Measurement of gas concentrations

Oxygen can be measured by the paramagnetic technique or the fuel cell (galvanic cell). Oxygen is paramagnetic because it has unpaired electrons in its outer shell, this causes it to be attracted into a magnetic field. Several devices have been manufactured in such a way that an electric current produced by this deflection is proportional to the concentration of oxygen measured. The fuel cell is in effect a battery that requires oxygen for a current to flow. Oxygen reacts with electrons at a gold cathode, while the electrons are produced at a lead anode. Carbon dioxide, nitrous oxide and volatile agents can be measured in a variety of ways including infrared spectroscopy, photoacoustic spectroscopy, interferometry and by using the piezoelectric effect and Raman scattering. Infrared absorption is the most common. Infrared is absorbed by molecules with more than two different atoms (hence oxygen cannot be measured with infrared). Each molecule has a unique fingerprint pattern of infrared absorption that follows the Beer-Lambert law. Infrared is shone at a gas chamber of known length containing a sample gas. The absorption of infrared is then proportional to the concentration of the gas. Several different gases can be measured simultaneously by having a rotating filter allowing peak wavelengths of absorption for the gases to be measured. Sampling of all gases can be either side stream or main stream. Side stream is a lighter weight, but has a lag time (the time from sampling to detection) and requires about 150 ml/min of fresh gas to be sampled (this can be returned if a circle system is being used).