Strange dial on the side of C-Class W203 air cleaner housing

[Orb the Impaler]

Afternoon all.

Yesterday I noticed that there is a little knob with a window showing "75% 100%" on the side of the airbox in Mrs Orb(s) car (2002 W203 C220i Sportcoupe with scabby rust spots).

Inside the airbox it doesn't appear connected to anything (it's marked 50mbar).

What does it do? What is it for? Is it some kind of "change me!" gauge for the air cleaner?

"Curious of Rochdale"

[mercedes tech]

it tells you if the airfilter is blocked,

[Orb the Impaler]

Oh? So if it says 100% (which it does) then I need to get me to GSF.....

[Howitzer]

It measures the induction depression (Amount of suck) between the air filter and engine.

Dave!

[Orb the Impaler]

Well I've just changed the filter (it was minging) and both filters on my SL (they too were minging and full of dead leaves). The SL is definitely better - it's doing mental, snaky 100 yard long wheelspins