to the same assignee as the present invention and incorporated herein by reference preferably the infrared emitter 300 is either a solid state or a black body radiator with an appropriate filter if required the infrared emitter 300 irradiates to the infrared detector 302 through a cross section of sampled vapor running through the vapor return line 34 the infrared detector 302 is either solid state pyro electric infrared pir or thermopile the attenuation in the infrared spectrum 306 caused by the absorption of infrared by hydrocarbons is detected by the detector 302 a signal representing the attenuation is sent to the control system 50 to determine the hydrocarbon concentration of the vapor 310 returning through the vapor return line 34 the infrared emitter 300 contains a window 304 through which the infrared spectrum 306 emitted by the infrared emitter 300 passes the primary purpose of the window 308 is to provide a barrier to prevent the infrared emitter 300 from being contaminated by the vapor when emitting a signal representing such attenuation to the control system 50 in order for the infrared spectrum 306 to pass through for detection by the infrared detector 302 the window 308 allows light of the infrared spectrum 306 to pass through the wavelength of the infrared spectrum 306 wavelengths is approximately 4 micro meters and the hydrocarbon vapor is sensed at approximately 3 3 to.