A check is done prior to each flight over the sector. The total Pax and F/D Oxygen pressure, which are contained in two separate bottle systems, has the pressure recorded and then corrected for local OAT temperature. This corrected pressure is then compared with the amount needed for
flight over the Y1 Route depending on the passenger cabin payload and flight deck complement. There are 304 seats plus up to 11 cabin crew (normal complement of 10) and the infants equates to depending on the number of passengers on the day to x amount of pressure required. Similarly for the F/D there needs to be y amount of pressure depending on whether there are 2, 3 or 4 occupants on the flight deck. If the pressure is below that required there are two choices, either a top up or alternatively it is the long way around.