INFRARED CARBON DIOXIDE ANALYSER

Typical Number in Hospital: 1

Cost Bands: 4

References: 9

Gases may be identified by the extent to which they absorb infrared light. Instruments are available which utilize this property to estimate carbon dioxide content in breathing gases, but other gases may also be identified. This is a type of absorption spectrometer.

Infrared radiation is passed through two parallel chambers to a detection device (usually a Golay or Luft cell) which records the difference in infrared absorption in the two chambers. A chopping disc causes the infrared light to pass through each chamber in turn causing alternating displacement of a diaphragm in the Golay cell. The diaphragm will be displaced to one side or other depending on the relative quantities of infrared light reaching the absorption chambers. An electronic circuit used to amplify, demodulate and linearize the output is connected to the capacitance displacement transducer attached to the diaphragm.

One of the chambers is filled with a reference gas while the other is filled with a background gas plus a small flow-through of the gas being sampled. The modification in absorption is used to detect the molar fraction of the sample gases. Problems arise where the absorption spectra of the gases being tested overlap (e.g. nitrous oxide and halothane, carbon dioxide and carbon monoxide). The device requires a long stabilization period and the results will be affected by changes in atmospheric pressure.

Such instruments can be used for the analysis of gases in the lung function laboratory, and in the metabolic computer used in the intensive care department.