be confirmed that the hydrometer being used is of the correct type. There are two types of hydrometer in use, the load line survey hydrometer, and the draught survey hydrometer, the latter being used in this case.
A draught survey hydrometer, calibrated in air, measures the apparent density of the water and is used for determining the weight of the cargo onboard. A load line hydrometer, calibrated in a vacuum, is used to determine the relative density of the water and determine the displacement of the vessel at a given load line. A dock water density reading taken with a draught survey hydrometer showing 1.015 t/m3 1.017 t/m3
, would show when taken with a
load line hydrometer, both are correct, the difference between the two of 0.002 is known as the “air buoyancy correction”, however as we want to determine the weight of the cargo on the vessel the density read from the draught survey hydrometer will be used. The draught survey hydrometer is calibrated at a standard temperature. Any deviation from
the standard temperature does not require a temperature correction, as the changes in the volume of the vessel and the hydrometer will generally cancel each other out. It must also be ensured that the hydrometer is calibrated regularly to ensure its accuracy, as they can become chipped, damaged and / or contaminated with a film on their surface during use, altering their mass. Hydrometer manufacturers generally recommend they are calibrated after one year, and then every two or three years thereafter.
When determining the dock water density a sample of the dock water should be taken beneath the surface of the water, clear of overboard discharges and deck runoff. In order to ensure an accurate reading the receptacle used to sample the dock water, and the hydrometer, must be clean. When taking the reading the hydrometer must not touch the side of the receptacle and be gently spun to break the surface meniscus of the water. The density reading is then read from the hydrometer stem at the bottom of the remaining meniscus. A number
40 | The Report • September 2018 • Issue 85
of readings should be taken from each sample, with the average of the readings determined. A number of separate dock water samples should be taken from around the vessel and measured, with the mean of all the average densities used for the draught survey. If an error is made when determining the dock water density, using a density lower than the actual density will lead to an apparent cargo shortage, with an apparent cargo surplus when a density higher than the actual density is used.
BALLAST
A number of errors can occur when determining the ballast onboard. The tank soundings themselves may be incorrect due to difficulties in obtaining accurate soundings when using a sounding rod and rope, with the inherent possibility of reading incorrect depths from the rope due to a lack of markings and the wetness of the rope. When using a metal sounding tape it may be difficult to determine the actual water level from the metal tape. Further, inaccurate soundings may
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