Teledyne 9110TH - Nitrogen Oxides Analyzer - Manual (Page 280)

Model 9110TH NOx Analyzer Principles of Operation
Teledyne Analytical Instruments 260
8.2.3. AUTO ZERO
Inherent in the operation of any PMT is a certain amount of noise. This is due to a
variety of factors such as black body infrared radiation given off by the metal
components of the reaction cell, unit to unit variations in the PMT units and even the
constant universal background radiation that surrounds us at all times. In order to
reduce this amount of noise and offset, the PMT is kept at a constant 7° C (45° F) by a
thermo-electric cooler (TEC).
While this intrinsic noise and offset is significantly reduced by cooling the PMT, it is
not eradicated. To determine how much noise remains, the 9110TH/M diverts the
sample gas flow directly to the exhaust manifold without passing the reaction cell once
every minute for about 5 seconds (Figure 8-4). During this time, only O
3
is present in
the reaction cell, effectively turning off the chemiluminescence reaction. Once the
chamber is completely dark, the 9110TH/M records the output of the PMT and keeps a
running average of these AZERO values. This average offset value is subtracted from
the raw PMT readings while the instrument is measuring NO and NO
X
to arrive at a
auto-zero corrected reading.
O
3
Generator
REACTION CELL:
O
3
only (when gas
flow is diverted
during Auto-Zero)
O
3
Scrubber
Sample
Gas
(All NO
2
converted to NO)
Exhaust
Molybdenum Converter
NO
2
+ Mo NO + Mo
y
O
2
at 315 o C
NO/NO
X
Valve
NO + NOx
Auto-Zero
Valve
Exhaust
(Gas flow alternately diverted
between Exhaust and Reaction
Cell during Auto-Zero)
Figure 8-4: Reaction Cell During the AutoZero Cycle