TECHNOLOGY FOCUS - INSTRUMENTATION AND TESTING - Preferred Components: A flow of liquid through the first liquid line and/or the second liquid line and/or through the liquid unit is pneumatically producible, preferably by creation of a difference in pressure between the first liquid storage and the second liquid storage. The first and/or the second valve is a pneumatic actuated valve. The first liquid storage exhibits an interface for connecting a pneumatic line, over which the first liquid storage is acted upon with pressure. The pneumatic line is additionally connected with the pneumatic actuated first valve, so that a pressurization of the first liquid storage simultaneously affects the pneumatic actuated first valve. The first and/or the second valve is a back-pressure valve (150). The system exhibits at least one flow path for transporting liquid from the first liquid storage into the liquid unit. The first back-flow valve is arranged in the flow path in such a manner that its passage direction is in the direction of the flow path to the liquid unit. The system exhibits at least one further flow path for transporting liquid from the liquid unit into the second liquid storage. The second back-flow valve is arranged in the flow-path in such a manner that its passage direction is in the direction of the flow path to the second liquid storage. The system exhibits individual interchangeable modules. A first interchangeable module exhibits the liquid unit, the liquid line and at least a liquid discharge pipe from the liquid unit. A second interchangeable module comprises the first and the second liquid storage and the interface for connecting the first liquid storage and/or the second liquid storage to the liquid supply line and/or the liquid discharge line. The second module exhibits first and the second valve, preferably the first and the second back-flow valve or the first and the second pneumatic actuated valve. The second module exhibits a further interface for connecting at least each first liquid storage and each second liquid storage to respective pneumatic line of a pneumatic system. The pneumatic system is configured in such a manner that a pressure prevailing in each pneumatic line is adjustable, in order to generate a predetermined pressure on the first or second liquid storage connected to the pneumatic line. The interface for connecting at least each of first liquid storage to respective pneumatic line, exhibits a shutter element (120), preferably a diaphragm, a membrane or a filter, which closes the liquid storage, preferably against the penetration or removal of microorganisms or withdrawing of liquid from the liquid storage, in a liquid tight manner, but gas permeable manner. All first and second liquid storages, the liquid unit, all liquid lines from the first liquid storage to the liquid unit, and all liquid discharge pipe from the liquid unit are grouped in a cassette sealed against the environment, preferably surrounded by a liquid or gas tight housing, where the cassette exhibits an interface for connecting to a pneumatic system. The liquid within the first liquid storage and/or the second is contained in a flexible gas- and liquid impermeable container, which is connected to the first liquid line. The system exhibits at least one flow path for transporting liquid from one or more first liquid storage by the liquid unit into one or more second liquid storage, where the system has a measuring unit for determining a flow, preferably a mass or a volume flow, of the liquid through the flow path. The system further exhibits a control unit, which is configured to regulate pressure applied to the first and second liquid storage, on the basis of at least one measuring value made from the measuring device. The liquid unit exhibits a first liquid line section connected with the first liquid storage and a second liquid line section connected with the second liquid storage. The first liquid line section and the second liquid line section are combined to a first point to a third liquid line section, which is branched itself at a second point of the third liquid. At least the fourth liquid line section ends in the functional unit. At least a sixth liquid line section goes away from the functional unit, which is connected with the first second type liquid storage. The fifth liquid line section is connected with a second second-type liquid storage. The liquid unit is configured as micro fluidic chip, which has at least one functional unit and at least one micro-channel portion as liquid supply line, leading to the functional unit, and at least one micro-channel portion as liquid discharge line, leads away from the functional unit. The liquid unit has passive elements for controlling the liquid flow. The first module and/or the second module comprises a temperature control device, preferably a controllable thermoelectric element or a controllable heating element. The system further comprises a signal detection device and/or a signal generating device to detect a characteristic, preferably an optical or magnetic characteristic, of an analyte present in the at least one functional unit of the fluid unit or a reaction product of the analyte.