The Scavenger PSA and SA models are designed to operate in automatic cleaning mode only. The units reverse automatically upon reaching a tank wall or every few minutes if there is no such encounter. Its oil-filled, computer controlled drive motor system allows it to operate continuously. The on-board pump motor enables the PSA model to remove solids as large as 1 1/2-in. through its 2-i n.-dia hose. The Scavenger LR and (pump-on-board counterpart) PLR models are designed for automatic as well as radio remote controlled cleaning (here a specific area needs special attention). Unless the remote control buttons are activated (manually) thes e Scavenger models will reverse automatically when a vertical surface is reached. They are 60% larger, 60% faster, and --with one more motor and twice the suction--more powerful than the (P)SA Model. These models also have a timer and periodically execute 90-deg turns.