Teledyne 9110T instruction manual PMT Cooling System, Typical Thermo-Electric Cooler

Model 9110T NOx Analyzer Principles of Operation

signal, it is possible to test most of the signal handling and conditioning circuitry on the PMT preamplifier board. See section 12.7.12.2 for instructions on performing this test.

∙The optical test (OTEST) feature causes an LED inside the PMT cold block to create a light signal that can be measured with the PMT. If zero air is supplied to the analyzer, the entire measurement capability of the sensor module can be tested including the PMT and the current to voltage conversion circuit on the PMT preamplifier board. See Section 12.7.12.1 for instructions on performing this test.

13.5.2. 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 utilizing a type of electronic heat pump called a thermo-electric cooler (TEC). A TEC is a solid-state active heat pump which transfers heat from a heat absorbing “cool” side to a heat releasing “hot” side via a series of DC powered semiconductor junctions. The effectiveness of the pump at moving heat away from the cold side is reliant on the amount of current flowing through the semiconductor junctions and how well the heat from the hot side can be removed.

ELECTRICAL CONDUCTOR

HEAT ABSORBING COLD JUNCTION

N P N P N P N P

HEAT RELEASING HOT JUNCTION

+―

ELECTRICAL INSULATOR

DC POWER

SOURCE

Figure 13-23: Typical Thermo-Electric Cooler

In the case of the 9110T, the current flow is controlled by the TEC Control PCA which adjusts the amount of current applied to the TEC based on the temperature sensed by a thermistor embedded in the PMT’s cold block. The higher the temperature of the PMT, the more current is pumped through the TEC. The “hot” side of the TEC is cooled by a constant flow of ambient air that is directed across a set of heat sinks by a fan.

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