From a teacher we got a tip for checking the accuracy of the spirometer. The calibration can be controlled with two drain tubes of known content, pressed into each other and closed off. One tube section is filled with water, the other remains empty. Connect the spirometer to the empty tube at the closed side (spirometer 0378i or BT82i). Start a measurement and slide the empty tube into the water-filled tube section. The trapped air escapes through the spirometer. Because the volume of this air is known, the calibration of the spirometer can be established. Make sure that no water gets into the spirometer.