The J.U.M. Engineering Model 109A uses two individual Heated FID detectors and two individual signal amplifiers with Zero and Spane manual adjustments. A sample is introduced into one FID for a THC reading. The same sample is also run through a heated non-methane cutter which eliminates all hydrocarbons except methane, which is run through FID #2.  The analyzer subtracts the two numbers to give you a THC less methane reading. JUM has internal activated charcoal filter to create combustion air. If high levels of methane background  are suspected, a Zero air cylinder is recommended.