Honeywell ACM 150 Manual To Installation, Operation, And Maintenance page 42

ACM 150
The units of absorbance (A) are divided into milliabsorbance (mA) units, where
1000 mA = 1.0 A. When we calibrate for a chemical and collect that calibration
spectrum, we are determining the number of milliabsorbance units per ppm of
gas concentration for each wavenumber region where that chemical absorbs IR
radiation. Or, if the ACM 150 monitor has a 10 cm cell for % LEL/LFL ranges, the
relation may be expressed as milliabsorbance units per % concentration.
Analytical methods
Methods development
A method is a combination of individual calibration spectra for a group of chemicals.
When we develop a method, we pre-select the absorbance peak regions that will be
used to first identify and then quantify each chemical. A few chemicals have only one
usable absorption peak, while many have six or more usable peaks. The average is
three usable peaks for qualitative screening and for quantitative analysis.
For each absorbance peak region of each monitored chemical in a method, the
relationship between milliabsorbance units and concentration (usually in ppm) is
entered into a p-matrix that is used to compute the concentration of each gas in all
new and unknown air spectra collected by the ACM 150 monitor.
Sometimes chemicals within the same method have absorbance peaks that overlap
in the common peak regions. In the simplest expression of the Beer-Lambert Law,
the resulting absorbance peak is the sum of absorbances of the over-lapping
peaks from each chemical. The matrix uses more calibration data sets from other
wavelength regions to improve on this computation.
The computation accuracy is further improved by the use of advanced data fitting
statistical techniques, known by the broader term of Chemometrics. The technique
of PLS (partial least squares) is employed in the ACM 150 monitor to fit a data set
from the unknown air spectra to the "training" set in the method.
The relationship between the various peak heights in milliabsorbance units and
the ppm concentration is determined and becomes the basis for quantitative
identification of unknown air spectra.
To confirm that the methods correctly identify and accurately measure the
concentrations of their chemicals, the methods are tested using calibration spectra.
This simulates the analysis of unknown air samples using the methods developed to
measure the chemicals of interest.
It proves that the methods correctly identify and measure all of those chemicals, and
it is a valid test of the calibration accuracy.
Of special interest is the ability of the ACM 150 monitor to identify and measure
these chemicals at the alarm set points. Since the reference calibration spectra
were not collected with these set points in mind, the concentrations are not the
same. Nevertheless, the magnitude of the absorption is directly proportional to
the chemical concentration, and the wavenumber region of every absorption peak
is fixed. Therefore, spectra of the chemicals at any level of milliabsorbance units,
including the set point level, will be read accurately. This can be confirmed by
introducing any chemical at or near the set point level in your ACM 150.
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