Teledyne 9110EH - Nitrogen Oxides Analyzer - Manual (Page 181)

Model 9110EH Instruction Manual Theory of Operation
Table 10-2: M9110EH Valve Cycle Phases
Phase NO/ NO
X
Valve
Status
Autozero
Valve
Status
Time
Index
Activity Figure
NO
Measure Open to
AutoZero
valve
Open to
reaction
cell
0 - 2 s Wait period (NO dwell time).
Ensures reaction cell has been
flushed of previous gas.
2 - 4 s Analyzer measures chemilumi-
nescence in reaction cell.
Figure 10-2
NO
X
Measure
Open to
NO
2
converter
Open to
reaction
cell
4 – 6 s Wait period (NO
X
dwell time).
Ensures reaction cell has been
flushed of previous gas.
6 – 8 s Analyzer measures NO + O
3
chemiluminescence in reaction
cell.
Figure 10-2
Cycle repeats every ~8 seconds
AutoZero Open to
AutoZero
valve
Open to
vacuum
manifold
0 – 4 s Wait period (AZERO dwell time).
Ensures reaction cell has been
flushed of sample gas and
chemiluminescence reaction is
stopped.
4 - 6 s Analyzer measures background
noise without sample gas
Figure 10-4
Cycle repeats every minute
The M9110EH uses a 0.010” (10 mil) diameter orifice to achieve a flow rate of about
450-500 cm³/min. With nominal pressures of 28 and 4 in-Hg-A for the sample and reaction
cell pressures, respectively. The necessary ratio of sample to reaction cell pressure of 2:1 is
largely exceeded and accommodates a wide range of possible variability in atmospheric
pressure and pump degradation. Once the pump degrades and vacuum pressure exceeds
14 in-Hg-A, the ratio between sample and vacuum pressures is less than 2:1 and, hence, a
critical flow rate can no longer be calculated. At this point, the instrument will display an
invalid sample flow rate measurement (XXXX).
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 Table 10-2.
10.2.3. Sample Particulate Filter
To remove particles in the sample gas, the analyzer is equipped with a PTFE membrane
filter of 47 mm diameter (also referred to as the sample filter) with a 1 µm pore size. The
filter is accessible through the front panel, which folds down (after removal of the CE Mark
safety screw), and should be changed according to the maintenance schedule in Table 9-1.
M9110EH Rev 0 167