Teledyne 9110EH - Nitrogen Oxides Analyzer - Manual (Page 192)

Theory of Operation Model 9110EH Instruction Manual
10.3.1.2. Flash Chip
The flash chip is another, smaller EEPROM with about 64 kb of space, internally labeled as
DOS drive B:\. The M9110EH CPU board can accommodate up to two EEPROM flash chips. The
M200E standard configuration is one chip with 64 kb of storage capacity, which is used to
store the analyzer configuration as created during final checkout at the factory. Separating
these data onto a less frequently accessed chip significantly decreases the chance of data
corruption through drive failure.
In the unlikely event that the flash chip should fail, the analyzer will continue to operate
with just the DOC. However, all configuration information will be lost, requiring the unit to
be recalibrated.
10.3.2. Sensor Module, Reaction Cell & PMT
Electronically, the M9110EH sensor assembly (Figure 10-14) consists of several subassemblies
with different tasks: to detect the intensity of the light from the chemiluminescence reaction
between NO and O
3
in the reaction cell, to produce a current signal proportional to the
intensity of the chemiluminescence, to control the temperature of the PMT to ensure the
accuracy and stability of the measurements and to drive the high voltage power supply that
is needed for the PMT. The individual functions are described individually below, Section
11.6.6 shows the sensor assembly and its components.
10.3.2.1. Reaction Cell Heating Circuit
The stability of the chemiluminescence reaction between NO and O
3
can be affected by
changes in the temperature and pressure of the O
3
and sample gases in the reaction cell. In
order to reduce temperature effects, the reaction cell is maintained at a constant 50° C, just
above the high end of the instrument’s operation temperature range.
Two AC heaters, one embedded into the bottom of the reaction cell, the other embedded
directly above the chamber’s exhaust fitting, provide the heat source. These heaters
operate off of the instrument’s main AC power and are controlled by the CPU through a
power relay on the relay board (Section 10.3.4). A thermistor, also embedded in the bottom
of the reaction cell, reports the cell’s temperature to the CPU through the thermistor
interface circuitry of the motherboard (Section 10.3.5).
10.3.2.2. Photo Multiplier Tube (PMT)
The M9110EH uses a photo multiplier tube (PMT) to detect the chemiluminescence created by
the reaction of NO with O
3
in the reaction cell.
A typical PMT is a vacuum tube containing a variety of specially designed electrodes
(Figure 10-14). Photons from the reaction are filtered by an optical high-pass filter, enter
the PMT and strike a negatively charged photo cathode causing it to emit electrons. A high
voltage potential across these focusing electrodes directs the electrons toward an array of
high voltage dynodes. The dynodes in this electron multiplier array are designed so that
each stage multiplies the number of emitted electrons by emitting multiple, new electrons.
The greatly increased number of electrons emitted from one end of electron multiplier are
collected by a positively charged anode at the other end, which creates a useable current
signal. This current signal is amplified by the preamplifier board and then reported to the
motherboard.
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