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As is well known, mass flow meters are widely used in various industries – industrial, agricultural, livestock farming, fisheries, food processing, pharmaceuticals, and the chemical industry – and they play a crucial role in these fields. So how can it be ensured that mass flow meters will not cause any errors in their operation? Easy to overlook! Are there methods for calibrating mass flow meters as well? Don’t do anything reckless! There’s no time for regret! Mass flow meters are generally used only for measuring liquid flow rates, and their calibration is usually carried out in a laboratory (with flow standard devices) under specific conditions; this is what is commonly referred to as offline calibration. We know that the purpose of calibrating a mass flow meter is to determine its metrological performance in actual use. There are the following issues with laboratory identification: ① The testing medium is different from the actual medium used in measurement, resulting in different physical property parameters ； ②The operating conditions during calibration, such as temperature and pressure, are not the same as those under which the flow meter is actually used ； ③There are also significant differences in the installation methods of flow meters depending on the site. The above explanations indicate that the calibration or performance evaluation of mass flowmeters conducted in the laboratory differs from the actual performance of these flowmeters in use, which is far from meeting the goals of accurate measurement. For example, when a mass flow meter is used for trade settlement measurements, such a difference is likely to be the source of disputes between buyers and sellers, and must be eliminated. Online calibration of mass flow meters is a type of calibration using actual fluid, and it has many particularities or requirements compared to laboratory calibration. These can be summarized as follows: (1) There must be a fixed or movable flow standard device for calibrating the mass flow meter. In addition to the device’s accuracy meeting the required standards, its operating temperature range, pressure resistance, and compatibility with the medium also need to satisfy the requirements for online calibration. If it is a movable flow standard, the device must also be lightweight and flexible ； ⑵During the calibration process, the fluid must be kept in a sealed state to ensure that the calibration conditions remain unchanged, as well as to guarantee the safety at the calibration site ； ⑶The flow of fluid must not be interrupted during the calibration process ； ⑷If the flow meter under inspection is to be calibrated using a mobile standard device, the site where the flow meter is located must have the necessary interfaces, valves, as well as appropriate facilities for recovery and cleaning, space, and power supply.