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Teledyne API - Model T200H/T200M Operation Manual
8.2.3. AUTO ZERO
Exhaust
Sample
Gas
07270D DCN7141
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 T200H/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
effectively turning off the chemiluminescence reaction. Once the chamber is completely
dark, the T200H/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
Molybdenum Converter
NO
+ Mo
NO + Mo
2
(Gas flow alternately diverted
between Exhaust and Reaction
Cell during Auto-Zero)
Exhaust
Figure 8-4:
Reaction Cell During the AutoZero Cycle
to arrive at a auto-zero corrected reading.
X
NO/NO
Valve
NO + NOx
o
O
at 315
C
y
2
(All NO
converted to NO)
2
Auto-Zero
Valve
REACTION CELL:
O
only (when gas
3
flow is diverted
during Auto-Zero)
Principles of Operation
is present in the reaction cell,
3
X
O
Generator
3
O
Scrubber
3
269
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