and 24 each include a circular base 26 having a plurality of apertures 28 which communicate with the surrounding atmosphere in inlet port 18 base 26 is an integral part of domed shaped lid 14 annular sidewalls 30 form a receptacle which is adapted to receive an improved removable filter unit 32 according to the invention discussed in detail below removable filter unit 32 is held tightly within annular sidewalls 30 by a twistable cover 34 which engages a plurality of locking members 36 disposed around the periphery of the respective port referring next to fig4 an exploded view is shown which demonstrates the cooperation between a selected port 18 22 or 24 with an improved filter unit 32 and twistable cover 34 twistable cover 34 includes four locking flanges 37 which extend from a lip member 38 extending from an outer sidewall 39 locking flanges 37 are positioned to be rotated into locking members 36 formed around the periphery of the respective port 18 22 or 24 a gripping member or handle 40 is provided on twistable cap 34 to facilitate its rotation within the respective annular sidewalls 30 fig5 a c are respective top side and bottom views of an improved filter unit 32 according to the invention filter unit 32 includes.