A method for measuring the volumetric changes in a substance which is part of a sample provided within a vessel with opened top mouth, wherein the mouth is closed by a plug having a hole through which a transparent tube with opened ends and graduation mark passes, the tube having its bottom end inside the vessel and its top end outside the vessel, the vessel being filled with a translucent liquid which further fills a tube length until it forms a level of the translucent liquid, a light emitter/receptor set (10, 11) being comprised by a light emitter provided at one side of the tube and a light receptor at the opposite side of the tube, said light emitter and receptor being optically aligned, wherein the method comprises the step of: Providing a marker immiscible with said translucent liquid and with an opacity different from the one of said translucent liquid over said level of translucent liquid, Providing said light emitter/receptor set (10, 11) along the tube and over said marker, Moving said light emitter/receptor (10, 11) set downstream until said marker interposes between said light emitter and said light receptor, thereby determining a variation in the intensity of the light traversing the tube, Defining at that point a zero "0" time and an initial position or baseline position, Moving said light emitter/receptor set (10, 11) downstream along a specified distance until a first position, and recording at that point a height value, Waiting for said marker, in a downstream direction, to interpose between said light emitter and said light receptor in said first position, thereby determining a new variation in the intensity of the light reaching the light receptor, Recording said height value and the time the marker took for interposing between the light emitter and the receptor; Moving said light emitter/receptor set (10, 11) downstream until a second position, and recording at that point the new height value, Waiting for said marker, in a downstream direction, to interpose between said light emitter and said light receptor, thereby determining a new variation in the intensity of the light reaching the light receptor, Recording at that point said height value and the time the marker took for interposing between the light emitter and the receptor; Moving said light emitter/receptor set (10, 11) downstream until a third position, and recording at that point a new height value, Waiting for said marker, in a downstream direction, to interpose between said light emitter and said light receptor and, when the marker is not able to interpose between the light emitter and the receptor in a specified time, establishing the second position as the last position; Recording the distance between the initial position and the last position, and Recording the time elapsed between the time zero "0" and the time accumulated until the last position. Providing a marker immiscible with said translucent liquid and with an opacity different from the one of said translucent liquid over said level of translucent liquid, Providing said light emitter/receptor set (10, 11) along the tube and over said marker, Moving said light emitter/receptor (10, 11) set downstream until said marker interposes between said light emitter and said light receptor, thereby determining a variation in the intensity of the light traversing the tube, Defining at that point a zero "0" time and an initial position or baseline position, Moving said light emitter/receptor set (10, 11) downstream along a specified distance until a first position, and recording at that point a height value, Waiting for said marker, in a downstream direction, to interpose between said light emitter and said light receptor in said first position, thereby determining a new variation in the intensity of the light reaching the light receptor, Recording said height value and the time the marker took for interposing between the light emitter and the receptor; Moving said light emitter/receptor set (10, 11) downstream until a second position, and recording at that point the new height value, Waiting for said marker, in a downstream direction, to interpose between said light emitter and said light receptor, thereby determining a new variation in the intensity of the light reaching the light receptor, Recording at that point said height value and the time the marker took for interposing between the light emitter and the receptor; Moving said light emitter/receptor set (10, 11) downstream until a third position, and recording at that point a new height value, Waiting for said marker, in a downstream direction, to interpose between said light emitter and said light receptor and, when the marker is not able to interpose between the light emitter and the receptor in a specified time, establishing the second position as the last position; Recording the distance between the initial position and the last position, and Recording the time elapsed between the time zero "0" and the time accumulated until the last position.