Time (HOT) is the length of time an aircraft can wait after being treated prior to takeoff. Holdover time is influenced by the fluid dilution, ambient temperature, wind, precipitation, humidity, aircraft skin material, aircraft skin temperature, and other factors. If the Holdover Time is exceeded the aircraft must be re-treated before takeoff. Lowest Operational Use Temperature (LOUT) is the lowest temperature at which a de/anti-icing fluid will adequately flow off aircraft critical surfaces and maintain the required anti-icing freezing point buffer which is 7 °C (13 °F) below outside air temperature (OAT). In the United States, the Federal Aviation Administration (FAA) publishes official Holdover Time and Lowest Operational Use Temperature tables for all approved deicing fluids, and revises them annually. For Type I fluids, the Holdover Time listed in the FAA tables ranges from 1 to 22 minutes, depending on the above-mentioned situational factors. For Type IV fluids the Holdover Time ranges from 9 minutes to 160 minutes. Dilution Deicing fluids work best when they are diluted with water. For example, undiluted Dow UCAR Deicing Fluid (Type I ethylene glycol), has a freezing point of −28 °C. Water freezes at 0 °C. However, a mixture of 70 percent deicing fluid and 30 percent water freezes below −55 °C. This is known as the eutectic concentration, where the freezing point of the mixture is at its lowest point, and lower than either of the component substances. Depending on the manufacturer, deicing fluids may be sold in concentrated or pre-diluted formulations. Dilution, where necessary, must be done according to ambient weather condition and the manufacturer's instructions in order to minimize costs while maintaining safety. The dilution of a particular sample of fluid (and hence its freezing point) can be easily confirmed by measuring its refractive index with a refractometer, and looking up the result in the deicing fluid manufacturer's tables. Layer Thickness Thickened fluids (types II, III, and IV) are intended to remain on surfaces after application in order to provide anti-icing protection. They are also designed to slough off during the takeoff run so that they do not negatively affect flight performance. In order to obtain these objectives they must be applied at the correct thickness. For a typical type IV fluid, a layer thickness of between 1 mm and 3 mm is required, however each manufacturer will document their own requirements. Standards compliance Manufacturers of aviation deicing fluids must certify