GAS CONCENTRATION MEASURING METHOD

PURPOSE:To measure even low concentration gas highly accurately by finding a linear equation of an operation value to an atmospheric pressure from two measurement values of gas concentration under plural different atmospheric pressures of not less than one atmospheric pressure. CONSTITUTION:Measurement gas is introduced into/substituted in a gas cell, and a discharge valve is closed, and a gas pressure is raised. When the pressure reaches a certain pressure A not less than one atmospheric pressure, the introduction is stopped, and outputs from an infrared sensor 5 are sampled, and an average value X is stored in a memory 21. In succession, the pressure is lowered to become pressures B and C not less than one atmospheric pressure, and the outputs of the sensor 5 are sampled respectively, and average values Y and Z are stored in memories 22 and 23. Last, the gas in the gas cell is substituted with air, and the output average value N is stored in a memory 24. Operations 27 are carried out respectively on values of X/N, Y/N and Z/N, and these values are stored in memories 29-31. Operations 32 are carried out on the two values X/N and Y/N among these values, and a specific linear equation is found. When an operation result obtained by substituting a pressure C in this equation coincides with the operation value Z/N, this equation, that is, concentration measurement by the sensor 5 is correct, so that an operation unit 35 can carry out an operation on concentration.