Balance An instrument used for the precise measurement of small weights or masses in amounts ranging from micrograms up to a few kilograms.

Balances are differentiated according to design, weighing principle, and metrological criteria . For a given weighing task, a balance is selected primarily for its maximum weighing load (Max) and for the finest graduation or division (d) of its weight-reading device (scale dial, digital display, readout).

Balances can be roughly differentiated from scales by their resolution or number of scale divisions, n = Max/d. Balances typically have a resolution of more than 10,000 divisions, and scales for the most part have less.

A traditional mechanical balance consists of a symmetric lever called a balance beam, two pans suspended from its ends, and a pivotal axis (fulcrum) at its center. The object to be weighed is placed on one pan, whereupon the balance is brought into equilibrium by placing the required amount of weights on the opposite pan. Thus the weight of an object is defined as the amount represented by the calibrated standard masses that will exactly counterbalance the object on a classic equal-arm balance. Although this is not self-evident with modern balances and scales, the measurement of weight continues to be based on this original understanding.

The substitution principle represented the conclusive step in the evolution of the mechanical balance. Substitution balances have only one hanger assembly, incorpo­rating both the load pan and a built-in set of weights on a holding rack. The hanger assembly is balanced by a counterpoise which is rigidly connected to the other side of the beam. The weight of an object is determined by lifting weights off the holding rack until the balance returns to an equilibrium position within its angular, differential weighing range. Small increments of weight in between the discrete dial weight steps are read from the projected screen image of a graduated optical reticle which is rigidly connected to the balance beam.

The evolution of electronic (more accurately, electromechanical) balances started in the late 1960s and has extended over several generations of electronic technology.

Among a number of technical possibilities, one operating principle, electromagnetic force compensation, emerged early as the standard in high-precision weighing. First described by K. Angstrom in 1895, the principle of electromagnetic force compensation became feasible for technical application as a result of the advancements in solid-state electronic components.

In every electromechanical weighing system, there are three basic functions: (1) The load-transfer mechanism, composed of the weighing platform or pan, levers, and guides, receives the weighing load on the pan as a randomly distributed pressure force and translates it into a measurable single force. (2) The electromechanical force transducer, often called load cell, converts the mechanical input force into an electrical output, for example, voltage, current, or frequency. (3) The electronic signal-processing part of the balance receives the output signal, converts it to numbers, performs com­putation, and displays the final weight data on the readout.

Besides improved accuracy, reliability, and speed of operation, the main benefits from this technology are human-engineered design for optimized interaction between operator and instrument, and numerous operating conveniences such as push-button zero setting, automatic calibration, built-in computing capabilities for frequently used work procedures, and data output to printers and computers.

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عن Akram Amir El Ali

استاذ الكيمياء التحليلية ومصمم غرافيك

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