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Theory of Operation Teledyne API - Model 200EH/EM Operation Manual
196
Finally, the M200EH/EM requires a steady, high under-pressure, which cannot be achieved reliably over
extended periods of time with small vacuum pumps. The external pump used for the M200EH/EM has a very
long lifetime and duty cycle and provides a very good vacuum for its entire lifetime. However, the pump is too
large to fit into the chassis of the analyzer.
10.3.2. SAMPLE GAS FLOW
The sample gas is the most critical flow path in the analyzer, as the medium has to be routed through a variety
of valves and tubes for the measurement of zero offset and concentrations of both NO and NO
X
(and possibly
the drying of the gas if the optional sample dryer is installed). At any point before and in the reaction cell, the
integrity of the sample gas cannot be compromised.
Sample gas flow in the M200EH/EM analyzer is not a directly measured value, but is rather calculated from the
sample pressure using the flow principle across a critical orifice. In general, the differential pressure ratio
between sample pressure and reaction cell pressure needs to exceed 2:1 to allow critical flow. The actual flow
rate is then only dependent on the size of the orifice and the upstream pressure. Refer to Section 10.3.3 for a
detailed d
escription of the instrument’s method of gas flow rate co
ntrol.
10.3.2.1. NO/NO
x
and AutoZero cycles
For the routing of the sample gas flow, the analyzer uses a variety of valves. The NO/NO
X
valve directs the
sample gas either directly to the reaction cell or through the unit’s NO
2
converter, alternating every ~4 s. The
AutoZero valve directs the sample gas stream to completely bypass the reaction cell for dark noise
measurement once every minute, which is then subtracted as a measurement offset from the raw concentration
signal. The valve cycle phases are summarized in the following table.
Table 10-2: M200EH/EM Valve Cycle Phases
PHASE
NO/ NO
X
VALVE
STATUS
AUTOZERO
VALVE
STATUS
TIME
INDEX
ACTIVITY FIGURE
0 - 2 s
Wait period (NO dwell time).
Ensures reaction cell has been
flushed of previous gas.
NO
Measure
Open to
AutoZero
valve
Open to
reaction cell
2 - 4 s
Analyzer measures chemilumi-
nescence in reaction cell.
Figure 10-10-2
4 – 6 s
Wait period (NOX dwell time).
Ensures reaction cell has been
flushed of previous gas.
NOX
Measure
Open to
NO2
converter
Open to
reaction cell
6 – 8 s
Analyzer measures NO + O3 chemi-
luminescence in reaction cell.
Figure 10-10-2
Cycle repeats every ~8 seconds
0 – 4 s
Wait period (AZERO dwell time).
Ensures reaction cell has been
flushed of sample gas and chemi-
luminescence reaction is stopped.
AutoZero
Open to
AutoZero
valve
Open to
vacuum
manifold
4 - 6 s
Analyzer measures background
noise without sample gas
Figure 10-10-4
Cycle repeats every minute
04521C (DCN5731)