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ACM 150
has been thoroughly purged of the last sample and replaced with a fresh sample.
As the ACM 150 monitor samples and analyzes areas, another bypass flow continues
for all areas not currently sampled. This flow is drawn through the Sample Valves by
the Bypass Venturi Pump. The flow rate is controlled and balanced by a flow control
orifice located in each Sample Valve. This assures a continuous flow through all of
the sample tubes to maintain fresh, representative samples of air.
The user may program the sample Areas to be analyzed in any order, and even
have some of the Areas analyzed more than once per complete scan cycle. This
is determined by the Order of Scan. Some areas may also be scanned in groups,
known as Composite Sample Groups. This is done to accelerate the scan cycle.
Refer to
Setup Menus
Background cycle
In order to sustain a permanent calibration for all monitored chemicals, it is
necessary to periodically establish a baseline reference for the infrared spectrum.
This is done by taking a Background or reference spectrum. This spectrum must
not contain any of the measured chemicals. In fact, it should be free of infrared
absorbing gases to the extent that it is practical. For that reason Background gas is
nitrogen.
Once the Background spectrum is taken, the spectra collected for each analysis (the
"scans") are compared to the latest Background. If the air samples were exactly the
same as the Background gas, the result would be a flat baseline along the x-axis
of the ratioed spectrum. But, the gases are never exactly the same, because the
air samples always contain some infrared absorbing gases, such as water vapor
and carbon dioxide. The water vapor in the air will always be higher than the water
vapor in the Background gas (it must be in order for the analysis methods to work
properly). The peaks in the analysis spectrum contain the absorbance peaks of the
chemicals present in the air sample. This includes the normal gases in the air, any
measured chemicals which are currently present in the air sample and any other
chemicals that exist in that air sample ("chemical strangers" with unknown odors,
etc.). The analysis spectrum compared to the Background spectrum results in
the absorption "fingerprint" of all chemicals present in the air samples. This is the
spectrum used for analysis by the analytical methods.
Valve switching and timing
ACM 150 flow cycles identifies the valves energized during each cycle. The flow
schematic for these valves, including Sample Valves 1-10 and Sample Valves 11-
20, 21-30 and 31-40 may be obtained on request. All valves are shown in the de-
energized state. For 3-way valves, the de-energized state is C—common to NO—
normally open, which changes to C—common to NC—normally closed when the
valve is energized. The 2-way valves are normally closed, and energized to open.
for more information.
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