The G-99 Air Filter Gage provides a visual indication of the need to replace the air filters in forced heating and cooling systems.
The gage is installed between the blower and the filter where a slight vacuum exists due to the air flow resistance of the air filter.
Air flowing into the gage, around the calibration screw, lifts a vane in proportion to the negative pressure in the blower compartment.
As the air filter loads, the vacuum increases, raising the vane to indicate a filter change is necessary.
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