Teledyne 9110TH - Nitrogen Oxides Analyzer - Manual (Page 296)

Model 9110TH NOx Analyzer Principles of Operation
Teledyne Analytical Instruments 276
flow condition (2:1 pressure ratio) through the sample gas critical flow orifice (Section
8.3.3). If the temperature/pressure compensation (TPC) feature is turned on (Section
8.8.3), the output of this sensor is also used to supply pressure data for that calculation.
The actual pressure value is viewable through the analyzer’s front panel display as the
test function SAMP. The flow rate of the sample gas is displayed as SAMP FLW.
8.3.10.3. Vacuum Pressure Sensor
An absolute pressure transducer connected to the exhaust manifold is used to measure
the pressure downstream from and inside the instrument’s reaction cell. The output of
the sensor is used by the CPU to calculate the pressure differential between the gas
upstream of the reaction cell and the gas downstream from it and is also used as the
main diagnostic for proper pump operation. If the ratio between the upstream pressure
and the downstream pressure falls below 2:1, a warning message (SAMPLE FLOW
WARN) is displayed on the analyzer’s front panel (Section 6.2.2) and the sample flow
rate will display XXXX instead of an actual value. If this pressure exceeds 10 in-Hg-A,
an RCEL PRESSURE WARNING Is issued, even though the analyzer will continue to
calculate a sample flow up to ~14 in Hg.
Also, if the temperature/pressure compensation (TPC) feature is turned on (Section
8.8.3), the output of this sensor is used to supply pressure data for that calculation. This
measurement is viewable through the analyzer’s front panel as the test function RCEL.
8.3.10.4. O
3
Supply Air Flow Sensor
A mass flow meter connected between the Perma Pure ® dryer and the O
3
generator
measures the flow rate of O
3
supply air through the analyzer. This information is used
to validate the O
3
gas flow rate. If the flow rate exceeds ±15% of the nominal flow rate
(250 cm³/min), a warning message OZONE FLOW WARNING is displayed on the
analyzer’s front panel (Section 6.2.2) and the O
3
generator is turned off. As second
warning, OZONE GEN OFF, is displayed. This flow measurement is viewable
through instrument’s front panel display as the test function OZONE FL.
8.3.11. DILUTION MANIFOLD
Certain applications require to measure NO
X
in sample gases that do not contain any
oxygen. However, the molybdenum NO
2
converter requires a minimum amount of
oxygen to operate properly and to ensure constant conversion efficiency. For these
special applications, the analyzer may be equipped with a dilution manifold (Figure
8-13) to provide the instrument with an internal sample stream that contains about 2.5%
O
2
. This manifold is mounted between converter housing and vacuum manifold on a
small mounting bracket. If the dilution manifold is to be mounted in the 9110TH/M
analyzer.
The manifold is equipped with two orifice holders that control the flow of the O
2
-free
sample gas and the bleeds in a small amount of zero air before the combined sample
stream goes to the NO/NO
X
valve for measurement. The zero air is produced by an
external zero air scrubber cartridge, mounted on the rear panel.
The dilution manifold is not temperature controlled, although the residual heat of the
NO
2
converter housing provides some temperature stability. Tight temperature stability
is not critical to the dilution application.