The balance is used to measure the mass of a body or substance or also their weight, since between mass and weight there is a well defined relationship. In the laboratory the balance is used to carry out quality control activities, to prepare mixtures of components in predefined proportions and to determine specific densities or weights. Its origins lie more than 3500 years before Christ, in Egypt.
The scales differ from each other by the design, the principles used and the metrology criteria they use. At present it could be considered that there are two large groups: the mechanical scales and the electronic scales.
Some of the most common are the following:
- Spring balance: Its operation is based on a mechanical property of the springs, which is that the force exerted by a spring is proportional to the spring’s elasticity constant [k] multiplied by its elongation [x] [F = -kx].
- Sliding weight scale: It has two known masses that can be moved on scales – one with a macro graduation and the other with a micro graduation -; when placing a substance of unknown mass on the tray, its weight is determined by sliding the doughs on the mentioned scales until the equilibrium position is obtained.
- Analytical balance: It works by comparing masses of known weight with the mass of a substance of unknown weight. It is built based on a bar or symmetrical lever that is supported by a blade-like support at a central point called fulcrum.
- Upper plate balance: This type of balance has a loading plate placed on the top, which is supported by a column that is held vertically by two pairs of guides that have flexible couplings. The effect of the force, produced by the mass, is transmitted from some point of the vertical column either directly or by some mechanism to the load cell.
- Replacement balance: It is a unique saucer scale. An unknown mass is placed on the weighing plate that is balanced by removing, on the counterweight side, masses of known magnitude, using a mechanical cam system until an equilibrium position is reached.
Electronic balances involve three basic elements:
- The object to be weighed on the weighing plate exerts a pressure that is distributed randomly on the surface of the plate.
- A measurement transducer, known as the load cell, produces an output signal proportional to the load force, in the form of changes in voltage or frequency.
- A digital analog electronic circuit that finally presents the weighing result in digital form.
What is required to successfully install and use a Balance?
- Have an environment that does not have air currents, sudden changes in temperature and is free of dust.
- Have a perfectly level inn. A high inertia platform is ideal, isolated from the structures located in the neighborhood, to reduce the effect of the vibrations emitted by certain equipment such as centrifuges and refrigerators. It must have a sufficient area to install the balance and that auxiliary equipment with which it interacts in the weighing processes. In the same way, the space required by the interconnection cables, electric current, connection to the information system, the printer, etc. must be provided.
- Prevent the installation of equipment that produces high magnetic fields or vibrations such as centrifuges, electric motors, compressors and generators.
- Avoid being under the direct influence of air conditioning systems – air currents – and sunlight.
- Have an electrical outlet in good condition, equipped with a grounding pole provided with switches, which complies with the electrical regulations in force in the country or the laboratory.
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