| | | Communication with the Mass Flow Communicator software (see Section 4 for further information) is via the RS-232 or RS-485 (depending on the instrument). In addition to reduced drift and offset by the components, microprocessor-based digital electronics offer the major advantage that all the required processes can be controlled by software (flash-programmed, in other words, the electronics can be updated). Data relevant to this (calibration curves, correction functions, control functions, etc.) can be stored in the memory. | | | The "valve authority" is important for the MFC control properties in the system. It should not be below a value of 0.3 ... 0.5. Meaning of formula symbols: kVa Flow-rate coefficient of system without installed MFC in m3/h kVs Flow-rate coefficient of the MFC with final control element fully open in m3/h (Ap)0 Pressure drop through the entire system (Ap)V0 Proportion of this that drops through the MFC with the valve fully open. The system section should not be designed in terms of its flow-rate coefficient kVa, so that at the required nominal flow rate, the vast majority of the available pressure is used there and thus the selected nominal valve diameter is too large. In this case, the valve authority is too small, in other words, only a small proportion of the valve working range is used. This considerably impairs the resolution and control quality of the MFC. If the system section is designed to be too "small", increasing the nominal valve diameter will not help, and the remedy is either to increase the supply pressure or increase the kVa value, e.g. by means of a larger pipe diameter. | | |
| | | Direct-acting plunger-type proportional valves (see Section 9, Proportional valves) from the Burkert valve range are used as final control elements in all MFC series. Constructive measures, in particular on valves in the MFCs for low flow rates (Types 8710 and 8712), ensure that the moving plunger is guided with low friction. Together with the PWM actuation, this ensures a constant, almost linear characteristic curve along with high response sensitivity. Both of these features are important for optimal function in the MFC's closed control loop. The nominal diameters of the valves result from the required nominal flow rate Qnom, the pressure conditions in the application and the density of the operating gas. Using these data, a proportional valve can be selected whose flow-rate coefficient kVs allows the maximum flow of at least the required nominal flow rate in the specified pressure conditions and in compliance with the flow-rate equations: | | |