Teledyne 9110E - Nitrogen Oxides Analyzer - Manual (Page 193)

Model 9110E Instruction Manual Theory of Operation
M9110E Rev B 179
Figure 10-14: Schematic of Basic PMT Design and Functionality
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 increases random noise (dark noise). The gain
voltage of the PMT used in the M9110E is usually set between 450 V and 800 V. This
parameter is viewable through the front panel as test function HVPS (Section 6.2.1) and
does not need to be changed unless the PMT or the HVPS itself is changed. For information
on when and how to set this voltage, see Section 11.6.5.
The PMT is housed inside the sensor module assembly (Figure 11-7). This assembly also
includes the high voltage power supply required to drive the PMT, an LED used by the
instrument’s optical test function (Section 6.7.5), a thermistor that measures the tempera-
ture of the PMT and various components of the PMT cooling system including the thermo-
electric cooler (TEC).
10.3.2.3. PMT Cooling System
The performance of the analyzer’s PMT is significantly affected by temperature. The warmer
it is, the noisier its signal becomes and significant variations in PMT temperature will render
the concentration signal useless, as it will directly reflect those variations.
The core part of the M9110E PMT cooling system is a thermoelectric cooler (TEC), which
keeps the PMT temperature at a constant, low level (7±2°C) for optimum performance.
Thermoelectric coolers are solid state (semi-conductor) heat pumps, which transfer heat
from one side to the other when a DC current is passed through its circuits. Heat absorbed
at the cold junction (the PMT cold-block) is pumped to the hot junction (the cooling fins) at
a rate proportional to the current passing through the TEC circuit and the number of
junctions. The TEC is just one of many components in the M9110E’s PMT cooling system as
shown in Figure 11-7.
10.3.2.4. TEC Control Board
The TEC control printed circuit assembly is located under the slanted shroud, next to the
cooling fins and directly above the cooling fan. Using the amplified PMT temperature signal
from the PMT preamplifier board, it sets the drive for the thermoelectric cooler. The warmer
the PMT gets, the more current is passed through the TEC.