Do you know how an analytical balance works?

The balance is a laboratory instrument that measures the mass of a body or chemical substance, using as a means of comparison the force of gravity that acts on the body.

It must be taken into account that the weight is the force that the gravitational field exerts on the mass of a body, such force being the product of the mass due to the local acceleration of gravity. The local term is included to emphasize that the acceleration depends on factors such as geographical latitude, height above sea level and the density of the earth, at the place where the measurement is made. This force is measured in Newton. Did you know? The scale has other names, including scale and weight.

What is the balance used for?

The balance is used to measure the mass of a body or substance or also the weight of them, since between mass and weight there is a well defined relationship. In the laboratory, the balance is used to perform quality control activities -with devices such as pipettes-, to prepare mixtures of components in predefined proportions and to determine specific densities or weights.

Some types of balances

The balances are differentiated by the design, the principles used and the metrology criteria they use. At present it could be considered that there are two large groups: mechanical scales and electronic scales.

What are the most common mechanical balances?

1. Spring balance

Its operation is based on a mechanical property of the springs, which consists in that the force exerted by a spring is proportional to the elasticity constant of the spring [k] multiplied by the elongation of the spring [x] [F = -kx].

2. 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; by placing a substance of unknown mass on the tray, its weight is determined by sliding the masses over the mentioned scales until the equilibrium position is obtained. At that moment, the reading is taken by adding the amounts indicated by the position of the masses on the aforementioned scales.

3. Analytical balance

It works by comparing known weight masses with the mass of a substance of unknown weight. It is built on a symmetrical bar or lever that is supported by a blade-type support at a central point called fulcrum. At its ends there are stirrups or ferrules that are also supported by blades that allow them to oscillate smoothly. Two dishes are suspended from there. In one the masses or certified weights are placed and in the other those that it is necessary to analyze. The whole set has a locking or locking system that allows the main lever to stand steadily when it is not used or when the counterweights need to be modified. It has an external box that protects the balance from interferences, such as air currents, that may occur in the place where it is installed. At present, it is considered that an analytical balance is one that can weigh ten thousandths of a gram (0.0001 g) or one hundred thousandths of a gram (0.00001 g); They have a capacity that usually reaches up to 200 grams.

4. Top plate balance

This type of scale has a load plate placed in the upper part, which is supported by a column that is kept in vertical position by two pairs of guides that have flexible couplings.

5. Substitution balance

It is a unique dish scale. An unknown mass is placed on the weighing plate, which is balanced by removing, on the counterweight side, masses of known magnitude, using a mechanical system of cams until an equilibrium position is reached.

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