A method for checking a connection between a compressed air outlet of a blood treatment apparatus and a pressure measuring line of an extracorporeal blood tubing set, wherein the method comprises the steps: - providing a blood treatment apparatus having: - a compressed air line; - a compressor in fluid communication with the compressed air line for generating pressure and an air flow within the compressed air line; - a compressed air outlet in fluid communication with both the compressed air line and an exterior of the blood treatment apparatus, wherein the compressed air outlet is designed to be connectable with a pressure measuring line; and - a pressure sensor which is arranged to measure the pressure prevailing at the compressed air outlet or in the compressed air line; - setting up a pressure and an air flow in the compressed air line and/or at the compressed air outlet, by means of the compressor; - measuring a pressure prevailing at the compressed air outlet or in the compressed air line or measuring a pressure change over time by means of the pressure sensor; and - evaluating an increase of the measured pressure or an increase of the measured pressure over time, each time by means of comparing the measured pressure or a measured pressure increase with a threshold value (P-V), wherein the compressor is already switched off as soon as the measured pressure has reached the threshold value (P-V), wherein the pressure measuring line is open at one end not connected with the compressed air outlet or leads into a volume of a bubble trap or a venous blood chamber, wherein the method is completed before the start of a treatment of a patient for which the extracorporeal blood tubing set is intended to be used. - providing a blood treatment apparatus having: - a compressed air line; - a compressor in fluid communication with the compressed air line for generating pressure and an air flow within the compressed air line; - a compressed air outlet in fluid communication with both the compressed air line and an exterior of the blood treatment apparatus, wherein the compressed air outlet is designed to be connectable with a pressure measuring line; and - a pressure sensor which is arranged to measure the pressure prevailing at the compressed air outlet or in the compressed air line; - a compressed air line; - a compressor in fluid communication with the compressed air line for generating pressure and an air flow within the compressed air line; - a compressed air outlet in fluid communication with both the compressed air line and an exterior of the blood treatment apparatus, wherein the compressed air outlet is designed to be connectable with a pressure measuring line; and - a pressure sensor which is arranged to measure the pressure prevailing at the compressed air outlet or in the compressed air line; - setting up a pressure and an air flow in the compressed air line and/or at the compressed air outlet, by means of the compressor; - measuring a pressure prevailing at the compressed air outlet or in the compressed air line or measuring a pressure change over time by means of the pressure sensor; and - evaluating an increase of the measured pressure or an increase of the measured pressure over time, each time by means of comparing the measured pressure or a measured pressure increase with a threshold value (P-V),