The weighing instrument is mainly composed of three parts: the load-bearing system, the force transmission conversion system, and the indication system. Weighing instruments can be divided into three categories: mechanical scales, electronic scales, and electromechanical scales according to their structural principles.

    In the verification of weighing instruments, especially the verification of large weighing instruments and the verification of automatic weighing instruments, sometimes the above-mentioned direct weighting method cannot be used, and some methods that are not in the quality verification system table need to be adopted.

    Among them, the most commonly used method is the "standard weight substitution method". After theoretical analysis, it can basically meet the principle requirement of "the measurement uncertainty is not greater than 1/3 of the maximum allowable error MPE of the weighing instrument", and also It has the characteristics of being easy to use and has practical value in the verification of weighing instruments.

    For weighing instruments with a maximum weighing capacity of several tons or more than tens of tons, the earliest verification method is to use a 20kg weight, which is manually transported to check the maximum weighing capacity step by step. However, due to the workload and strong limitations of this method, another verification method of large scales uses a large weight of 500kg to 1000kg cylindrical or rectangular, and is equipped with corresponding transportation and lifting equipment, which constitutes a platform check and balance.

    Equipped with a sufficient amount of standard weights, the weights are transported back and forth by the weighing truck several times to realize the verification of the maximum weighing capacity. This method is only suitable for occasions where the road is short, but due to the very high cost, the application is relatively small.

    Another method is to be equipped with quantitative standard weights, find a suitable "substitute" at the verification site, and use the standard weight substitution method to achieve the verification of the maximum weighing instrument.

    The standard weight substitution method is to use a constant load instead of the standard weight for verification because the number of standard weights prepared cannot reach the amount required for the maximum weighing capacity of the weighing instrument when verifying a large scale.