Teledyne T200H, Руководство пользователя, Страница: 271 / 353

Teledyne API - Model T200H/T200M Operation Manual Principles of Operation
269
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 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
3
is present in the reaction cell,
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
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
07270D DCN7141