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ACM 150
Permanent calibration
Controlling variables
The ACM 150 monitor eliminates or minimizes these variables so that their effects on
calculating the concentration are insignificant, in the following manner:
•
The path length is fixed, e.g. at 10 M or 5 M. All reference calibrations and
analyses are adjusted in your ACM 150.
•
The resolution of the FTIR analyzer is set at 4 wavenumbers and all reference
calibrations and analyses use the same 4 wavenumber resolution. The FTIR
analyzer resolution can be changed, but it will cause error messages if operated
at a resolution other than 4 wavenumbers.
•
Temperature and barometric pressure changes have a small effect on the
gas concentration and may ignored without affecting accuracy of the ACM
150 monitor. In this installation, the temperature should be nearly constant in
an indoor office-type facility. A cell heater should be added if there is a wide
fluctuation. The potential effect of changes in the absolute "gauge" pressure of
the sampled air is greater than temperature changes.
All reference calibration spectra are collected at 0 psig or 14.7 psia (1.0 atmospheric
pressure). The ACM 150 monitor uses an absolute pressure transducer to assure that
all analyses are done at 0 psig (14.7 psia).
•
The relationship between concentration and absorbance becomes increasingly
non-linear as the concentration increases and approaches saturation (which is
0% transmittance).
•
As the concentration increases from, for example, 300 ppm to 1000 ppm the non-
linearity increases and the accuracy of the reading decreases. The non-linearity
varies for each chemical, for each absorbance wavelength and with cell path
length. For the weakest IR absorbing chemicals and long cell path lengths, the
non-linearity may be insignificant over the range of 0-1000 ppm. For the strongest
IR absorbing chemicals, readings over 100 ppm may be significantly non-linear
and less accurate.
Use of the background spectrum
It is critical that calibrations taken on one ACM 150 are valid for computing accurate
concentrations on any other ACM 150. In that way, calibration spectra are valid for
all ACM 150s and older models as well. This is accomplished by using a
reference(or background) spectrum to which each analysis spectrum is ratioed. In
the ACM 150 monitor a reference spectrum is collected every 2 hours while the gas
cell is filled with nitrogen that is void of any monitored gases. For the next two
hours each air spectrum collected is ratioed to the latest reference spectrum. If the
gas composition of the air were identical to the background, the result would be a
flat line on the x-axis. All variables that effect the spectrum are cancelled in this
manner, because each air spectrum has same characteristics as the reference
spectrum.
This cancels the effects of drift of the FTIR detector, IR source intensity changes,
reduction of IR radiation in the gas cell as particles accumulate, etc. Each of these
variables change slowly and has no effect on the accuracy of the measurement over
a two hour period. This has been verified over years of application experience.
With "instrument error" cancelled by the background reference, the only factor
effecting the amount of absorbance at any wavenumber is the concentration of the
vapor phase chemical that absorbs at that wavenumber. If no gas is present, the
absorbance will be 0, i.e. a flat line on the x-axis. If the gas is present, it will absorb
in direct proportion to its concentration.
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