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Some reasons that may cause differences in observed
gauge heights and potentially affect the accuracy of measurement are:
gauge hatches situated on improperly-secured man- ways; sediment
build-up in tanks; obstacles or deadwood in tanks; and changes in tank
configuration.
In the event that differences are found, the gauger
must determine whether to use the innage or the ullage method to
measure the levels in the tank. This decision will depend on the cause
of the discrepancy. For example, if the difference in
reference height is due to a buildup of residue on the tank floor, the
ullage should be used. If the difference is caused by an
increase in reference height as a result of improper setting of the
tank top, the innage method should be used. If it is
established that the tank configuration has changed, a protest should
be filed, and the official capacity tables amended.
Certification of Capacity and
Wedge Tables
Some vessel owners or operators may use a set of
capacity tables prepared for a class of vessel based on one sister
vessel. Capacity tables used to determine cargo volumes should be
certified, preferably by the shipbuilder, for accuracy of use on board'
the particular vessel for which they are issued; however, if the only
tables available are not certified, they may be used. In all
cases, the innage/ullage report should note the name of the vessel they
were prepared for; the name of the vessel they were used on; the name
of the certifying ship builder; and, if the tables were only certified
by the vessel owner or operator, an explanation why there is no
shipbuilder certification, as well as the name of the shipbuilder.
Missing
Capacity Tables
All parties involved, including
the vessel's owners, should be notified immediately and a letter of
protest issued when the vessel's capacity tables cannot be
located. Copies of the tables should be obtained at the
earliest possible opportunity. In such situations,
measurement data must be obtained as usual and retained until the
tables become available and calculations can be performed.
Lack
of and Poor Maintenance of Equipment
Before any custody transfer
occurs, vessel operators, gaugers, inspectors, and others involved in
marine bulk cargo transactions must be aware of the specific
requirements for, and the condition of, all measuring equipment and
devices used in the transfer. Equipment that is known to be defective,
out of calibration, or in poor operating condition must not be used.
All equipment, whether automatic or manual, must con- .form to the
latest edition of the API Manual of Petroleum Measurement Standards
(MPMS), unless all parties involved in a specific measurement activity
have previously agreed on an alternative.
Draft
Readings and Trim and List Corrections
DRAFT
READINGS
Draft marks are displayed in
US Customary (feet and inches) or SI (metric) units. The numbers for US
Customary units are 6 inches high and are spaced 6 inches
apart. Readings are made from the bottom of each number and
are estimated to the nearest inch. The numbers for metric
units are displayed in even decimeters, are 10 centimeters high, and
are spaced 10 centimeters apart. Readings are made from the
bottom of the numbers and estimated to the centimeter
Draft readings must be taken
before and after loading and discharging. They are usually
used to determine the following:
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The depth of the vessel in the water.
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The trim and list of the vessel.
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Whether the vessel is loaded correctly.
Draft readings can also be
used as an
alternative method for determining the weight of the cargo loaded on
board the vessel by means of a dead-weight survey. To
accomplish
tills, the salinity of the water in which the vessel is floating must
be determined and the vessel's port and star- board, fore, aft, and
midships drafts taken, averaged, and recorded. The vessel's
deadweight/displacement scale is then entered using the average draft
and the salinity. This gives the total weight on board the
vessel
at the time of observation. Subtract the vessel's fuel,
water,
stores, and constant from the total weight on board to determine the
weight of the cargo on board.
Sections of text taken from
API - MPMS Chapter 17 Marine Measurement
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