The present invention relates to a sampling unit (100; 300; 400; 600; 700) for liquid samples adapted to be fitted into a system with pressure variations, which system contains or transports a liquid. The sampling unit comprises a wall section (317; 417; 617; 717), which partly surrounds a cavity (301; 401; 601; 701) filled with liquid, an opening (303; 403; 603; 703) equipped with a blocking element (305; 405; 605; 705; 720), which prevents the liquid from flowing into or out of the cavity at a substantially stable pressure in the system, but allows the liquid to flow into the cavity when the pressure in the system increases from a resting pressure to an operating pressure, and out of the cavity when the pressure in the system drops back from the operating pressure to the resting pressure. The sampling unit may be used to collect a fuel sample.