This head cover is designed to fit over the cell side when transporting the sensor head separate from the unit, protecting the cell membranes.
It is also useful when calibrating the cells with the head outside of the unit - put the cover on and feed oxygen into the loop orifice until the sensor reading stabilises.
If you want a cover that allows air flow through the inside of the rebreather head, allowing it to dry out, you should go for
this design.