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Model 200E Instruction Manual Theory of Operation
upstream pressure, the more gas molecules pass through the orifice. The flow rate of the
gas is also unaffected by small degradations in pump efficiency due to age as long as the
2:1 pressure difference is maintained.
Table 10-3: M200E Gas Flow Rates
Location Purpose Orifice Diameter
Flowrate
(nominal)
Sample gas inlet of
reaction cell Controls rate of flow of sample gas into
the reaction cell.
0.010” (0.25 mm) 500 cm³/min
O
3
supply inlet of
reaction cell.
Controls rate of flow of ozone gas into
the reaction cell.
0.004” (0.10 mm) 80 cm³/min
Dry air return of
Perma Pure
®
dryer
Controls flow rate of dry air return /
purge air of the dryer.
0.004” (0.10 mm) 80 cm³/min
Vacuum manifold ,
auto-zero port.
Controls rate of sample gas flow when
bypassing the reaction cell during the
auto-zero cycle. 0.010” (0.25 mm) 500 cm³/min
Vacuum manifold,
IZS exhaust port
Controls rate of flow of zero purge gas
through the IZS option (when installed
and enabled) when inactive.
0.004” (0.10 mm) 80 cm³/min
Note that the diameter of the critical orifice may change with temperature because of
expansion of the orifice material (ruby) and, hence, the most critical flow orifices in the
M200E are maintained at a constant temperature inside the reaction cell. These are the
sample and O
3
flows. Table 10-3 shows the flow rates for each of the critical flow orifices of
the M200E. 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 M200E
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.
044100102 Rev A 169