A solenoid dosing pump is a form of positive displacement pump which uses a diaphragm and solenoid assembly to displace the fluid into the discharge line.
The solenoid 'drive' consists of an electromagnet and spring assembly, which is activated/deactivated with a series of electrical impulses. As the solenoid is activated, the electromagnet pushes the diaphragm, which displaces the fluid. As the solenoid is deactivated, the spring mechanism returns the diaphragm, allowing more fluid from the suction line to enter the chamber ready for delivery.
The dosing aspect is determined by programming the pump to a rate of delivery. The rate can be arbitrarily set by the operator or according to an external signal e.g. from an analytical probe measuring pH. The pump can then dose its fluid proportionally - In this case it may be dosing caustic soda according to correct acidic effluent.
Some solenoid dosing pumps also allow a variable stroke length and stroke rate. This allows a very high 'turndown' i.e. a very high ratio between the maximum and minimum potential flow rates.