The metering pump is a pump specially used for metering and transporting liquid. Most metering pumps use plunger pump and diaphragm pump structure (see reciprocating pump), but also useful gear pump structure, because they can maintain constant flow independent of discharge pressure. The metering capacity of a metering pump is determined by the amount of fluid pumped out each time. Under a certain effective diaphragm area, the volume flow of metering pump output fluid is proportional to stroke length L and stroke frequency F:V_A*F*L. The metering pump can realize two-dimensional adjustment of metering pump output by adjusting stroke length L and stroke frequency F when the metering medium and working pressure are determined. Although stroke length and frequency can be used as adjustment variables, stroke length is generally regarded as coarse adjustment variable and stroke frequency as fine adjustment variable in engineering application: adjust stroke length to a certain value, and then achieve fine adjustment by changing its frequency to increase the flexibility of regulation. In relatively simple applications, the stroke length can also be set manually, and only the stroke frequency can be used as a variable to simplify the system configuration. Following is an introduction to several different control modes of metering pumps.

Base-type control mode of metering pump
In some special occasions, such as pH adjustment, metering pump acts as actuator, adding acid or alkali under the control of regulator. In order to simplify the system configuration and improve the reliability, the embedded control system with microprocessor as the core is directly integrated into the metering pump, so only an external pH sensor is needed to form a complete regulation system. The concept of base intelligent metering pump can also be applied to control other process parameters, such as redox potential (ORP) and residual chlorine concentration regulation.

Procedural control mode of metering pump setting
Because of the internal integration of microprocessor, the regulation and operation performance of some metering pump products has been fully improved. Besides following the external control commands to achieve real-time metering flow regulation, it also has many tasks, such as quantitative addition, time series trigger program addition, event sequence trigger program addition, time-event hybrid trigger program addition and automatic correction. The model can also provide useful information such as the total amount of fluid pumped out, the number of remaining strokes and the capacity of the fluid to be transported, the stroke length and other relevant working parameters of metering pumps. Time series trigger program mode makes metering pump add working medium quantitatively according to the set stroke number or time at the specified time according to the real-time clock and the pre-planned task list, and the time can be set flexibly in the cycle of every hour, day, working day, public holiday, one week and two weeks; while event sequence trigger program mode makes metering pump complete. According to the real-time trigger event, according to the pre-planned task list, the working medium is added quantitatively according to the set stroke number or time when a specific event occurs (contact input).

Fieldbus ProFibus Control Mode
In large-scale and highly automated applications such as petrochemical industry, metering pumps use digital communication protocol to transmit data between automation equipment at high speed and then form a network control system, which has large capacity and high reliability, and has become the mainstream of development. Following the introduction of the German DIN standard in the early 1990s, the field bus ProFibus became the European standard (EN50170) in the late 1990s. It has been widely used in the world and has become one of the representative protocols in field bus technology.

Using metering pump can accomplish three functions of transportation, metering and regulation, thus simplifying production process, realizing automation and semi-automation, and improving product quality. Using multi-cylinder or multi-metering pumps, several kinds of media can also be mixed into the reactor in an accurate proportion. The research and development of metering pump has gone beyond its traditional purely pursuit of capacity and material, and turned to the development direction of high precision, multi-function and intelligence. This trend has become the consensus of the world's major metering pump manufacturers.