A method to allow fluid communication to be established between an inner cavity of a drill string and a surrounding material in situ down in a drill hole during the use of a down-the-hole drill unit that includes the drill string formed from a number of drill rods coupled at their ends, a down-the-hole hammer drill fixed at one end of the drill string and a source of pressurised medium that delivers a medium under pressure to the down-the-hole hammer drill and that is connected to the second end of the drill string, which method comprises the following operational steps: (a) that a tube section (10:1) of the drill string is assigned at its outer cover one or several openings with an area of opening that has been determined in advance, (b) that a piston with an axial penetrating channel is arranged, (c) that the piston is arranged such that it can glide axially inside the cavity of the tube section (10:1), (d) that the piston is designed such that it, during drilling, allows driving fluid to be led through the channel from the source of pressure to the down-the-hole hammer drill when the piston is located at its most withdrawn position in the tube section (10:1) and in contact with the down-the-hole drill unit in a manner that allows fluid to flow, and that the piston is driven towards its most withdrawn position through influence of a hydrostatic force that the driving fluid in the channel exerts onto an end surface of the piston that faces the source of pressurised medium during drilling, (e) that the side of the piston that faces the source of pressure is assigned a first connector of a recovery means, which first connector can be coupled together in a retaining manner with a second connector that is a component of the said recovery means, (f) that, after drilling, a lifting arrangement is arranged at the surface level in connection with the drill hole, (g) that the second connector of the recovery means, fixed at the lifting arrangement that is arranged at the surface level, is caused to enter into retaining interaction with the first connector by being lowered down into the drill hole by means of the lifting arrangement, after which the piston is fished up out of the drill hole by means of the lifting arrangement and the a compartment that is formed in this way in the cavity of the tube section (10:1) can be used as a measurement compartment, and that the tube section (10:1) allows water from the surroundings to flow into the measuring compartment through one or several openings, thereby establishing fluid communication between the inner cavity of the drill string and the surrounding material. (a) that a tube section (10:1) of the drill string is assigned at its outer cover one or several openings with an area of opening that has been determined in advance, (b) that a piston with an axial penetrating channel is arranged, (c) that the piston is arranged such that it can glide axially inside the cavity of the tube section (10:1), (d) that the piston is designed such that it, during drilling, allows driving fluid to be led through the channel from the source of pressure to the down-the-hole hammer drill when the piston is located at its most withdrawn position in the tube section (10:1) and in contact with the down-the-hole drill unit in a manner that allows fluid to flow, and that the piston is driven towards its most withdrawn position through influence of a hydrostatic force that the driving fluid in the channel exerts onto an end surface of the piston that faces the source of pressurised medium during drilling, (e) that the side of the piston that faces the source of pressure is assigned a first connector of a recovery means, which first connector can be coupled together in a retaining manner with a second connector that is a component of the said recovery means, (f) that, after drilling, a lifting arrangement is arranged at the surface level in connection with the drill hole, (g) that the second connector of the recovery means, fixed at the lifting arrangement that is arranged at the surface level, is caused to enter into retaining interaction with the first connector by being lowered down into the drill hole by means of the lifting arrangement, after which the piston is fished up out of the drill hole by means of the lifting arrangement and the a compartment that is formed in this way in the cavity of the tube section (10:1) can be used as a measurement compartment, and that the tube section (10:1) allows water from the surroundings to flow into the measuring compartment through one or several openings, thereby establishing fluid communication between the inner cavity of the drill string and the surrounding material.