Abstract: A ventilator is disclosed having a power unit for driving the ventilator which includes fluidic timing control means, gas input means and gas output means. The gas input means is connected to the timing control means via pressure regulating means. Display means are provided to the timing control means. The timing control means is connected to the output means and is alternately conductive and non-conductive of gas from the input means to the output means. The ratio of the periods of conductivity to non-conductivity remain fixed even where the frequency of alternation varies to maintain a fixed ratio of the inspiratory to expiratory phase time.