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A
Outage is empty space saved to reduce the weight of the cargo
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B
Liquids expand as temperature rises — outage is the empty space left in the tank to accommodate thermal expansion and prevent over-pressurization or structural failure of the tank
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C
Outage is required to allow the driver to monitor liquid levels while driving
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D
Outage prevents ice from forming in cold weather
Why this is the answer
OUTAGE (also called ULLAGE) is the unfilled space left at the top of a liquid cargo tank. PHYSICAL REASON: Most liquids expand significantly when heated. The volume of liquid at its loading temperature may be substantially less than the volume it will occupy at a higher temperature encountered during transport or at the destination. If a tank is filled to 100% capacity at a cool loading temperature, warming during a summer transport can cause the liquid to expand beyond the tank's design volume — leading to: dangerous overpressure; valve or seal failure; spills from pressure relief venting; catastrophic tank rupture in extreme cases. DIFFERENT PRODUCTS, DIFFERENT OUTAGE REQUIREMENTS: Lighter petroleum products (gasoline, naphtha) expand significantly with temperature and require more outage than denser products; cryogenic liquids (liquid nitrogen, LNG) require careful management of boil-off and expansion; water-based products expand less than hydrocarbons; hazardous liquids typically have federally specified outage minimums. WEIGHT LIMIT CONSIDERATION: Outage also helps manage axle weights — a tank filled to maximum volume with a dense chemical may exceed axle weight limits before the tank is visually 'full'; the driver and carrier must calculate by weight, not just volume.
Source: FMCSA CDL Manual, Chapter 10, Tank Vehicles, Outage