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Survey Documentation 

Open Measurement

Closed / Restricted Measurement

Data Collection

Special Considerations

Precautionary Notes





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Survey Documentation - Closed and Restricted Measurement

A closed measurement system is designed to allow cargo measurements to be taken with no vapours  escaping to the atmosphere.  A restricted measurement system is designed to allow measurements to be taken with minimum vapours  being allowed into the atmosphere.  The two basic categories of dosed or restricted system measurement equipment used on marine tank vessels are "portable manual" and "fixed automatic."

Manual equipment can be used to obtain levels of liquid cargo and free water, cargo temperatures, and samples; whereas automatic systems are primarily used to obtain the levels and temperatures of liquid cargo only.  While either type of equipment can be used for custody transfer measurements, it must be understood that not all automatic equipment was designed and installed on vessels for that purpose. 

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Some systems were designed to be used for shipboard operational purposes only (i.e., for determination of proper trim and stability and cargo loading/discharging).  Accordingly, both parties should be aware of the limitations of any shipboard measurement system and agree on the method of measurement to be used to determine the "official" custody transfer volumes.

If a closed or restricted measurement system is to be used for marine custody transfer measurements, the accuracy of the equipment used should fall within the tolerances set forth in API MPMS Chapters 2.8, 3.1A, 3.4, 7.3, and 7.4.  However, because of various vessel designs, and physical installation of the equipment used, accuracies other than those described therein may be the maximum achievable. The measurement accuracies designed into the system must be warranted by the manufacturer.


This section describes the equipment to be used and the procedures to be followed when measuring cargoes on ships that have manual closed or restricted systems.

Manual Closed and Restricted Equipment

Manual equipment consists of a Portable Measurement Unit (PMU) which must be carried from tank to tank to obtain the appropriate measurements through a Vapor Control Valve (VCV) located at each tank. Generally, PMUs and VCVs made by the same manufacturer are designed to be used together.  However, equipment made by different manufacturers may be used together with an appropriate adapter.

Vapor Control Valve - vapour control valve (VCV)These valves are generally found on standpipes, flanges, existing ullage hatches, expansion trunks, or fitted flush to the vessel's deck.

Note: is an illustration of a vessel that has been retrofitted for a PMU gauge location using existing gauge tables for "open" measurements. They are designed to allow attachment of the portable measurement or sampling device using a securing device or adaptor.  By operating the VCV according to the manufacturer's instructions, the PMU probe, sampler tape, and/or sampler can be lowered into the tank through the VCV whether the vessel's inert gas system (lGS) is putting positive pressure into the tanks or not.

Vapor control valves come in varying diameters from 1 in. (25.4 mm) to 4 in. (101.6 mm).  The valve sizes and types are specified by the manufacturer and the vessel owner. However, if the VCV is too narrow, it will not allow adequate sampling to be conducted.

The location and size of the VCV is critical to the ability to be able to measure tank contents and to take sufficient samples.  In order to be able to measure small quantities in a tank when the vessel is not on an even keel, a VCV must be located as close as possible to the bulkhead that is in the direction of the vessel's normal operating trim and list when the vessel is in an OBQ/ROB condition.  In placing the VCV, care must be given to assure its location will not cause the measurement equipment to touch the tank bulkhead when in use. - tank top with  PMUs not attached to VCV - tank top with  PMUs attached to VCV

It may be noted that since many vessels have been retrofitted with vapor control valves that are not in the exact location as the existing "open" gauge points, tank capacity tables should be adjusted to take in consideration any new gauge location for PMU equipment. Also, the vapor control valve locations should be placed in accordance MPMS Chapter 2.8B.  If the tables have not been adjusted for these location changes, some corrective action may have to be taken to obtain correct measurements.  Such corrective action must take into consideration the use of adapters that allow the use of different manufacturers' portable measurement units with varying vapor control configurations.

Sections of text taken from API - MPMS Chapter 17 Marine Measurement

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