A catheter securement device holds a medical article such as a catheter hub or a connector fitting in position upon the body of a patient and at least inhibits longitudinal movement of the medical article. The assembly aid has a retainer with recesses or apertures for holding keys. According to the corresponding method, due to the directional element, which is cylindrically inclined downwards from the outer edge the flotation mechanism directs the gas-slurry suspension that is formed in a downward slanting direction towards the side wall of the cell. An achromatic lens is disposed within the housing between the display panel and the objective lens.