Whats the little dial on the inlet manifold?

[G-A-R-Y]

I have been cleaning my new car (engine bay) and found a little dial on the right hand side, I think its something to do with the inlet manifold. It was on 70% and showing red but I twiddled it and it's gone back to 0.
Is it similar to a device on my tractors that tell you if the air cleaner is blocked.

W210 320 CDI

 

[Dieselman]

Yes.

 

[AMGBoy]

yes tells you when the air filter is blocked and needs replaceing

 

[G-A-R-Y]

I turned it and it's gone back to 0.
Have I broke it or will it go back up on it's own.
Was 70% OK?

 

[C43AMG]

Make a note. You must not twiddle with things you don't understand.

 

[jaymanek]

I think you will find that as you drive it it will go back to what it was....

 

[Carrotchomper]

How does that dial work then?!

 

[Dieselman]

It measures the vacuum on the engine side of the air filter. As the filter blocks the vacuum rises, so a high number is bad..