A dishwasher is a mechanical device for cleaning dishes and eating utensils. Dishwashers can be found in restaurants and private homes. Unlike manual dishwashing, which relies largely on physical scrubbing to remove soiling, the mechanical dishwasher cleans by spraying hot water, typically between at the dishes, with lower temperatures used for delicate items. A mix of water and detergent is circulated by a pump. Water is pumped to one or more rotating spray arms, which blast the dishes with the cleaning mixture. Once the wash is finished, the water is drained, more hot water is pumped in and a rinse cycle begins. After the rinse cycle finishes and the water is drained, a heating element in the bottom of the tub heats the air to dry the dishes. Sometimes a rinse aid is used to eliminate water spots for streak-free dishes. Mechanical dishwashing devices were developed in the mid- to late-1800s. A domestic dishwasher was invented in the 1920s, but it did not see widespread use. Dishwashers became more common in wealthy homes in the 1950s. By the 1970s dishwashers had become common. Present-day machines feature a drop-down front panel door, allowing access to the interior. The inside of a dishwasher in the North American market are either stainless steel or plastic. Mid-to-higher end North American dishwashers often come with hard food disposal units, which behave like miniature garbage (waste) disposal units that eliminate large pieces of food waste from the wash water. Many new dishwashers feature microprocessor-controlled, sensor-assisted wash cycles that adjust the wash duration to the quantity of dirty dishes. Large heavy-duty dishwashers are available for use in commercial establishments (e.g. hotels, restaurants) where a large number of dishes must be cleaned. Washing is conducted with 65-71 C / 150-160 F temperatures and sanitation is achieved by either the use of a booster heater that will provide the machine 82 C / 180 F “final rinse” temperature or through the use of a chemical sanitizer.