NOVELTY - Measuring device (1) for measuring an ultra low gas flow, working by the principle of liquid displacement, comprises: at least one cell (2) comprising a gas inflow device (3); and a gas compartment device (4) with a predefined inner geometric physical volume and active volume, and having one gas accumulating end (5) and one lifting end (6).    USE - In a measuring device system for a lab scale process system for simulation of biogas production (claimed), for measuring a biogas flow, preferably a biomethane gas flow and other small gas flows from a biosample, e.g. in anaerobic biodegradation process. The device is useful as a standalone device to provide on-line, real-time monitoring of biogas production from small scale biogas digesters.    ADVANTAGE - The measuring device has high measuring accuracy even at a low flow rate of a biogas, simple design, and has low material and construction cost.    DETAILED DESCRIPTION - Measuring device (1) for measuring an ultra low gas flow, working by the principle of liquid displacement, comprises: at least one cell (2) comprising a gas inflow device (3); and a gas compartment device (4) with a predefined inner geometric physical volume and active volume. The gas compartment device (GCD) has one gas accumulating end (5) and one lifting end (6), the device GCD also defines a geometric gas collecting point inside the device GCD during a gas filling cycle, the gas collecting point moves further and further from the gas accumulating end to the lifting end during the gas filling cycle, and the cell (2) comprises a holder (7) having a pivoting element (8) enabling the GCD to pivot upwards when the gas collecting point is positioned at the lifting end and the lifting force is larger than the down-pressing force at the lifting end, thereby releasing all of the accumulated gas in GCD, and then pivoting back to its initial standby position for new receipt and storage of gas during another gas filling cycle until next releasing sequence, and the cell also comprises a sensor (9) to generate a signal and/or change the state of a signal when GCD is not in its initial standby position, where the gas storing capacity of the inside of GCD is larger at the gas accumulating end than at the lifting end and the gas accumulating end has a higher vertical position than the lifting end at the initial standby position. An INDEPENDENT CLAIM is included for a lab scale process system for simulation of biogas production, comprising: at least one digester with good agitation; optionally at least one additional measuring device or sensor for measuring gas composition on-line or in spot check in regular basis; and the measuring device (1) or a measuring device system that is immersed in a wet space.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of the measuring device (1).    Cell (2)    Gas inflow device (3)    Gas compartment device (4)    Gas accumulating end (5)    Lifting end (6)    Holder (7)    Pivoting element (8)    A sensor (9)