Definition
A mechanical balance is an instrument that achieves mass measurement through mechanical structure based on the principle of lever balance. It is usually composed of core components such as beams, fulcrums, load-bearing blades, suspension systems and reading devices, and belongs to the classic static balance measuring equipment. In laboratory and industrial testing, mechanical balances are often used in situations where quality measurement accuracy is required due to their intuitive physics and reliable performance.
Principle
The working principle of mechanical balances is based on the first type of lever balance. When the beam is supported by a central fulcrum, the object to be measured and a known standard weight are hung at both ends, respectively. Ideally, when the beam reaches horizontal balance, according to the moment balance formula:
F₁ × L₁ = F₂ × L₂
where F is the force (usually the product of mass and gravitational acceleration), and L is the length of the arm. Since the gravitational acceleration is the same and the upper arms are equal in design, the mass of the object to be measured during equilibrium is equal to the mass of the standard weight. Some precision mechanical balances will adopt an unequal arm design or add compensation mechanisms such as a mobile code or chain to extend the measurement range or improve the resolution.
Measurement method
The typical measurement process of a mechanical balance follows a certain operating procedure. First, it is necessary to adjust the level to ensure that the balance base is in a horizontal state to avoid errors introduced by gravity direction deviation. The zero point is then adjusted so that the pointer points to the center of the scale when no load is adjusted by adjusting the balance nut at the end of the beam. During formal measurement, the object to be measured is placed lightly on the left disk, and the weight is gradually added to the right disk, and finely adjusted by moving the code or using the chain compensation mechanism until the beam returns to the balance position. The final mass value is the sum of the total value of the added weights and the mobile code readings. After the measurement, the load needs to be unloaded and the weight should be returned to its place.
Influencing factors
The measurement accuracy of mechanical balances is influenced by various factors. Environmental conditions such as air flow may cause the beam to shake or produce buoyancy effect, and temperature changes can cause thermal expansion and contraction of metal parts to change the length of the arm. The performance parameters of the instrument itself, such as index value, sensitivity, and repeatability, directly determine the accuracy of the measurement results. Operational factors should also not be overlooked, such as improper load placement that can introduce bias load errors, wear or contamination of the cutting edge, and affect rotational flexibility. In addition, external disturbances such as electromagnetic interference and vibration may also interfere with the judgment of the balance state.
Application
Mechanical balances play a role in several areas. In the field of basic scientific research and education, it is often used to demonstrate the principles of mechanics and conduct basic mass measurement experiments. In chemistry laboratories, it can be used for tasks that require high accuracy, such as preparing standard solutions and weighing reagents. In the metrology department, some high-precision mechanical balances are available as standard equipment for mass transfer. In addition, there is still room for application in jewelry, precious metal testing, and quality control in certain industries due to its stability and the need for power supply.
Selection
Choosing a mechanical balance requires a combination of measurement needs and actual conditions. The primary parameters are the weighing range and index value, which must ensure that they cover the mass of the object to be measured and meet the required measurement accuracy. Sensitivity is another key metric, reflecting the balance's responsiveness to small changes in mass. The construction materials of the balance, such as whether the beams are made of low expansion coefficient alloys, can affect long-term stability. Environmental requirements also need to be evaluated, such as the presence of anti-vibration tables or temperature-controlled facilities to meet the installation conditions of high-precision balances. Finally, it is necessary to consider the ease of operation, maintenance costs, and compliance with relevant national or international standards, such as the technical specifications for measuring instruments.