Teledyne T200, Руководство по эксплуатации, Страница: 289 / 425

Teledyne API
– T200 NO/NO
2
/NO
X
Analyzer Operation Manual Principles of Operation
287
The heated molybdenum reacts with NO
2
in the sample gas and produces a NO gas and
a variety of molybdenum.
Equation 13-4
) 315 (
2
C at O M xNO yMo xNO
z y
   
Once the NO
2
in the sample gas has been converted to NO, it is routed to the reaction
cell where it undergoes the chemiluminescence reaction described in Equation 13-1 and
Equation 13-2.
By converting the NO
2
in the sample gas into NO, the analyzer can measure the total
NO
X
) content of the sample gas (i.e. the NO p the converted NO
2
present). By
switching the sample gas stream in and out of the “moly” converter every 6 - 10
seconds, the T200 analyzer is able to quasi-continuously measure both the NO and the
total NO
X
content.
Finally, the NO
2
concentration is not directly measured but calculated by subtracting the
known NO content of the sample gas from the known NO
X
content.
13.1.4. 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, once every minute for about 8
seconds the T200 diverts the sample gas flow directly to the vacuum manifold without
passing the reaction cell.
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 T200 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 an Auto Zero corrected reading.
06858E DCN7057