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Open Measurement

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Survey Documentation - Open Measurement Equipment and Procedures

Open OBQ / ROB Measurement

OBQ and ROB volumes may be determined by either the innage or the ullage method.  Liquid material is usually innaged. Solid material must be ullaged.  ROB should be measured after lines (hoses) have been drained into the vessel.  By draining lines (hoses) to a single small tank, ROB may be measured more accurately.

When a vessel is out of trim, some OBQ and ROB quantities may not be measurable at the proper gauge points.  In these circumstances, more extensive methods of volume determination may be necessary, and additional measurements will usually be required.

Safety and operational considerations must always be factors in determining what actions can be taken, but in all situations, existing conditions and the specific actions taken to measure ROB and OBQ must be noted in the report. 

Page 4 of 4


Liquid cargo should only be trim and/or list corrected if the liquid is in contact with all bulkheads.  When the liquid is not in contact with all bulkheads, a wedge correction should be applied.  In all circumstances, the cargo documents should include the vessel's list and trim.  The nature of the material in the tank should be described in detail, and the conditions of measurement and other pertinent information should be noted.  For calculation of small quantities, refer to API M-PMS Chapter 17.4.

Note: Wedge, trim, and list corrections do not normally apply to sediment and sludge but may apply to solidified (non-liquid) cargo.  In addition, when the wedge formula or wedge tables are used, extreme care must be exercised to ensure that wedge does exist, that the measured material is not just a puddle under the gauge hatch, and that the formula used is applicable to the actual shape of the tank (that is, it accounts for the curve of the bilge).  Measures to be taken in such a case should include-but are not necessarily limited to-taking ROB measurements at more than one point in the tank.  This would verify the existence of a wedge and the extent of cargo solidification.

Open Temperature Determination The temperature of the cargo being measured is one of the most important elements needed to accurately determine its volume.  This section fully describes the equipment and procedures that should be used to manually obtain the cargo's temperature.

Open Temperature Measurement Equipment All temperature equipment must be safe for use with the material whose temperature is to be obtained.  The preferred method of obtaining temperatures of the liquid in a vessel's tanks is to use a portable electronic thermometer (PET).  Alternately, a mercury-in-glass thermometer with etched glass face may be used.

Thermometers used for custody transfer should be properly calibrated and their accuracy verifiable and traceable to a NlST standard thermometer and meet the requirements specified in API MPMS Chapter 7.


Thermometers are precision instruments and should be handled with care.  For a technical description of the specifications of each type, refer to API MPMS Chapter 7.

Field Verification of Temperature Equipment

All thermometers used for custody transfer measurements should be verified for accuracy before initial use, and at least once a year thereafter.  In addition, before each use or once per day (whichever is less frequent) the thermometer should be spot-checked.  For full details of thermometer verification, please refer to API MPMS Chapter 7.1

Mercury-in-Glass Thermometers

Glass stem thermometers should be verified for accuracy before initial use and at least once a year thereafter. In addition before each use or once a day (whichever is less frequent) the thermometer should be field checked by visually checking the glass capillary for breakage and separation of the mercury column.  Glass stem thermometers with abnormally worn etched faces or broken mercury column should not be used.  If the column is rejoined, it may be used provided that it successfully passes a bench inspection.  For additional technical details see API MPMS Chapter 7.1.

Portable Electronic Thermometers (PETs)

Before initial use, and at least once a year thereafter, all electronic thermometers shall be re-standardized in a laboratory or other qualified facility.  For full details see API MPMS Chapter 7.3.  In addition before each use, or once a day (whichever is less frequent), PETs should be spot-checked by comparing the ambient reading against an ASTM glass stem thermometer in liquid.  If the readings differ by more than 1°F or 0.5°C, the PET should be re-standardized before it is used for custody transfer.  For details on verification of the PET see API MPMS Chapter 7.3.

Open Temperature Measurement Procedures

Manual temperature measurement is the determination of the temperature of a liquid in a vessel's tank, using the appropriate devices. The primary considerations of accurately determining temperature are

  1. the size and location of cargo tanks,

  2. whether or not heat has been applied to the cargo,

  3. the atmospheric and seawater temperatures, and

  4. the degree of temperature stratification within the cargo. Temperatures should be taken and should be clearly designated as degrees Fahrenheit or Celsius, as appropriate.

Temperatures should be determined at the same time gauging is performed.  Temperatures should be taken in all tanks, and upper, middle, and lower temperatures should be taken in each tank whenever the liquid level is greater than 10 feet (3 metres).  For vessel tanks with less than 5000 barrels (795 cubic metres), a single temperature measurement at the middle of the liquid will suffice (see Table 4).  The total vessel volume should be corrected to the standard temperature on a tank-by-tank basis, using the average temperature determined for each tank.  By agreement of all parties involved, more or less than three temperatures may be taken to calculate an average tank temperature.

It may be noted that when temperature differentials greater than 5°F (3°C) are found, additional temperatures should be taken.  The number of additional temperatures will vary with the temperature differential.  However, they must always be equally spaced and averaged accordingly.

The immersion time required for the thermometer reading to reach equilibrium will vary depending on the type of liquid and equipment.  For more specific guidelines on immersion times, see Tables 5a and 5b and refer to API MPMS Chapters 7.1 and 7.3.

Portable Electronic Thermometers (PETs)

In addition to the steps described earlier, the following procedure is recommended for measuring temperatures with a portable electronic thermometer (PET):

  1. Attach an electrical ground between the thermometer and the tank before the hatch is opened. Check the ground to ensure that it is securely attached to the thermometer.

  2. Set the temperature range selector as appropriate.

  3. Lower the sensing probe to the predetermined level.

  1. Raise and lower the probe 1 foot (0.3 metre) above and below the predetermined level to allow rapid stabilization.

  2. After stabilization, read and record individual temperatures to the nearest 0.1°F or 0.°C,

  3. Determine the average tank temperature to a tenth of a degree.

  4. Round off and report the average tank temperature in accordance with the most recent edition of API MPMS round off and report average tank temperature to 1°F or 0.5°C  [round 0.5°F up]. Temperatures may be reported in units less than whole degrees by mutual agreement.

If the probe is allowed to remain stationary, contact with a convection current of colder oil will cause low readings. With a moving probe, however, the thermometer may be considered stabilized if the readout varies by no more than 0.2°F (0.1°C) for 30 seconds.

Mercury Thermometers In addition to the steps described earlier, the following procedure is recommended for measuring temperatures with a mercury thermometer.

  1. Lower the thermometer assembly through the gauge hatch to the required level.

  2. Repeatedly raise and lower the thermometer 1 foot (0.3 meter) above and below the required level so that the equilibrium temperature will be reached more rapidly.

  3. Withdraw the thermometer after the required immersion time.

  4. Round off and report the average tank temperature in accordance with the most recent edition of API MPMS Chapter 7.1 (round off and report average tank temperature to 1°F or 0.5°C  [round 0.5°F up]). Temperatures may be reported in units less than whole degrees by mutual agreement.

  5. Report the temperature to the nearest 1°F or 0.5°C.

  6. Repeat items a through e for every tank to be 'temperatured'

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

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