Selection principle of load cell

Selection principle of load cell

    In the weighing system, which weighing load cell to choose must be fully measured. The following describes several aspects that should be generally considered in the selection of the weighing load cell's structure, range, and accuracy level.

●Structural form selection

    Which structural form of load cell to choose depends mainly on the environment in which the weighing system is used and the structure of the scale body.

★Using environment of weighing system

     ※ The load cell working in high temperature environment should adopt high temperature resistant load cell, and heat insulation, water cooling or air cooling should be added in particularly harsh occasions.

     ※ For use in high-cold areas, a load cell with a heating device should be considered.

     ※ The influence of dust, humidity and corrosiveness:

       Stainless steel series products are suitable for environmental humidity> 80%RH or more, other acids and ammonia corrosive;

       Rubber seal series alloy steel products are suitable for environmental humidity <65%RH, no water infiltration, no other corrosive gases and liquids;

       Welding seal series alloy steel products are suitable for environmental humidity <80%RH, with smooth drainage and no other corrosive gases and liquids;

       Aluminum alloy series products are suitable for environmental humidity <65%RH, no water infiltration, no other corrosive gases and liquids

     ※ In the overhead weighing system, safety and overload protection should be considered.

     ※ In flammable and explosive environments, explosion-proof load cells or intrinsically safe load cells must be selected. The sealed enclosure of explosion-proof load cells should not only consider its airtightness, but also consider the explosion-proof strength, and the waterproof, moisture-proof and explosion-proof of the cable lead Sex and so on.

★Structural characteristics of the scale

     ※ The installation space of the carrier

       In some places where installation space is limited, the space limitation should be considered when selecting the load cell.

     ※ Easy to install and maintain

       Regardless of its reliability, any equipment must be considered for installation and maintenance. In addition to considering the convenience of installation, it is also necessary to consider whether it is convenient to maintain during use and whether it is convenient to replace the load cell.

     ※ The influence of lateral force

       When selecting a load cell, you must consider whether there is a lateral force in the scale body in use. The load cell designed according to the principle of shear stress has relatively strong ability to resist lateral force, while the ability of load cell designed according to the principle of normal stress is relatively weak.

     ※ The stiffness of the carrier, basic structure and accessories

       The stiffness of these structures will directly affect the amount of deformation, thereby affecting the accuracy of measurement.

     ※ The influence of temperature on the scale body.

       For outdoor weighing systems with a long carrier and a large area, such as truck scales and large tanks, the expansion coefficient of the carrier must be considered.

●Selection of load cell quantity and range

    ★Selection of the number of load cells

      The number of load cells is selected according to the purpose of the weighing system and the number of points that the scale needs to support (the number of support points should be determined according to the principle that the geometric center of gravity of the scale body coincides with the actual center of gravity). Generally speaking, if the scale body has several supporting points, only a few load cells are used.

    ★Selection of load cell range

      The selection of the load cell range can be determined based on the comprehensive evaluation of factors such as the maximum weighing value of the scale, the number of load cells to be selected, the self-weight of the scale body, the maximum possible partial load and the dynamic load. In theory, the closer the weighing value of the weighing system is to the rated capacity of the load cell, the higher the accuracy of the weighing. However, in actual use, due to the weight of the weighing system, tare and vibration, impact, partial load, etc. Therefore, the principle of selecting the limit of the load cell for different weighing systems is very different.

      As a general rule, there are:

      Scale body (weighing platform, container, etc.) + material (load to be weighed) ≤ rated load of selected load cell × number of equipped load cells × 70%

      The 70% coefficient is added in consideration of factors such as vibration, shock and eccentric load.

      It should be noted:

      ※ The rated capacity of the load cell should be as close as possible to the value in the manufacturer's standard product series, otherwise, non-standard products are selected, not only expensive, but also difficult to replace after damage.

      ※ In the same weighing system, it is not allowed to select load cells with different rated capacities, otherwise, the system will not work normally.

●Accuracy grade selection of load cell

    The accuracy level is an important performance index of the load cell, and it is an important link related to the measurement accuracy of the entire measurement system. The higher the accuracy level of the load cell, the more expensive it is. Therefore, the accuracy of the load cell only needs to meet the accuracy requirements of the entire measurement system, and it is not necessary to select too high.

    The choice of load cell grade must meet the following two conditions:

    ※ To meet the requirements of instrument input

       That is, the output signal of the load cell must be greater than or equal to the input sensitivity value required by the instrument.

    ※ Meet the accuracy requirements of the entire electronic scale

       An electronic scale is mainly composed of a scale body, a load cell, and an instrument. When selecting the accuracy of the load cell, the accuracy of the load cell should be slightly higher than the theoretical calculation value. If several load cells of the same type and the same rated capacity are used in a weighing system in parallel, the overall error is △, then △'=△√n, where △: the overall error of a single load cell; n: The number of load cells. When the tolerance of the system is 1, the comprehensive error (△) of the load cell, which is one of the main components of the non-automatic weighing instrument, can generally only reach a ratio of 0.7. Based on this and △'=△√n, it is not difficult to choose the required load cell accuracy.