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A hydraulic scale is a measurement device that utilizes the compression of hydraulic fluid by a test weight to indicate the mass of the test weight. This is typically achieved by having the test weight depress a piston or diaphragm which, in turn, compresses the fluid. The pressurized fluid is then fed into a Bourdon tube type analog gauge or an electronic sensor and display. In either case, the system is calibrated so that displayed value indicates the mass value of the test weight. Hydraulic scales range in complexity from simple, single piston suspended scales to large weigh bridge models featuring multiple load cells.
Fluids act in a predictable manner when compressed; each fluid type possesses specific, known characteristics when placed under compression. This allows for very accurate measurement of mass in hydraulic scale applications. All hydraulic scale types consist of one or more load cells and a means of translating the resultant pressure into a visible mass indication. A load cell is typically little more than a piston or diaphragm with a small, oil-filled compression chamber. When the test weight is brought to bear on the piston, it moves down and compresses the oil in relation to the size of the test weight.
This pressurized oil is then used to indicate the mass of the test weight. This indication may be displayed in several ways, the most simple of which is the Bourdon tube gauge. This gauge is usually used on small, single load cell scales and consists of a flat, C shaped tube connected via a series of linkages to an analog dial. When the oil pressure in the tube increases, it flexes and moves the needle of the gauge to indicate the mass.
In larger scales, a load receiver or platform is situated on top of an array of hydrostatic cells. When a load is placed on the platform, it depresses the all the cells evenly. All the hydraulic input lines from the load cells are brought together in a summing totalizer. This device is usually located away from the load platform and consists of a series of receiving cells which translate the incoming pressure into an electronic signal. This signal is then passed to a processor which shows the mass of the load on a digital display.
This type of hydraulic scale is generally used at weigh bridge installations which measure the mass of large trucks or railway rolling stock. The load platforms used on these scales may measure up to 120 feet (37 meters) in length. Scales of this type are generally capable of accurately measuring loads of 120 tons or more. In addition to the inherent accuracy of the hydraulic scale, it also features very few moving parts and is reliable and easy to maintain, thereby making it a popular choice for high traffic applications.
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