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How to determine specific gravity & density of cargo onboard chemical tankers ?

The range of chemicals shipped in bulk now has been increased enormously and to maintain product quality, the specific gravity and cargo density need to be often determined at loading and discharging ports .

Specific gravity expresses the weight of a unit volume of a matter compared with the weight of the same volume of water, both at a defined (but sometimes different) temperature. Expressions such as SG 20/200C are used in the petroleum industry. The first figure denotes the temperature of the product and the latter the temperature of the water used as a comparison. Specific gravity includes the effect of air displacement.

The areometer consists of a displacement body with a graduated scale . The accuracy in reading gives a maximum of three correct figures, the fourth figure being doubtful. This means an accuracy of 0, 1-0, 2 %. This accuracy is generally insufficient for cargo quantity determination. The areometer is, however, well suited as an instrument for a general check of cargo density on board. A set of areometers for density ranges 0, 7 to 1, 0 and 1, 3 to 1, 8 (alkalies and acids) should be on board.

The pycnometer consists of a small glass retort with an accurately determined volume (certified pycnometers available). The specific gravity is determined by weighing the pycnometer empty and filled with liquid. The result is obtained with five correct figures which is much better than the areometer principle. The determination is generally made at 20 0 C. The use of pycnometers on board is not practicable due to the need for a very sensitive weight scale.

Specific weight/density is part of all product specifications as a check on concentration, mixture etc. The SG/density falls with the rising temperature. Volume correction factors are used for recalculation (per degr. C or degr. F) or tables (for petroleum products) available from, e.g American Petroleum Industry (API).

A warning: It sometimes happens that cargo density and thus the quantity is determined at both the loading end and the discharge end by means of areometers. Obviously therefore there will be discrepancies in the two quantity determinations simply due to the relatively poor accuracy in the density readings. As a result questions regarding cargo claims may arise. The answer is to use the "as loaded density" (whether completely correct or not) and correct it for the change in temperature at the discharge end. This “calculated” density is then used in the quantity determination upon discharge. Thereby it can be established with good accuracy whether cargo has been lost or not.

There are also other specific gravity scales.

Further reference are available at :ASTM standard D 1298-67 (API 2547).

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