Teledyne 9110TH - Nitrogen Oxides Analyzer - Manual (Page 303)

Troubleshooting & Repair Model 9110TH NOx Analyzer
Teledyne Analytical Instruments 283
A typical PMT is a vacuum tube containing a variety of specially designed electrodes.
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.
Figure 8-18: Basic PMT Design
A significant performance characteristic of the PMT is the voltage potential across the
electron multiplier. The higher the voltage, the greater is the number of electrons
emitted from each dynode of the electron multiplier, making the PMT more sensitive
and responsive to small variations in light intensity but also more noisy (dark noise).
The gain voltage of the PMT used in the 9110TH/M is usually set between 450 V and
800 V. This parameter is viewable through the front panel as test function HVPS (see
Section 6.2.1). For information on when and how to set this voltage, see Section
11.6.3.8.
The PMT is housed inside the PMT module assembly (see Figure 10-18). This
assembly also includes the high voltage power supply required to drive the PMT, an
LED used by the instrument’s optical test function, a thermistor that measures the
temperature of the PMT and various components of the PMT cooling system including
the thermo-electric cooler (TEC).