Mass and weight are two different things though the word weight is used many occasions in the place of mass. Every object has a mass. It never changes. An object having a mass of 50 Kg will have the same mass whether it is on the surface of Earth or Moon.
Weight is used to describe the mass of the body or the force of the gravity acting on it. Actually weight is the measure of gravitational force acting on the object at a specific location. Tthe object with the mass of 50 kg will have the weight of 50 kgf on the surface of the Earth but only 8.5 kgf on the surface of the moon.
Effects by change of gravity
Now we know that change of gravity will have dynamic effect on the weight. A shipping container falling downwards with an acceleration force greater than 9.8m/sec will make the mass weight less. However move the container at same speed upwards will make the weight of the mass double.
During ocean passage the movements of the vessel will make considerable changes to the weight of the cargo within the containers.
A well secured cargo in container not only minimizes cargo damage and claim but will drastically minimize hazard due to shifting of cargo or leakage which can be fatal in case of dangerous goods.
Vessel’s motions at sea
What shall be declared for dangerous goods? Mass? Weight? Net or Gross?
Quantity of Dangerous goods shall be declared by volume or mass as appropriate.
Gross mass = mass of dangerous goods and package
Net mass = mass of dangerous goods
In IMDG Code whenever the mass of a package is mentioned, the gross mass is meant. However this does not include mass of tanks and containers.
Example of declaration
UN 1263 PAINT class 3 PG III (24 Deg C.c.c) 1 x steel drum 100 kg
UN 1263 PAINT class 3 PG III (24 Deg C.c.c) 1 x steel drum Gross mass 100 kg net mass 93 kg
Filling of packages
While filling of packages the quantity allowed is given in volume or by mass in IMDG Code. See example packing instruction below.
For gas it is altogether different case because there we deal with pressure 🙂