Thermo Environmental’s Model 10S analyzers are based on the chemiluminescent principle of detection. Their key features include selective detection of NO or NO_x (by  front panel mode switch), continuous monitoring with rapid response .The Thermo 10S analyzer is identical to the 10A, but has a thermo electrically cooled photomultiplier tube for  stability  . This is an old faithful work horse. We serviced one manufactured on March 4th, 1972 and it’s still working great.