The Filter Minder® filter monitoring device makes it easy for you to tell when it's time to change filters. 
Air filter restriction is 
The Filter Minder® can be found on a wide variety of
Air Filter Service Indicators - View the Filter Minder® Demo.
The Filter Minder® is installed on the air cleaner or air induction piping so that it has access to clean filtered air (between air filter and engine).
The air filter service indicator is a simple device that accurately measures the air induction system filter restriction. It measures and remembers the highest restriction at maximum air flow, which is called full load condition. This condition occurs during hard acceleration, or while pulling a trailer, etc.
The indicator is easy to read. As the air filter plugs, and restriction increases, the yellow position indicator moves in the clear window and locks at several points. When it reaches and locks in the red zone, the air filter should be changed and the indicator reset to zero. Reset the indicator by pushing the yellow reset at the end of the indicator.
Another type of air filter service indicator is the Filter Minder® Single Position Service Indicator. The clear window turns red when maximum recommended restriction has been reached.
Every air induction filter system has a clean filter or initial restriction reading. Depending on air flow volume, filter media, and air filter size, there may not be enough restriction to show a reading on the gauge with a new filter, or the position indicator may move one or two positions. Do not be alarmed if there is some restriction with a new filter as this is not unusual.
Air filters usually show low restriction through most of their service life. As the air filter nears the end of available service life, filter restriction will increase more rapidly, and the yellow position indicator on the Filter Minder® will move more rapidly towards the red zone. High humidity or rainy conditions may slightly increase air filter restriction.
The Filter Minder® is calibrated in inches of water vacuum per the engine manufacturer's recommendations.
Use of the Filter Minder® helps maximize filter service life, prevents excess fuel usage from overly restricted filters, and saves on operational costs.