MEASUREMENTS OF GAS EXCHANGE 



141 



As the CO2 absorbs infrared energy it tends to expand, exerting pres- 

 sure on the diaphragm (D). With no CO2 in the sample tube, the instru- 

 ment is adjusted to indicate equal pressures on the two sides of the 

 diaphragm. Thereafter, any CO2 in the sample tube will reduce the 

 amount of energy reaching the lower detector chamber, and the result- 

 ing pressure differential moves the diaphragm. The diaphragm and the 

 stationary electrode (E) form a condenser whose capacitance changes as 

 the distance between the plates changes. A radio frequency voltage is 

 applied across this capacitor, and a very small movement of the dia- 

 phragm produces a usable signal which is amplified and employed to 

 drive a recorder. 



i 



Reference tube 



W 



Sample tube 



W 



^ 



Detector 



^^ 



Fig. 10-4. Simplified diagram of the detection system in the Beckman 

 InfraRed Gas Analyzer. (Courtesy Beckman Instruments, Inc.) 



The fact that the gas to be analyzed is used in the detector makes 

 the instrument absolutely specific for this gas. Of course, as the reduc- 

 tion of energy in the sample tube depends upon the number of molecules 

 of gas in the light path, the instrument is sensitive to pressure and tem- 

 perature changes. Temperature control is provided in the instrument, 

 and it is not too difficult to prevent pressure surges in the external gas 

 stream. 



This gas analyzer responds extremely rapidly to very small variations 

 in CO2. The instrument in our laboratories has been used to monitor 

 the air in a room by pumping room air through the sample tube. The 

 analyzer will detect one more person walking into the room in less than 

 a minute. 



Analyzer tubes of several different lengths are available to permit the 

 use of the instrument for several different ranges of gas concentration. 

 If the detector chambers are charged with water vapor, we have a useful 

 indicator of humidity. Various industries have similarly used the infrared 

 analyzer to detect methane, ethane, and other gases. 



