Quoted: When the pressure reaches 15psi it then exceeds the weight of the regulator, causing a loud blast of escaping steam that lifts the weight on the vent tube, producing a sharp whistle. The first whistle takes the longest, about 7 minutes, and indicates the cooker is fully pressurized, after which the heat is reduced, and the time is counted beginning with the next whistle. These cookers operate with oscillating pressures, cycling through building pressure and then releasing it with a whistling sound of escaping steam. Cooks in India rely on the whistle noise as a handy built-in timer. Indian pressure cookers are somewhat unique in using amplitude and frequency of pressure, rather than the more familiar constant pressure Westerners find in our pressure cookers.