Patent ID: 7234450

Claim:
A gas density ratio detecting apparatus comprising: a measure-passage which plural kinds of gases flow into at separate timings from an upstream end thereof; a gas flow producing means connected with a downstream end of the measure-passage to decompress the measure-passage; an orifice provided in the measure-passage to restrict a flow passage area of the measure-passage; a separation-restricting means for restricting a separation of gases from an inner surface of the measure-passage downstream of the orifice, the separation-restricting means being structured in such a manner that the orifice has a diameter-changing portion having an inner diameter that decreases from a downstream end toward upstream to an intermediate portion and an upstream portion having a constant inner diameter from the intermediate portion to an upstream end; a measuring means for measuring a pressure determined by the orifice and the gas flow producing means or for measuring a flowrate of gas flowing through the orifice with the measure-passage decompressed; and a density ratio calculating means for calculating a density ratio between gases based on the pressure or the flowrate of gas measured by the measuring means with respect to plural kinds of gases.