Measurement of gaseous/volatile hydrocarbon concentrations

A method of measuring concentrations of hydrocarbons in a gas mixture, comprising:

• for one or more individual hydrocarbons, which constitute a subset of the gas mixture, sending infrared radiation from an infrared source through a sample of the gas mixture to an infrared detector and operating the infrared detector to measure absorbance of infrared radiation at a predetermined wavelength chosen to be associated with the individual hydrocarbon and at which the individual hydrocarbon has an absorption coefficient which is at least five times greater than the absorption coefficient of any other gas in the mixture at the same predetermined wavelength;
• for at least one other hydrocarbon or group of hydrocarbons which are part of the gas mixture but outside the said subset, sending infrared radiation from an infrared source through the sample of the gas mixture to an infrared detector and operating the detector to measure absorbance of infrared radiation at a predetermined wavelength chosen to be associated with the said other hydrocarbon or group of hydrocarbons and at which the said other hydrocarbon or group of hydrocarbons outside the subset and also at least one hydrocarbon within the subset absorb infrared radiation;
• operating a computer processor to
• receive the measurements of absorbance,
• calculate the concentrations of each hydrocarbon within the subset from the measured absorbances at the predetermined wavelengths associated with the individual hydrocarbons within the subset,
• calculate a predicted absorbance by each hydrocarbon within the subset at the wavelength associated with the said other hydrocarbon or group of hydrocarbons outside the subset,
• subtract the predicted absorbances from the measured absorbance at the said wavelength associated with the said other hydrocarbon or group of hydrocarbons outside the subset and thereby calculate absorbance by said other hydrocarbon or group of hydrocarbons outside the subset, and
• use the calculated absorbance by said other hydrocarbon or group of hydrocarbons outside the subset to calculate the concentration of the said other hydrocarbon or group of hydrocarbons in the sample.