The G-99 Air Filter Gauge provides a visual indication of the need to replace the air filters in forced air heating and cooling systems. The gauge 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 gauge, 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.

The gauge may be calibrated in a negative pressure range from 0.1 to 0.4 inch w.c. When properly calibrated in this range a vacuum increase of 0.10 to 0.15 inch w.c. will indicate a filter change is necessary.