Driving the Bartels mp6 micropump with the controller mp-x at 100 Hz with 250 V, flow rates of typ. 7 ml/min and backpressure of typ. 600 mbar can be achieved. The flow rate of the pump is a linear dependency on the pressure. At 0 mbar pressure the maximum pump rate can be achieved and at the maximum pressure the flow rate is decreased to 0 ml/min.
With low frequencies and amplitudes, the volume flow and pressure generation is rather weak due to the passive character of the valves inside the micropump. These valves are more effective with fast and high pressure changes induced by the actuator.
If the required flow rate should be small, the application of a restrictor is recommended. A restrictor will throttle down the max flow rate of the pump and will deliver the required flow rate. With this method, the pressure capability of the pump is not reduced and the flow is much more stable.
Find more details in the two attached application notes for low flow rates.