert gas from tank.

The pressure meter contains a screw type orifice which controls sudden change of pressure.

Also a Mesh Filter is fitted at the inlet of gas pressure to prevent dust coming in.

Indication of the meter applies within the range -10kPa (vacuum) ~ +30kPa (pressure).
CAUTION:
This portable gas pressure meter is developed to measure pressure of Inert-Gas filled in tank simply on deck locally. However, as the meter is low and delicate pressure type, careful handling is required. Also the meter is not constructed to fit permanently on deck for water-tightness and durability. So, it is recommended to remove it from tank gauging station and store at a suitable place after use.