Teledyne 9110E - Nitrogen Oxides Analyzer - Manual (Page 185)

Model 9110E Instruction Manual Theory of Operation
M9110E Rev B 171
stream, the output of the O
3
generator flows through a special filter between the generator
and the reaction cell.
Any NO
X
that may be produced in the generator (from reaction of O
2
or O
3
and N
2
in the air)
and may cause an artifact in the measurement, is calibrated out through the Auto-zero
functionality, which checks the background signal of the O
3
stream only once per minute.
10.2.8. Ozone Scrubber
Even though ozone is unstable and typically reacts to form O
2
, the break-down is not quite
fast enough to ensure that it is completely removed from the exhaust gas stream of the
M9110E by the time the gas exits the analyzer. Due to the high toxicity and reactivity of O
3
,
a special catalytic ozone scrubber is used to remove all of the O
3
exiting the reaction cell.
Besides its efficient destruction of O
3
, this catalyst does not produce any toxic or hazardous
gases as it only converts ozone to oxygen and it also does not pose an explosion hazard like
charcoal scrubbers.
The O
3
scrubber is located inside the NO
2
converter housing next to the NO
2
converter in
order to utilize residual heat given of by the converter heater. Even though the catalyst is
100% efficient at scrubbing ozone at room temperature, heating it significantly reduces the
necessary residence time (the amount of time the gas must be in contact with the catalyst)
for 100% efficiency and full efficiency can be maintained at higher gas flow rates. As this is
a true catalytic converter, there are no maintenance requirements as would be required for
charcoal-based scrubbers.
A certain amount of fine, black dust may exit the catalyst, particularly if the analyzer is
subjected to sudden pressure drops (for example, when disconnecting the running pump
without letting the analyzer properly and slowly equilibrate to ambient pressure). To avoid
the dust from entering the reaction cell or the pump, the scrubber is equipped with sintered
stainless steel filters of 20 µm pore size on either end and on some models, an additional
dust filter may be attached to the exhaust port.
10.2.9. Flow Rate Control - Critical Flow Orifices
In order to maintain constant flow rates for both the O
3
supply air and the sample gas, the
M9110E uses a variety of critical-orifice flow control assemblies (Figure 9-7) located at
these places in the pneumatic system of the instrument:
• Reaction cell, sample inlet (Figure 9-6)
• Reaction cell, ozone inlet (Figure 9-6)
• Vacuum manifold, AutoZero exit (Figure 10-10)
• Vacuum manifold, IZS exit (if installed (Figure 10-10)
• Permapure ozone air dryer, purge flow control (Figure 10-8)
• Permapure sample or combo dryer (if installed), purge flow control (Figure 10-8)
The most important component of each flow control assembly is the critical flow orifice.
Critical flow orifices are a simple means to regulate stable gas flow rates. They operate
without moving parts by taking advantage of the laws of fluid dynamics. By restricting the
flow of gas through the orifice, a pressure differential is created. This pressure differential,
created by the analyzer’s external pump, draws the gas through the orifice.