Air cones come in a set with one round cone for vents smaller than 7 1⁄2; diameter, and one square cone for vents smaller than 12 1⁄2; x 12 1⁄2;. When attached to the cone the meter automatically calculates air volume without the need to input vent size. The optional extension rod can be used with the meter alone, or the cone, eliminating the need for a ladder. For use with: Bluetooth IAQ Anemometer #900030