Infrared light source emits an infrared beam, and the beam is divided into two beams by a rotating chopper, one beam goes through the reference chamber which contains a gas such as nitrogen or argon that does not absorb light at the wavelength used in the instrument, the other beam go through the sample chamber, where target gas molecules will absorb some of the light. As a result, when the light emerges from the end of the sample chamber it has less energy than when it entered. It will also have less energy than the light emerging from the reference cell. The energy difference is detected by a detector. a pulsating flow is generated between the two detectors is converted by the microflow sensor into an electric signal which is dependent on the concentration of the sample gas.