The air release time is defined as the time in minutes after the air dispersed in oil has been removed again until 0.2% vol. remains. Air at a set pressure level is blown into the oil through a fixed nozzle over a set period of time.
Using the density, the dispersed air bubbles' escape from the oil is then tracked over time and recorded on a graph until the volume stays constant. The air release time is extrapolated from this graph. The method can be performed at four different temperatures (25°C, 50°C, 75°C, 80°C).