ABSTRACT
An elegant solution to the measurement of a mixture of gases and vapours in a single chamber is that applied in the Bruel and Kjaer monitor.
A wide-band infrared source is pulsed by the interposition of a rotating disc containing apertures in the form of slits. Three distinct pulsed frequencies in the sonic range are achieved by having three concentric rings of slits, with each ring containing a different number of apertures. Each of the three resulting pulsed wide-band infrared emissions are transmitted though a separate, narrow-band, optical filter at a wavelength specific to the principal absorption bands of CO2, N2O, and anaesthetic agents. The pulsed infrared light causes expansion and contraction by intermittent heating and cooling of the different component gases and vapours. As the three distinct infrared sources are pulsed at different frequencies, vibration frequency now becomes the discriminator between CO2, N2O, and anaesthetic agents. The amplitude of vibration at a particular frequency will vary with the concentration of the vapour. The principle is shown in Figure 39.13. O2 is not able to absorb infrared light, but as a result of its paramagnetic properties it is made to vibrate at its own identifiable frequency by a varying electromagnetic field. This principle of measurement is described later.
