Infrared radiation is emitted at the same intensity by two infrared radiation sources. The radiation from one source passes through a sample cell containing ambient air. Radiation from the other source passes through a reference cell con gen. After passing through either the taining a nonabsorbing gas such as detector cell con sample cell or the reference cell. the infrared radiation enters a metal is taining carbon monoxide which is divided into two sections by a thin that diaphragm. Any distention of the diaphragm produces a capacitance change such voltage output. The voltage output is used to activate a device. as a strip chart recorder, that displays carbon monoxide concentrations in parts per million. amount of infrared If the sample air doesn't contain carbon monoxide, the same adiation enters the sample section of the detector as enters the reference section Therefore, the same amount of radiation is absorbed by the carbon monoxide in the sample section of the detector as is absorbed in the reference section. Further more. pressure resulting from the absorption of radiation is equal in both sections the detector. The diaphragm which separates the detector section doesn't move because equal pressure is being exerted on both sides of it. Hence. no voltage is created to activate the analyzer's strip recorder. so a zero carbon monoxide chart concentration reading is displayed If the sample air contains carbon monoxide. less infrared radiation enters the because some of sample section of the detector than enters the reference section he radiation is absorbed the carbon monoxide in the sample cell. Therefore by les is produced in the detector's sample section than in the reference sec pressure the diaphragm by the sample tion. This causes less pressure to be exerted a section than by the reference section. thus creating a distention of the diaphragm The distention. which is directly proportional to the concentration of carbon hich is used to indicate the monoxide in the sample cell, creates a voltage output carbon monoxide concentration in parts per million on a strip chart recorder. The beam chopper simply creates an alternating current (AC) signal that can be easily amplified. This technique, while seemingly specific for co, suffers from interferences due to the broad band absorption of carbon dioxide, vapor. and hydrocarbons water (HC). Figure 8-10 shows the absorption of carbon monoxide and the broad band interferences