Teledyne T200, Руководство по эксплуатации, Страница: 287 / 425

Teledyne API
– T200 NO/NO
2
/NO
X
Analyzer Operation Manual Principles of Operation
285
13.1.2. CHEMILUMINESCENCE DETECTION IN THE T200 REACTION CELL
13.1.2.1. THE PHOTO MULTIPLIER TUBE (PMT)
The T200 uses a special kind of vacuum tube, called a photo-multiplier tube (PMT), to
detect the amount of light created by the NO and O
3
reaction in the reaction cell.
Photons enter the PMT and strike a negatively charged photo cathode causing it to emit
electrons. These electrons are accelerated by an applied high voltage and multiplied
through a sequence of similar acceleration steps (dynodes) until a useable current signal
is generated (see Section 13.5 for a more detailed description). The more light present
(in this case photons given off by the chemiluminescent reaction described above), the
more current is produced. Therefore the more NO present in the reaction cell the more
current is produced by the PMT.
The current produced by the PMT is converted to a voltage and amplified by the
preamplifier board and then communicated to the T200‟s CPU via the A

D converter
circuitry on the analyzer.
13.1.2.2. OPTICAL FILTER
A high pass optical filter, only transparent to wavelengths of light above 645nm, placed
between the reaction cell and the PMT (see Figure 13-1) in conjunction with the
response characteristics of the PMT creates a very narrow window of wavelengths of
light to which the T200 will respond.
PMT HOUSING
PMT
Optical
Filter
NO+O
3
Reaction
Cell
NO
O
3
Figure 13-1: Reaction Cell with PMT Tube and Optical Filter
The narrowness of this band of sensitivity allows the T200 to ignore extraneous light
and radiation that might interfere with the T200‟s measurement. For instance, some
oxides of sulfur can also be chemiluminescent emitters when in contact with O
3
but give
off light at much shorter wavelengths (usually around 260nm to 480nm).
06858E DCN7057