How can manufacturers express the accuracy of a meter?

Manufacturers typically express the accuracy of a meter as a percentage of the full scale reading. This method of expressing accuracy indicates how close the reading is to the actual value compared to the maximum range of the meter. For instance, if a meter has a full-scale reading of 100 units and is rated for an accuracy of ±2%, this means that the actual reading can deviate by 2 units at any point across the full scale. This form of measurement gives a clear understanding of how the meter will perform under various readings, making it an industry-standard for conveying accuracy.

Other methods listed, such as expressing accuracy as a percentage of the power supply or as a ratio of input to output, do not appropriately reflect how well the meter measures its specified range. An absolute value does not provide context for accuracy relative to the meter's capabilities and thus lacks the necessary information for evaluating the performance of the meter in practical applications. Hence, expressing accuracy as a percentage of the full face value or reading is the most effective and accepted method within instrumentation and process control.