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 (P) 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 (P) 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 (P) or an increase of the measured pressure over time, each time by means of comparing the measured pressure (P) or a measured pressure increase with a threshold value (P-V), wherein the compressor is already switched off as soon as the measured pressure (P) 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 (P) 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 (P) 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 (P) 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 (P) or an increase of the measured pressure over time, each time by means of comparing the measured pressure (P) or a measured pressure increase with a threshold value (P-V), <EOS>