User's Manual Part 3

10-16
JS
E.2. Theory and design, 3886 Multi-Gas System
E.2.1. Gas measurements and identification
The measurement of CO
2
,
N
2
O and anesthetic agents is based on the fact that different
gases
absorb infrared light at specific wavelengths. The analysis of respiratory gases by
the ISA gas analyzers are therefore performed by continuously measuring the infrared
light absorption in the gas flow through an infrared spectrometer. Oxygen, on the other
hand, does not absorb infrared light to the same extent as other breathing gases and is
therefore measured using alternative methods.
The gas analysis
The heart of the ISA gas analyzer is the multi-channel spectrometer which uses a
proprietary broadband infrared radiation source to transmit light through the gas sample.
Before reaching the gas sample, the light path is intersected by narrowband optical filters
that only allow light corresponding to selected wavelength peaks of the measured gases
to pass. At the other end of the light path, a sensor detects the portion of the light that is
not absorbed by the gas. The amplitude of the detector output is an inverse function of
the gas concentration.
Thus, at a concentration of zero, the amplitude is at its maximum.
If the gas sample is a mixture of several components that absorb light at the same
wavelength, such as a mixture of two anesthetic agents, the absorbed radiation will be
the sum of the absorption of the agents. To determine the concentration of each of the
individual gases, several filters have to be used. The ISA gas analyzers therefore uses
the SIGMA spectrometer, which contains up to nine different narrowband filters to
facilitate simultaneous measurement of CO
2
,
N
2
O and a mixture of any two of the five
anesthetic agents.
Figure 4-1. Gas absorption spectra.
The selection of the optical filters within the spectrometer is crucial to the characteristics
and performance of the gas analyzers. The ISA spectrometer uses the strong absorption
peaks at
and 4.5 µm for CO
2
and N
2
O measurements and five wavelengths in the 8 to
10 µm long
wave infrared range (LWIR) for the anesthetic agent calculations. The LWIR
contains strong absorption peaks for the anesthetic agents and negligible interference
from other common respiratory gases, such as alcohol and acetone, which could degrade