Pmt Cooling System; Thermoelectric Cooler (Tec); Figure 10-14: Basic Pmt Design - Teledyne 100E Instruction Manual

Uv fluorescence so2 analyzer

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Model 100E Instruction Manual

Theory Of Operation

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 M100E 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. Also

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

The core of the M100E 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

100E 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

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