Method and apparatus for determining a zero gas flow state in a bidirectional gas flow conduit

Apparatus for determining a state of zero gas flow in a conduit, the zero gas flow state occurring between a first period in which gas flows in one direction in the conduit and a subsequent, second period in which gas flows in the opposite direction in the conduit, said apparatus comprising:

• a first gas flow sensor for obtaining a first signal indicative of the amount of gas flow in the conduit, the first signal having one polarity when the gas flow is in one direction in the conduit and the opposite polarity when the gas flow is in the opposite direction in the conduit;
• a second gas flow sensor for obtaining a second signal indicative of the amount of gas flow in the conduit, said second signal having the same polarity for either direction of gas flow in the conduit;
• means for obtaining the derivatives of the first and second signals at successive points in time and for determining their signs;
• means for determining that the signs of the derivatives of the first and second signals are present in a predetermined combination of signs and thereafter determining a first point in time at which the sign of the derivative of the second signal becomes opposite to its sign in the predetermined combination and the sign of the derivative of the first signal remains the same as its sign in the predetermined combination, said means identifying a second point in time, prior to first point, as a state of zero gas flow in the conduit.