Measurement of Performance
Deicing fluid performance is measured by holdover time (HOT), which is the length of time an aircraft can wait after being treated prior to takeoff. Holdover time is influenced by the ambient temperature, wind, precipitation, humidity, aircraft skin temperature, and other factors. For Type I fluids, the holdover time is only about five to 15 minutes, so the aircraft must take off immediately or else wait to be deiced again. Type IV fluids generally provide a holdover time between 30 and 80 minutes.
The FAA publishes official Holdover Time Tables for all approved deicing fluids, and revises them annually.
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, of course, freezes at 0 °C. However, a mixture of 70% deicing fluid and 30% 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.
Read more about this topic: Deicing Fluid
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