Method and apparatus for the control and independent monitoring of an extremely small gas flow

An apparatus for independent monitoring of gas flow of a further gas delivered together with a respiratory gas to a respiratory path of a human or animal, said gas flow of said further gas being smaller than a gas flow of said respiratory gas, comprising:

• a first flow meter means for measuring gas flow of the respiratory gas;
• an additional flow meter means for measuring gas flow of the further gas;
• a monitor means for comparing an output signal of said first flow meter means indicative of gas flow of the respiratory gas to an output signal from said additional flow meter means indicative of the measured gas flow of the respiratory gas, and for generating an error signal in response to said comparison;
• a first controllable valve means associated with said first flow meter means for controlling gas flow of said respiratory gas, said first flow meter means output signal corresponding to actual quantity of respiratory gas flow;
• a second controllable valve means for controlling delivery of said further gas to said respiratory path, said second controllable valve means having a second flow meter means for measuring the gas flow of said further gas;
• said additional flow meter means being arranged between an output of said second valve means and a location at which the respective gas flows through the first and second valve means are united;
• and
• said monitoring means triggering an alarm given a deviation of the output signals of said additional flow meter means and said first flow meter means from one another beyond a prescribable value.