Filter
Top Products
Principles Of Operation Model T101 Instruction Manual
264
Figure 10-14. 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 T101 is usually set between 450 V and 800 V. This
parameter is viewable through the front panel as test function HVPS (Section 4.2.1). For
information on when and how to set this
voltage, see Section 9.6.2.
The PMT is housed inside the PMT module assem
bly (Figure 10-13). 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).
10.4.5. PMT COOLING SYSTEM
The performance of the analyzer’s PMT is significantly affected by temperature.
Variations in PMT temperature are directly reflected in the signal output of the PMT.
The signal to noise ratio of the PMT output is radically influenced by temperature as
well. The warmer The PMT is, the noisier its signal becomes until the noise renders the
concentration signal useless. To alleviate this problem a special cooling system exists
that maintains the PMT temperature at a stable, low level
10.4.5.1. Thermoelectric Cooler (TEC)
The core of the T101 PMT cooling system is a solid state heat pump called a
thermoelectric cooler (TEC). Thermoelectric coolers transfer heat from a one side of a
special set of semiconductor junctions to the other when a DC current is applied. The
heat is pumped at a rate proportional to the amount of current applied. In the Model
T101 the TEC is physically attached to a cold block that absorbs heat directly from the
PMT and a heat sink that is cooled by moving air (see Figure 10-15). A Thermocouple
embedded into the cold block generates an analog voltage corresponding to the current
temperature of the PMT. The PMT Preamp PCA conditions and amplifies this signal
then passes it on to the TEC Control PCA
07266B DCN6485