Paramount's DDC canister has a patented water seal lid and does not normally use an O-ring. The canister is plumbed with an equalizer line going into the pool that allows the water to remain in the canister at the same level as the pool. The water then seals the clear lid when the pump is on. This design allows for easy removal of the lid, unlike other types of canisters that have O-ring seals with threaded lids or clamps. In some cases a company may not have plumbed in the equalizer line or a special application such as negative edge basins. This may require an O-ring to obtain the proper seal of the lid. If this is the case then part number 005-152-0120-00 is available.