City of San Diego Water Department - Landscape Watering Calculator Impact Rotors Impact rotors use the force of the water hitting a spring loaded arm to turn the sprinker.  The mist and overspray caused by the impact arm wastes water and is a disadvantage of this type of sprinkler.  The rotors are made of plastic or brass and cover areas from 15 to 65 feet wide, depending on the water pressure and size of the nozzle.  Average operating pressures range from 35 to 85 p.s.i. and impact systems can be up to 75% uniform.