Before strain gage-based load cells became the method of choice for industrial weighing applications, mechanical lever scales were widely used. Mechanical scales can weigh everything from pills to railroad cars and can do so accurately and reliably if they are properly calibrated and maintained. The method of operation can involve either the use of a weight balancing mechanism or the detection of the force developed by mechanical levers. The earliest, pre-strain gage force sensors included hydraulic and pneumatic designs. In 1843, English physicist Sir Charles Wheatstone devised a bridge circuit that could measure electrical resistances. The Wheatstone bridge circuit is ideal for measuring the resistance changes that occur in strain gages. Although the first bonded resistance wire strain gage was developed in the 1940s, it was not until modern electronics caught up that the new technology became technically and economically feasible. Since that time, however, strain gages have proliferated both as mechanical scale components and in stand-alone load cells.
Various load cell types are preferred, relative to the needs of the laboratory or operational environment. When you need to convert force into a measurable electrical output, the load cell or transducer, is the best application. Strain gage load cells are accurate within 0.03 to 0.25%. Used for experimental stress analysis and electrical measurement of resistance to strain, these load cells are used in most industrial applications. When precision mechanical balances are required, and where intrinsic safety and optimal hygiene is essential, pneumatic type load cells are a better fit. In cases where the operation is in a remote location, the most applicable load cell type is still the hydraulic load cell because a power supply is not needed.
Load Cell Operating Principles
Load cell designs can be distinguished according to the type of output signal generated (pneumatic, hydraulic, electric) or according to the way they detect weight (bending, shear, compression, tension, etc.
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