Method for analysis of a liquid with a sampling unit or with a sampling unit cluster comprising at least two sampling units (110, 115, 130), which sampling unit and sampling unit cluster are adapted to be fitted into a system with temperature variations, which system contains or transports a liquid to be analysed, wherein the sampling unit (100, 115, 120, 130) comprises a wall section partly surrounding a cavity for being filled with a liquid, and an opening through which the liquid in the cavity may flow out of the cavity when the liquid expands at a temperature increase, and through which opening the liquid in the system may flow into the cavity when the liquid in the cavity is compressed at a temperature drop, which opening is equipped with a channel (120, 121, 122, 123) that may hold a specified volume of liquid and which channel (120, 121, 122, 123) is arranged to be in contact with the liquid in the system, wherein the method comprises the steps: a) of filling the cavity in the sampling unit (100, 115, 120, 130) with a starting liquid; b) of placing or fitting the sampling unit or the sampling unit cluster in the system, in which the sampling unit (100, 115, 120, 130) comes into contact with the liquid to be analysed; c) of subjecting the system to temperature variations between a resting temperature and an operating temperature and vice versa; d) of allowing the liquid in the channel (120, 121, 122, 123) to flow out of the channel (120, 121, 122, 123) and into the channel (120, 121, 122, 123) at a temperature variation, which is smaller than the temperature variation between the resting temperature and the operating temperature; e) of allowing the liquid both in the channel (120, 121, 122, 123) and in the cavity to flow out of the channel (120, 121, 122, 123) and the cavity and into the channel (120, 121, 122, 123) and the cavity at a temperature variation between the resting temperature and the operating temperature; f) of allowing mixture of the liquid inside the cavity by way of the liquid's in- and out-flow; g) of removing the sampling unit or the sampling unit cluster from the system after a time period, h) of removing the liquid accumulated in the sampling unit or in the sampling unit cluster, and i) of analysing the level of a substance in the liquid from the sampling or the sampling unit cluster with an analysis method adapted to the substance to be analysed. a) of filling the cavity in the sampling unit (100, 115, 120, 130) with a starting liquid; b) of placing or fitting the sampling unit or the sampling unit cluster in the system, in which the sampling unit (100, 115, 120, 130) comes into contact with the liquid to be analysed; c) of subjecting the system to temperature variations between a resting temperature and an operating temperature and vice versa; d) of allowing the liquid in the channel (120, 121, 122, 123) to flow out of the channel (120, 121, 122, 123) and into the channel (120, 121, 122, 123) at a temperature variation, which is smaller than the temperature variation between the resting temperature and the operating temperature; e) of allowing the liquid both in the channel (120, 121, 122, 123) and in the cavity to flow out of the channel (120, 121, 122, 123) and the cavity and into the channel (120, 121, 122, 123) and the cavity at a temperature variation between the resting temperature and the operating temperature; f) of allowing mixture of the liquid inside the cavity by way of the liquid's in- and out-flow; g) of removing the sampling unit or the sampling unit cluster from the system after a time period, h) of removing the liquid accumulated in the sampling unit or in the sampling unit cluster, and i) of analysing the level of a substance in the liquid from the sampling or the sampling unit cluster with an analysis method adapted to the substance to be analysed.