The HME works best when it is as close as possible to the mask.  To work efficiently, the HME must be close to the nose/mouth area of your face.  The HME will not work well on a mask that has a tube that extends out before connecting to the main tube. The HME needs to be attached to the swivel fitting right at the mask.  The diameter of the HME is 15 mm on one end and 19 mm on the other end.   

The HME has 2 different diameters, the large end connects to your mask and the narrow end connects to the tube.

Click on the attachment below to view assembly.