The AV900 despite its age has 'fresh gas compensation' . The bellows movement will be automatically increased or decreased with fresh gas flow from the flow meters.
High fresh gas flow = smaller bellows movement.
Low fresh gas flow = larger bellows movement.
In compensating ventilators the numbers on a bellows canister are only an indication of output accuracy when compensation is switched off.
The only true to assess the output accuracy of a ventilator is to use use an external flow measuring device.