Some dive computers are able to calculate decompression schedules for breathing gases other than air, such as nitrox, pure oxygen, trimix or heliox. The more basic nitrox dive computers only support one or two gas mixes for each dive. Others support many different mixes. When multiple gases are supported, there may be an option to set those which will be carried on the dive as active, which sets the computer to calculate the decompression schedule and time to surface based on the assumption that the active gases will be used when they are optimal for decompression. Calculation of tissue gas loads will generally follow the gas actually selected by the diver, unless there is multiple cylinder pressure monitoring to enable automatic gas selection by the computer.