The pump’s motor drives a piston to create a vacuum that pulls chemicals into the liquid end of a metering pump from external tanks. Alternating piston strokes create pressure that closes the inlet valve, opens the outlet valve, and forces the chemical out to the process. Within the liquid end is a diaphragm, which acts as a barrier between the piston and the process fluid. Sometimes a diaphragm is mechanically connected to a piston. Sometimes a diaphragm is hydraulically connected. The piston's pumping motion is applied to hydraulic fluid, which causes the diaphragm to flex back and forth as the piston reciprocates. The movement of the piston flexes the diaphragm – the more the diaphragm flexes, the higher the flow rate for the pump.