Honeywell ACM 150, Manual To Installation, Operation, And Maintenance

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ACM 150
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ACM 150 operating principle
Basic operation
The ACM 150 monitor can provide qualitative identification and quantitative analysis
of the chemicals in their gaseous state. Each time the ACM 150 monitor scans an
air sample, it collects multiple spectra of that sample, typically 8 or more. The FTIR
spectrometer takes about 1.2 seconds to collect a single scan. The spectra collected
for each air sample are averaged, and the resulting spectrum is what the ACM 150
monitor uses to analyze the air sample.
Except for the few chemicals that do not absorb infrared radiation, the vapor phase
of every chemical has a unique absorbance spectrum. An absorbance spectrum is
a plot of absorbance on the y-axis and wavenumber on the x-axis. Wavenumbers
are expressed as cm
-1
because the wavenumber is the inverse of wavelength in
centimeters. These spectra cover what is known as the mid-infrared or fingerprint
region of the infrared spectrum.
The air sample spectra collected by the ACM 150 monitor contains all of the
identifying absorption peaks of the gases that make up that air sample. However, the
ACM 150 monitor will only report the gases that are set up as active for that sample
area.
Wavenumber precision
The wavelength location of any chemical’s absorbance peaks is fundamental and
never changes, being fixed by the quantum mechanics of the molecule. The FTIR
analyzer is able to reproduce the exact wavenumber location for each gas.
Absorbance and gas concentration
Absorbance is the extent to which the intensity of a beam of infrared radiation is
decreased on passing through a sample of air in the ACM 150 monitor’s gas cell.
Transmittance is an expression of the intensity (or “radiant power”) of the beam of
infrared radiation passing through the same sample of air. If no gases are present
at a given wavenumber, the transmittance is 1.0 or 100% and the absorbance is 0.
The relationship between absorbance (A) and transmittance (T) is expressed as: A
= - log T Absorbance, as defined by the Beer-Lambert Law, is directly
proportional to the concentration of the absorbing chemical (provided other key
variables are held constant), while transmittance is not directly proportional.
Therefore, absorbance
is used as a basis of measuring the concentration of gases present in the ACM 150
monitor’s air samples. Variables other than concentration that effect absorbance are:
• Path length of the sample cell
• Resolution of the FTIR analyzer
• Temperature and pressure of the sample
• Non-linearity of the direct proportional relationship
• Characteristics of a particular FTIR analyzer and gas cell