This is the replacement foam pre-filter, inlet HEPA filter and cabinet filter for the Respironics Millenium oxygen concentrators.
The pre-filter and plastic cartridge HEPA filter are installed inside of the cabinet on the Millenium 5 liter oxygen concentrators. There is typically a foam pre-filter installed on the inlet side of the plastic cartidge for the HEPA filter. Refer to the illustration below for details on how it is installed. If your OPI indicator is alarming, a clogged filter could be the problem.
The inlet filter and the foam pre-filter are accessed by removing the panel cover on the back of the unit.
The inlet filter can be seen here with the foam pre-filter installed onto the top. Both filters can be easily removed by simply lifting them up off of the inlet opening.