When comparing FTA vs IR for process applications, its important to remember that few analyzers react the same way to all substances.
An infrared sensor is a narrow-band instrument. It can usually discriminate between the substance of interest and background gases but it does not respond to gases outside of it's narrow range of vision.
These variations in infrared sensor response may not pose a significant problem when measuring an atmosphere containing a single substance. But when asked to measure a mixture of different vapors, infrared instruments will usually fail miserably.
Flame temperature analyzers will react accurately to most flammable substances and will usually measure both single substances and mixtures with the same high degree of accuracy. Unlike some sensors, flame temperature analyzers were developed for one specific purpose—to directly measure flammability.