Inversion in D.C. We were contacted by a Washington D.C. contractor regarding the Perma-Liner air inversion unit. The air inversion unit does all of the work! The certified installer calibrates the correct inversion pressure while the liner inverts itself through the existing host pipe. The certified installer controls the air inversion speed to ensure proper installation. Please visit www.youtube.com/PermaLiner to see live installation videos. If you are in the Washington D.C. area and are interested in the Perma-Lateral air inversion unit, please call us at anytime.