Teledyne 9110EH - Nitrogen Oxides Analyzer - Manual (Page 186)

Theory of Operation Model 9110EH Instruction Manual
As the pressure on the downstream side of the orifice (the pump side) continues to drop,
the speed of the gas flowing through the orifice continues to rise. Once the ratio of
upstream pressure to downstream pressure is greater than 2:1, the velocity of the gas
through the orifice reaches the speed of sound and remains constant, regardless of any
further pressure difference. As long as that ratio stays at least 2:1, the gas flow rate is
unaffected by fluctuations, surges, or changes in downstream pressure because such
variations only travel at the speed of sound themselves and are therefore cancelled out at
the downstream exit of the critical flow orifice.
Figure 10-9: Flow Control Assembly & Critical Flow Orifice
The actual flow rate of gas through the orifice depends entirely on the size and shape of the
aperture in the orifice and the upstream pressure. The larger the hole or the higher the
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: M9110EH 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
M9110EH are maintained at a constant temperature inside the reaction cell. These are the
172 M9110EH Rev 0