Teledyne 6200E - Sulfides Analyzer - Manual (Page 223)

Model 6200E Instruction Manual Theory Of Operation
M6200E Rev: A1 223
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 M6200E 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). For information on when and how to set this voltage,
see Section 11.6.3.
The PMT is housed inside the PMT module assembly (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 M6200E 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 6200E 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