The draft meter is used to measure the draught, fresh water, ballast water and fuel oil storage capacity of the ship before and after loading or unloading, and to measure the water density at the port of the ship's berth with the ship's draft as the main tool, and then use the drainage meter or load meter provided by the ship, relevant static hydraulic curve chart, water tank meter and calibration table and other marine charts Calculating the weight of the cargo carried by the ship, in English, is checking draft or draft survey, that is, to calculate the weight by measuring the draft.

The "Archimedes principle" is applied to the water gauge weight, that is, "the buoyancy of an object immersed in a liquid is equal to the weight of the liquid discharged by the object." There is a certain correspondence between the draught and the displacement of the ship. When the ship is designed and built, it will engrave a water gauge mark on the side of the ship, and make a chart of the volume of the water entering part of the ship with different draughts, i.e. load scale, for checking and calculation. In this way, the weight of the cargo can be calculated by observing the draught before and after loading and unloading and measuring the weight of the ship's materials.

Draft weight measurement is a scientific method with certain accuracy, which has been widely used in the world. The calculation results can be used as the basis for delivery settlement, claim settlement, claim, freight calculation and customs clearance and tax calculation. The method of draft weight measurement introduced in this book is currently the internationally accepted method. If the accuracy of ship tabulation is within 1 ‰, the accuracy can be within 5 ‰.

The advantages of water gauge weight are time-saving, labor-saving and cost saving. It can avoid the error factors of loading and unloading loss and calculate the weight of the whole ship quickly. It plays a positive role in expanding bulk transportation, reducing costs, improving transportation efficiency, accelerating transportation turnover and port distribution, and promoting foreign trade.

The calculation process of draft weight is complex and there are many objective factors that affect the weight calculation results, such as: the size of wind and waves; the shape of the ship is complex, the hull deformation; the change of port water density; the trim and heeling of the ship; the change of ballast water, fuel oil, fresh water and other supplies on the ship, which directly affect the accuracy of weight calculation. Therefore, the surveyor must have the knowledge of mathematics, physics, principles of ship structure, shipping and other sciences, and have a certain level of foreign language to complete this work. At the same time. The result of the draft weight measurement is directly related to the economic interests of all parties concerned. Therefore, in the practical work, the appraisers of the draft weight measurement should not only be careful, but also have a strong ability to communicate with people, so as to coordinate and safeguard the legitimate economic rights and interests of all parties concerned.

Draft weight is generally applicable to the weight of bulk solid commodities with low value, difficult weight balance and bulk. At present, it has been widely used in international trade in bulk coal, salt, ore, sea sand, pig iron, scrap steel, petroleum coke, sulfur, phosphorous lime soil and other commodities. With the development of international trade, the transportation of chemical fertilizer, cement, grain and other goods has also changed from packing transportation to bulk transportation, so as to save packaging and other costs. It is also required to use water gauge to measure weight. In recent years, the tabulation technology and automatic measurement technology related to water gauge weight have made great progress. Computer calculation has gradually replaced manual calculation, and the efficiency and accuracy of water gauge weight are getting higher and higher. As a convenient and quick way of weighing, it will be more and more widely used.