Ore shipments are transported from the mines to the loading vessel by various means, including rail, truck or by barge, and a certain loss may occur during transit and during loading operations. In many cases the weight of the cargo loaded is measured by taking the draft of the vessel, but the weight so ascertained is not absolutely reliable, particularly when the vessel is moving in a swell or when there is a movement of the water due to wind.
In calm weather and under normal conditions measurements draft readings are generally reliable. Shipped weights are sometimes provided by a weighbridge, prior to shipment, but such weights are not necessarily evidence of the quantity loaded, by reason of scale miscalibration and the loss which may take place during the actual loading operations, particularly if the ore is dry and dusty. Similarly, methods of determining the arrived weight at the port of discharge vary and are subject to the same considerations.
The quantity outturned may also vary due to the drying out of the or in wet weather the ore is likely to contain an excess over and above its natural moisture content. In some mines the pit being worked may be deeper than the level of an adjacent river and the ore extracted may therefore have an excessive moisture content. It is usual, however, for the moisture content of the ore to be determined at the time of shipment by a chemical analysis of samples, and it is customary for the ore to be bought on the calculated dry weight.
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