We do switch cylinders, Neil, alternating between the front and back after each refill; we also switch cylinders if we are pushing into the red zone deeply [mainly to avoid the fridge alarm going off in the middle of the night!]. I usually refill with 19-22kg of gas, so, even though the gauge says empty, there's around 2kg of gas left in the 'empty' bottle, which is consistent with the chart that Charlie at Autogas provided when he fitted the system
Steve