A disposable cassette configured for attaching to a fluid supply pump, the cassette comprising: a pumping chamber having a wall, a fluid inlet and a fluid outlet and configured to receive a fluid from a fluid storage unit via the fluid inlet, and a diaphragm structure coupled to the pumping chamber, the diaphragm structure comprising a movable element configured to move with changes in fluid pressure within the pumping chamber and thereby cause a change in a sensed measurement variable sensed by at least one sensing arrangement (103A, 104A) coupled to the fluid supply pump without contacting the sensing arrangement (103A, 104A), the amount of movement of the movable element being related to the amount of change in fluid pressure; wherein the diaphragm structure comprises a deformable element connected between a perimeter of the movable element and a side wall; the deformable element configured to deform in response to the changes in the fluid pressure within the pumping chamber; the cassette being characterized in that it comprises: a coupler connecting the diaphragm structure to the pumping chamber; wherein the diaphragm structure comprises a cavity disposed between the pumping chamber and the movable element, the coupler having an outer wall inserted in an aperture in the wall of the pumping chamber, said outer wall of the coupler having openings for establishing a fluidic connection between the cavity and the pumping chamber such that the cavity receives a portion of the fluid introduced into the pumping chamber through the openings. a pumping chamber having a wall, a fluid inlet and a fluid outlet and configured to receive a fluid from a fluid storage unit via the fluid inlet, and a diaphragm structure coupled to the pumping chamber, the diaphragm structure comprising a movable element configured to move with changes in fluid pressure within the pumping chamber and thereby cause a change in a sensed measurement variable sensed by at least one sensing arrangement (103A, 104A) coupled to the fluid supply pump without contacting the sensing arrangement (103A, 104A), the amount of movement of the movable element being related to the amount of change in fluid pressure; wherein the diaphragm structure comprises a deformable element connected between a perimeter of the movable element and a side wall; the deformable element configured to deform in response to the changes in the fluid pressure within the pumping chamber; the cassette being characterized in that it comprises: a coupler connecting the diaphragm structure to the pumping chamber; wherein the diaphragm structure comprises a cavity disposed between the pumping chamber and the movable element, the coupler having an outer wall inserted in an aperture in the wall of the pumping chamber, said outer wall of the coupler having openings for establishing a fluidic connection between the cavity and the pumping chamber such that the cavity receives a portion of the fluid introduced into the pumping chamber through the openings. a coupler connecting the diaphragm structure to the pumping chamber; wherein the diaphragm structure comprises a cavity disposed between the pumping chamber and the movable element, the coupler having an outer wall inserted in an aperture in the wall of the pumping chamber, said outer wall of the coupler having openings for establishing a fluidic connection between the cavity and the pumping chamber such that the cavity receives a portion of the fluid introduced into the pumping chamber through the openings.