Teledyne 9110EH - Nitrogen Oxides Analyzer - Manual (Page 187)

Model 9110EH Instruction Manual Theory of Operation
sample and O
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flows. Table 10-3 shows the flow rates for each of the critical flow orifices of
the M9110EH. See Appendix B for spare part numbers of these orifices.
In addition to controlling the gas flows, the two critical flow orifices at the inlets of the
reaction cell also maintain an underpressure inside the reaction cell, effectively reducing the
number of molecules in the chamber and therefore increasing the chemiluminescence yield
as the likelihood of third body quenching is reduced (Section 10.1.5.1). The M9110EH
sensitivity reaches a peak at about 2 in-Hg-A, below which the sensitivity drops due to a
low number of molecules and decreased yield in the chemiluminescence reaction.
The other components of the flow control assemblies are:
• A sintered stainless steel filter, which prevents particulates from entering the
reaction cell and potentially plug the orifice. Note that very fine sub-micron particles
may still accumulate on that filter and slowly clog up either the filter or the orifice
over time. See Chapter 11 on trouble-shooting sample flow for more information.
• Two O-rings are located before and after the critical flow orifice to seal the gap
between the walls of the assembly housing and the critical orifice and force all gas
through the orifice opening.
• A spring applies mechanical force to form the seal between the o-rings, the critical
flow orifice and the assembly housing and to prevent the components to float up and
turn on sudden pressure drops.
10.2.10. Pneumatic Sensors
NOTE
The M9110EH displays all pressures in inches of mercury absolute (in-Hg-A), i.e.
absolute pressure referenced against zero (a perfect vacuum).
The M9110EH uses three pneumatic sensors to verify gas streams. These sensors are located
on a printed circuit assembly, called the pneumatic pressure/flow sensor board, located just
behind the sensor assembly.
10.2.10.1. Vacuum Manifold
The vacuum manifold is the central exit port for all analyzer pneumatics. All gas streams of
the analyzer exit through this assembly and connect to the instrument’s pump. Figure 10-
10 shows the standard configuration. Configurations will vary depending on the optional
equipment that is installed. An IZS option, for example, will add another FT8 connector and
orifice assembly to the manifold, an optional sample dryer may add a Tee-fitting so that two
¼” tubes can be connected to the same port shown in the Figure.
At this time, the vacuum manifold does not yet support the orifice holder shown in Figure 9-
7. To exchange the critical orifice installed in the vacuum manifold, the user needs to either
blow the orifice out with reversed pressure or remove the entire manifold for this task.
However, orifices installed in the vacuum manifold should not have to be cleaned under
normal circumstances.
M9110EH Rev 0 173