What does 'outage' mean in tank loading, and why does it matter?

Outage is the empty space left in a tank when loading — typically about 2% of tank volume for most liquids. The space allows the liquid to expand without pressurizing the tank when temperature rises during transport. Liquids expand approximately 1% in volume for every 10°F (about 6°C) of temperature increase, so a tank filled in cool morning air and warmed in afternoon sun can build dangerous pressure if no outage was left. Different products have different expansion coefficients and therefore different required outages: gasoline and propane more, water less. Overloading without outage can rupture the tank or force liquid through pressure relief valves. The required outage is set by regulation and by the shipper's specifications.