Measurements
should be taken in the same units used in the tables, If measurements
must be taken in other units, the conversion factors given in Appendix
C should be used. When capacity tables are not calculated to
the prescribed graduations 1/8thin., 1mm, or 0.01ft), and when gauge
readings fall between the values in the tables, interpolation will be
necessary.
The
observed reference height should be compared with the reference height
given in the tables and recorded (see Appendix B3). Trim
corrections must be carefully applied when the vessel is not on even
keel. Additional corrections are required when the vessel is
listing. Trim and list corrections must be carefully applied
as
indicated in the appropriate vessel tables (see Appendix B.4, B.5, and
B.7).
The
trim and list corrections given in the tables shall be applied only
when the liquid is in contact with all bulkheads in the tank but is not
in contact with the top of the compartment. When the free
water of cargo surface does not touch all bulkheads of the tank, wedge
tables or a wedge formula may be used to determine the volumes present
in the tank.
VOLUME
CORRECTION
TABLES
See SN's Ullage Reports
The volume correction tables
are necessary to convert the observed cargo volumes being measured to
standard volumes at standard temperatures such as 60°F or
15°C. When making these conversions, always be sure to use
the correct tables according to the sales contract or as otherwise
agreed. Generally, Tables 6A, 24A, and 54A are to be used for
crude oils, 6B, 24B, and 54B are to be used for products, and 6D, 24D,
and 54D are to be used for lubricating oils. It must be noted however,
that not all crude oils and products fall into the common categories
and other tables such as Tables 6C, 24C, and 54C may provide a better
"fit" for calculation purposes. If you are not familiar with
the specific cargo being measured, confirm with principals which table
should be applied.
OBSERVATIONS,
MEASUREMENTS, AND CALCULATIONS
After all observations and
measurements shown on the checklist (see API MPMS Chapter 17.1) have
been made, quantity calculations must be performed. Volumes should be
determined according to the steps as follows:
Total Observed Volume (TOV) 
Free Water (FW) = Gross Observed Volume (GOV)
x Volume Correction Factor (VCF
or Ctl) = Gross Standard Volume (GSV)
 Sediment and Water
(S & W) = Net Standard Volume (NSV)
It may be noted that the
parties involved should agree whether to calculate the ship volumes to
a GSV or NSV end volume and which values should be used for the
required calculations (i.e., ship or shore S&W, FW, API, etc.
values).
It may be noted that if the
cargo being measured does not have an S&W content (as is the
case with most clean products), the GSV would then equal the NSV of the
cargo.
Total Calculated Volume (TCV) =
GSV + FW
of
the cargo being measured

Total Observed Volume
(TOV). Determine the total observed volume by
entering the appropriate capacity table for each vessel's tank with the
measured innage/ullage readings, with trim and list corrections (if
any) applied according to the instructions in the tables. As
an alternative, the trim and list corrections may be applied as
adjustments made to the volumes, based on observed gauges. The capacity
tables will indicate the correct method of applying trim and list
corrections.

Free Water (FW).
Determine the free water volumes by entering the capacity tables, as
described in step a, using the obtained watercut measurements.

Gross Observed Volume
(GOV). Subtract the free water volume from total observed
volume determined in steps a and b to calculate gross observed volume.

Gross Standard Volume
(GSV). Calculate the gross standard volume by multiplying the
gross observed volume by the volume correction factor. The volume
correction factor is obtained as appropriate from API MPMS Chapter 11.1

Net Standard Volume
(NSV). When required, calculate the cargo's NSV volume by
subtracting its S&W content from the calculated GSV
volume. Therefore, if the cargo has no S&W, the
vessel's NSV would be equal to its GSV.

Total Calculated Volume
(TCV). Add any free water (FW) determined to be on board to
the calculated gross standard volume (GSV) to calculate the total
calculated volume (TCV).
VOLUME
OF SHIP'S PIPELINES
Each ship should have a diagram
on which the location of loading and discharging piping is clearly
indicated. The diagram should show the diameter of each line and the
volume of each line when full or provide sufficient information to
allow the calculation of these volumes. Knowledge of the lines used
during the cargohandling operation should make it possible to
determine which pipes could contain oil at any point in the cargo
operation. It must be understood, however, that the actual
pipeline volumes usually cannot be quantified at all times.
Sections of text taken from
API  MPMS Chapter 17 Marine Measurement
