3.3.3 Gauging Procedures-Free Water Gauging for free water on a marine vessel is similar to a shore tank. Water finding paste is applied to the bob of the gauge tape, and an innage gauge is taken. Water that comes into contact with the water paste will cause the paste to change color. The water cut can be read from the gauge tape bob. 3.3.3.1 Trim Correcting Free Water If free water is detected on a vessel that is out of trim, the free water will need to be corrected for trim. This can be done in the usual manner by reference to the trim correction tables; however, care must be exercised.
Where a vessel is out of trim and the liquid material does not touch all four bulkheads, wedge tables or the wedge formula should be used. When the nature of the material is nonliquid, it is preferable to take a number of innages at different locations and average them. This is not always possible. If the material is non-liquid and it is only possible to obtain a gauge from one location, it should be assumed that the material is lying evenly on the bottom of the tank and no correction should be applied to the gauge. OBQ / ROB can only be qualified as Liquid, Non-liquid, or Free Water. Terminology such as ‘pumpable’, ‘un-pumpable’, ‘reachable by ships equipment’ must not be used. 3.4 Conclusion Gauging Never use an outage (ullage) tape and bob to take innage gauges. Never take a gauge if the floating roof is in the critical zone. You must use a gauge tape and bob assembly that have been calibrated as a unit.
Liquid level
Free water level
Free water in a wedge condition Trim corrections do not apply
If the trim correction is greater than the innage, wedge tables or a wedge formula should be used to calculate the free water volume. 3.3.4 Gauging Procedures – OBQ / ROB A small quantity in a tank prior to loading is called the On-Board Quantity (OBQ) and what remains on board after completion of discharge is called Remaining On Board (ROB) . Gauging OBQ / ROB quantities adds a new element, as the material may be liquid or nonliquid. OBQ/ ROB quantities are usually innaged. If an ullage is needed, subtract the innage from the measured gauge height. In some cases, part or all of the OBQ / ROB may be water or wet sediments. Water finding paste should be used when gauging OBQ / ROB.
Free water level
Liquid level
Free water touching all bulkheads Trim corrections applicable
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