Abstract : This article introduces the core issues and solutions related to flow control in flow meter testing equipment. On the hardware side, it briefly describes the connection between the computer and the frequency converter. On the software side, it describes the development of host computer software using the VC++ communication control MSComm, providing detailed code and solutions to problems encountered in practice. The software and hardware are organically combined using RS-485 communication to successfully achieve the project objectives. Keywords : MSComm control; frequency converter; flow control. Flow meter manufacturers need to verify the parameters of these instruments during production and calibration to determine their properties and provide users with the corresponding parameters for use. There are many methods for determining instrument parameters. In the actual flow calibration method, the testing device must provide the flow rate at several testing points within the corresponding instrument's flow range. The following describes the implementation of flow rate changes at several testing points in the testing device. 1 Project Requirements and Implementation Methods The project provides equipment such as a signal acquisition unit, standard meter, pipes of corresponding diameter, frequency converter, and fan. To achieve fully automated control of the entire calibration process, we used the VC++ communication control MSComm to develop host computer software to control the frequency converter. The frequency converter then controls the frequency of the fan, thereby controlling the fan's output power to achieve changes in gas flow rate. 2 System Design and Implementation In this system, the hardware connection is from the computer's RS-232 interface to the frequency converter via an RS-232/485 converter. The system uses an ABB ACS400 frequency converter, and the communication protocol in the VC++ communication software strictly adheres to the ACS400 frequency converter's communication protocol. [b][align=center]For more details, please click: Automatic Control of Flow Rate Changes in Flowmeter Calibration Device[/align][/b]