When determining the weight of cargo in a draft survey, the volume of seawater eliminated by the vessel is multiplied by the density of seawater to calculate the displacement. Therefore, the influence of seawater density on cargo weight is not small.
In addition, there are currently two types of seawater hydrometers in use around the world, and there are no internationally unified rules as to which one to use.
For more information on the difference between the two types of hydrometers, please refer to the MARINE NOTICE issued by the Australian Federal Department of Transport.
The difference between the two types of hydrometers is mainly whether air buoyancy correction is applied.
The density measured by either is also the correct value.
However, when using different type of hydrometer for loading and unloading, it is important to realize that there is a difference in each measurement between loading and unloading.