Stability & CargoPage 9 of 15

Two chains on one certificate: the metric chain to weight in air

Why it matters One factor read from the wrong column moves the cargo figure by tens of tonnes; nobody at the table sees it in the working.

Words on this page

Certificate of quantity: the paper from the terminal that states the cargo's volume and quality figures.
Metric chain and barrel chain: the two ways of working one cargo: in m³ at 15 °C, or in barrels at 60 °F; each has its own table and density.
Gross Observed Volume (GOV): the oil measured in the tanks at its own temperature, free water already taken out.
Volume correction factor (VCF): the number that shrinks or swells the measured volume to the standard temperature.
Table 54B and Table 6B: two tables of factors: 54B brings m³ to 15 °C using the density; 6B brings barrels to 60 °F using the API gravity.
Gross Standard Volume (GSV): the oil's volume brought to the standard temperature.
Density at 15 °C: how heavy one cubic metre of the oil is at the standard 15 °C.
Relative density at 60/60 °F: how heavy the oil is next to water, both at 60 °F; the older name is specific gravity (SG 60/60); a different figure from the density at 15 °C, for the barrel chain.
API gravity: the oil industry's scale for how light an oil is, used to enter Table 6B.
Weight correction factor (WCF): the density at 15 °C less the air buoyancy correction; the volume times this is the weight in air.

An everyday picture

A recipe card is printed in two columns. On the left, grams and degrees Celsius; on the right, ounces and degrees Fahrenheit.

Both columns describe the same cake. You may bake from either, but every figure must come from one column.

Take the oven setting from the right, 350, and use it in the left column. That is 350 °C, and the cake burns.

The certificate is that card. The m³ column is the ship's chain; the barrels are the terminal's.

The rule

Find the chain the ship's figure is in. A volume in m³ weighed against a density at 15 °C is the metric chain.
Take the factor that chain uses. For m³ it is Table 54B, entered with the density at 15 °C. Table 6B, entered with the API gravity, is for barrels at 60 °F; it stays in its own column.
GSV = GOV × that factor. The volume is now at 15 °C.
Take the density from the same column: the one at 15 °C, not the 60/60 °F figure. Take the air buoyancy correction off it; that gives the WCF.
Weight in air = GSV × WCF. That is the cargo figure.

Formula

GSV = GOV × VCF (Table 54B, with the density at 15 °C)
WCF = density at 15 °C − air buoyancy correction
Weight in air = GSV × WCF

Worked example

Gross Observed Volume	120900 m³
Volume correction factor, Table 54B, entered with the density at 15 °C	0.9738
Volume correction factor, Table 6B, entered with the API gravity at 60 °F	0.9741
Density at 15 °C in vacuo	0.8712 t/m³
Relative density at 60/60 °F	0.8717

Metric chain: VCF = 0.9738 (Table 54B) with the density at 15 °C = 0.8712 t/m³; Table 6B's 0.9741 and the 60/60 °F 0.8717 stay with the barrelsFirst choose the column; the two factors and the two densities are not interchangeable.
GSV = 120900 × 0.9738 = 117732.4 m³The measured volume is brought to the standard temperature with the metric table.
WCF = 0.8712 − 0.0011 = 0.8701 t/m³We take the small air allowance off the density.
Weight in air = 117732.4 × 0.8701 = 102439 tWe multiply the standard volume by that factor; this is the weight in air.

Answer102439 t

Common mistake

Using the Table 6B factor, 0.9741, on the m³ volume.

Table 6B brings barrels to 60 °F and is entered with the API gravity; it is the other column. On the m³ it gives 117768.7 m³ and 102471 t, 32 t too many. The working looks right, and the ship and the terminal end up with different numbers.

Note for mariners The two densities differ in the fourth decimal for one oil because they are referred to different temperatures; the barrel chain gives the same weight, in long tons, when every figure in it is its own.