The mp6-series micropumps always work corresponding to the pump curve, which is based on the maximum flowrate (at zero pressure) and the maximum pressure (at zero flowrate). The curve of pressure and flowrate combinations that are directly achievable with the mp6 micropump is a straight line between these two values, it depends on further components within the fluidic system. When the pump is running and the outlet is closed so that no flow rate is possible the pump will generate the max pressure. When the outlet is open the pump can achieve the max flow rate. If there is a back pressure on the outlet, the flow rate is reduced by this. Note that each tubing and component connected to the pumps fluidic system will have an own fluidic resistance (like electrical resistors in an electric circuits), which can be compared to a certain back pressure, and the max flow rate is slightly reduced.
Same applies to a pressure on the inlet.
With forward pressure the pump is not reduced in performance and can add its flow rate to the pressure induced flow rate.
In case that the inlet has a negative pressure like a slight vacuum, the pump would be reduced a bit as it has to overcome this before a flow rate is realized. Thus, a negative inlet pressure can be compared to a backpressure on the outlet.
An additional thing to know is that the pump is designed fluidically open, that means liquid can pass the pump due to differential pressure. In forward direction this is easy as the internal pump valves open. In reverse direction there is a leak flow, as the pump valves are not completely tight. If you want to prevent back flow we recommend using a checkvalve like our mp-cv.
At the moment we don't have a valve that prevents flow in forward direction, as this requires a certain opening pressure of the valve, but we are currently in the process of designing one.
However, with such a valve the pump needs to open the valve first, i.e. create some pressure which will reduce the max flow rate, before a resulting flow can happen.
If your current application is sensitive to the existing inlet pressure, it is always possible to compensate the pump by decreasing the amplitude a bit.