Teledyne 9110EH - Nitrogen Oxides Analyzer - Manual (Page 179)

Model 9110EH Instruction Manual Theory of Operation
10.1.5.3. Light Leaks
The M9110EH sensitivity curve includes a small portion of the visible light spectrum
(Figure 10-3), hence, it is important to make sure than the reaction cell is completely
sealed with respect to light. To ensure this, all pneumatic tubing leading into the reaction
cell is either opaque (vacuum exit tubing) in order to prevent light from entering the cell or
light penetration is prevented by stainless steel filters and orifices (gas entries).
10.2. Pneumatic Operation
CAUTION
It is important that the sample airflow system is leak-tight and not pressurized
over ambient pressure. Regular leak checks should be performed on the analyzer
as described in the maintenance schedule, Table 9-1. Procedures for correctly
performing leak checks can be found in Section 11.5.
10.2.1. Pump and Exhaust Manifold
NOTE
Relative Pressure versus absolute pressure. In this manual vacuum readings are
given in inches of mercury absolute pressure (in-Hg-A), i.e. indicate an absolute
pressure referenced against zero (a perfect vacuum).
The gas flow for the M9110EH is created by an external pump (Figure 10-5) that is pneumati-
cally connected through a 6.4 mm / 0.25” tube to the analyzer’s exhaust port located on
the rear panel (Figure 3-2). This pump creates a vacuum of approximately 5 in-Hg-A at one
standard liter/minute, which is provided to various pneumatic components by a vacuum
manifold located just in front of the rear panel (Figure 3-2 and Error! Reference source
not found.). Gas flow is created by keeping the analyzer’s sample gas inlet near ambient
pressure, usually by means of a small vent installed in the sample line at the inlet, in effect
pulling the gas through the instrument’s pneumatic systems.
There are several advantages to this external pump / pull-through configuration.
• By using an external pump, it is possible to remove a significant source of acoustic
noise and vibration from the immediate vicinity of the sensor. The PMT can act as a
“microphone”, amplifying noise and vibration within the chassis. This is one of the
main reasons, why the M9110EH has an external pump.
• Pumping heats and compresses the sample air, complicating the measurement
process if the pump is upstream.
• Most importantly, however, certain physical parts of the pump itself are made of
materials that might chemically react with the sample gas. Placing the pump
downstream of the reaction cell avoids these problems.
See Section 3.1 for information on connecting and operating the external pump.
M9110EH Rev 0 165