Method for analysis of a liquid with a sampling unit (100; 300; 400; 500; 610; 620; 630) for a liquid sample, preferably for a fuel intended for a combustion engine, which sampling unit (100; 300; 400; 500; 610; 620; 630) is adapted to be fitted into a system which contains or transports a liquid, wherein the sampling unit (100; 300; 400; 500; 610; 620; 630) comprises a wall section (304; 404; 504'; 604) partly surrounding a cavity (301; 401; 501'; 601), which is adapted to be filled with the liquid, a first opening (303; 403; 503'; 603), through which liquid in the cavity may flow out of the cavity and a second opening (307; 407; 507'; 507"; 607) through which liquid may flow into the cavity (301; 401; 501'; 601), wherein the first opening (303; 403; 503'; 603) is equipped with a first shutter element (305; 405; 505'; 605; 605'; 605"), which opens when the liquid's temperature in the cavity increases and the liquid expands and an over-pressure is created in the cavity, and wherein the second opening (307; 407; 507'; 607) is equipped with a second shutter element (309; 409; 509'; 609; 609'; 609"), which opens when the liquid's temperature in the cavity drops and a negative pressure is created in the cavity, characterised in that the method comprises the steps: a) of filling the cavity (301; 401; 501'; 601) of the sampling unit (100; 300; 400; 500; 610; 620; 630) initially with a liquid; b) of placing or fitting the sampling unit in a system, in which the sampling unit (100; 300; 400; 500; 610; 620; 630) comes into contact with a liquid to be analysed; c) of subjecting the liquid in the system to temperature variations between a resting temperature and an operating temperature and vice versa; d) of allowing the liquid in the cavity (301; 401; 501'; 601) to flow out of the cavity, using the first shutter element (305; 405; 505'; 605; 605'; 605"), when the temperature of the liquid in the cavity (301; 401; 501'; 601) increases and the liquid expands and an over-pressure is created in the cavity (301; 401; 501'; 601); e) of allowing the liquid in the system to flow into the cavity (301; 401; 501'; 601), using the second shutter element (309; 409; 509'; 609; 609'; 609"), when the liquid's temperature in the cavity (301; 401; 501'; 601) drops and a negative pressure is created in the cavity (301; 401; 501'; 601); and f) of allowing mixture of the liquid inside the cavity (301; 401; 501'; 601) by way of the liquid's in- and out-flow. a) of filling the cavity (301; 401; 501'; 601) of the sampling unit (100; 300; 400; 500; 610; 620; 630) initially with a liquid; b) of placing or fitting the sampling unit in a system, in which the sampling unit (100; 300; 400; 500; 610; 620; 630) comes into contact with a liquid to be analysed; c) of subjecting the liquid in the system to temperature variations between a resting temperature and an operating temperature and vice versa; d) of allowing the liquid in the cavity (301; 401; 501'; 601) to flow out of the cavity, using the first shutter element (305; 405; 505'; 605; 605'; 605"), when the temperature of the liquid in the cavity (301; 401; 501'; 601) increases and the liquid expands and an over-pressure is created in the cavity (301; 401; 501'; 601); e) of allowing the liquid in the system to flow into the cavity (301; 401; 501'; 601), using the second shutter element (309; 409; 509'; 609; 609'; 609"), when the liquid's temperature in the cavity (301; 401; 501'; 601) drops and a negative pressure is created in the cavity (301; 401; 501'; 601); and f) of allowing mixture of the liquid inside the cavity (301; 401; 501'; 601) by way of the liquid's in- and out-flow.