This type incorporates a combined tubular body and indicator humidity plug. It is screwed into the equipment container from the outside. Air enters the desiccator through an annular inlet channel, which is covered by a strip of Polytetrafluorethylene (PTFE) coated wire mesh to prevent the ingress of rain, dust, insects, etc.
A tube is connected at one end to the annular channel, and the other end is covered by metal gauze to prevent particles of desiccant blocking it. The direction of airflow depends on the relative pressure differentials between the inside of the equipment and the surrounding atmosphere. If the external pressure is greater than the internal pressure, moist airflows from outside, through the annular channel and breather tube, to the desiccant. Dry air flows from the desiccant through a second tube to the equipment interior. If the pressure differential is reversed, the direction of the airflow is also reversed until pressure equalization is achieved.