An arrangement in a capillary driven fluid system for metering a predetermined volume of sample fluid, the arrangement comprising: a sample reservoir arranged to receive a sample fluid, a first channel which is in fluid communication with the sample reservoir and which branches off into a second channel ending at a first capillary trigger valve and a third channel ending at a second capillary trigger valve, wherein the second channel and the third channel together have a predetermined volume, and the first channel is arranged to draw sample fluid from the sample reservoir by use of capillary forces to fill the second and the third channel with the predetermined volume of sample fluid, a capillary pump being fluidically connected to the sample reservoir via a first flow resistor, wherein the first flow resistor has a flow resistance which is selected to control the flow rate from the sample reservoir to the capillary pump such that the sample reservoir is emptied after the second and third channels have been filled with sample fluid, a buffer reservoir arranged to receive a buffer fluid, a fourth channel, wherein the second capillary trigger valve is fluidically connected to the buffer reservoir via the fourth channel, the fourth channel being arranged to draw buffer fluid from the buffer reservoir by use of capillary forces, and to open the second capillary trigger valve as buffer fluid in the fourth channel reaches the second capillary trigger valve, whereby a fluid path including the fourth channel, the third channel and the second channel is opened up from the buffer reservoir to the first capillary trigger valve, and a first control circuit comprising a first fluidic circuit which fluidically connects the first capillary trigger valve to the buffer reservoir, the first fluidic circuit being arranged to draw buffer fluid from the buffer reservoir and open the first capillary trigger valve as buffer fluid reaches the first capillary trigger valve, the first control circuit being arranged to open the first capillary trigger valve after the sample reservoir has been emptied, whereby a capillary driven flow arises in said fluid path, thereby causing the predetermined volume of sample fluid in the second and third channels to flow out through the first capillary trigger valve, and enter a sixth channel C6, wherein the sample fluid and/or the buffer fluid at least partly is in gaseous communication with surroundings of the arrangement such as to allow gas mixed within the sample fluid and/or buffer fluid to escape from the arrangement, wherein the gaseous communication with surroundings occurs through a gas permeable sheet, or the gaseous communication with surroundings occurs through a further valve fluidically connected to the second capillary trigger valve. a sample reservoir arranged to receive a sample fluid, a first channel which is in fluid communication with the sample reservoir and which branches off into a second channel ending at a first capillary trigger valve and a third channel ending at a second capillary trigger valve, wherein the second channel and the third channel together have a predetermined volume, and the first channel is arranged to draw sample fluid from the sample reservoir by use of capillary forces to fill the second and the third channel with the predetermined volume of sample fluid, a capillary pump being fluidically connected to the sample reservoir via a first flow resistor, wherein the first flow resistor has a flow resistance which is selected to control the flow rate from the sample reservoir to the capillary pump such that the sample reservoir is emptied after the second and third channels have been filled with sample fluid, a buffer reservoir arranged to receive a buffer fluid, a fourth channel, wherein the second capillary trigger valve is fluidically connected to the buffer reservoir via the fourth channel, the fourth channel being arranged to draw buffer fluid from the buffer reservoir by use of capillary forces, and to open the second capillary trigger valve as buffer fluid in the fourth channel reaches the second capillary trigger valve, whereby a fluid path including the fourth channel, the third channel and the second channel is opened up from the buffer reservoir to the first capillary trigger valve, and a first control circuit comprising a first fluidic circuit which fluidically connects the first capillary trigger valve to the buffer reservoir, the first fluidic circuit being arranged to draw buffer fluid from the buffer reservoir and open the first capillary trigger valve as buffer fluid reaches the first capillary trigger valve, the first control circuit being arranged to open the first capillary trigger valve after the sample reservoir has been emptied, whereby a capillary driven flow arises in said fluid path, thereby causing the predetermined volume of sample fluid in the second and third channels to flow out through the first capillary trigger valve, and enter a sixth channel C6, wherein the sample fluid and/or the buffer fluid at least partly is in gaseous communication with surroundings of the arrangement such as to allow gas mixed within the sample fluid and/or buffer fluid to escape from the arrangement, wherein the gaseous communication with surroundings occurs through a gas permeable sheet, or the gaseous communication with surroundings occurs through a further valve fluidically connected to the second capillary trigger valve.