The bob and tape are lowered into the tank until the bob is within a short distance of the tank bottom.
This can be determined by comparing the length of tape unwound from the reel with the reference height
of the tank.
Continue to unwind the tape slowly until the tip of the bob just touches tank bottom or datum plate. The
bob must not rest on a rivet or other obstruction. If the tape is lowered too far, the bob will tilt and an
incorrect gage reading will be obtained as the product cut shown on the tape will be too high. The tape
reading at the reference point should be compared with the reference depth of the container to make
sure that the gage is accurate.
The tape should be withdrawn after at least 30 seconds, and the product cut on the tape should be read
and recorded as the innage gage. The water cut on the bob is read and recorded as the BS&W cut.
Two identical readings are obtained to ensure that the measurement is accurate. The same gaging
equipment and gaging hatches are used in obtaining both opening and closing gages, and the tape is
lowered to the same depth for both gages.
Outage Method of Gaging. The outage method is used for gaging barges and tankers. All safety
procedures apply. Gaging procedures for the outage method are as follows:
Apply product indicating paste to the length of the bob.
The bob and tape are lowered into the tank until the bob touches the surface of the product.
When the bob is motionless, the tape is lowered slowly until the bottom of the bob is 2 or 3 inches below
the surface of the product and an even inch graduation mark on the tape is at the reference point. The
reading on the tape at the reference point is recorded as the tape reading.
The tape is withdrawn quickly, and the product cut on the bob recorded as the bob reading. The scale is
read to the nearest 1/4 inch.
The bob reading is added to the tape reading to obtain the outage gage as shown in the following example:
Tape reading at reference point
21 feet, 6 inches
3 3/4 inches
21 feet, 9 3/4 inches
An outage gage can be converted to an innage gage if the tank being gaged is calibrated only in innage
measurements. The procedure is as follows:
Use the outage gage above (21 feet, 9 3/4 inches) and an assumed reference height of 50 feet.
The outage gage is 21 feet, 9 3/4 inches (point A to B). The reference height is 50 feet (point A to C).
The innage gage will be the distance from points B to C.
To convert outage to innage simply subtract the outage from the reference height:
Reference Height - Outage
= Innage Gage
- 21 ft, 9 3/4 inches = 28 ft, 2 1/4 inches
Bob cuts for BS&W in the outage method must be done with an innage bob as described previously.
Volume Correction. The volume of liquid petroleum products changes because of changes in
temperature. Therefore, gaged volumes in excess of 3,500 gallons must be corrected to account for this
change of volume. When the observed temperature is above 60F, the observed volume is reduced to 60F
because expansion has taken place. However, when the observed temperature is below 60F, the observed
volume is increased to the volume at 60F because contraction has taken place.
DA Form 3853-1 will be used to record volume correction data. All tanks have individual strapping
charts or tank calibration tables which show the volume of product in the tank per foot, per inch, and even
per fractions of an inch, usually 1/8 inch. Using the physical gage (product and BS&W) measurements, the
gager can determine the total volume of product in the tank. By using the same tank table, the gager
determines the volume of water in the tank. The volume, and only the volume, of petroleum products is left.
You must remember to subtract volume from volume, never inches from inches. The resulting figure is at
the observed temperature and is ready to be corrected to volume at 60F. To determine the multiplier
necessary to convert the volume at observed temperature to the volume at 60F, one would need an
average tank temperature and the API gravity of the product.