Pneumatic seal keeps ground water from flowing into the open manholes. Just inflate tube with 5-10 lbs. of air or nitrogen to form a water-tight seal. No clamps or sealing compounds required. Series 2730 (reversible 27″ & 30″) fits most communications manhole openings. Model 3336 (33″ & 36″) fits many Electric Utility manholes. Custom sizes are available. All aluminum fabrication.