Air filter box % dial

[avantdi]

Hi all
What is this dial for?. It seems to have an adjustable knob on it.

W210 E220CDI 02 reg but manufactured in late 01 for 02 delivery

 

[panason1c]

This dial displays in percentage terms the amount of restriction of air flow through the air filter due to dust contamination..........the knob is for resetting it to zero after fitting a new filter.

 

[avantdi]

Thanks for that Panason1c