Teledyne T200, Руководство по эксплуатации, Страница: 321 / 425

Teledyne API
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Analyzer Operation Manual Principles of Operation
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The PMT preamplifier PCA also operates two different tests used to calibrate and check
the performance of the sensor module.

The electrical test (
ETEST
) circuit generates a constant, electronic signal intended
to simulate the output of the PMT (after conversion from current to voltage). By
bypassing the detector’s actual 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.
Figure 13-20: Typical Thermo-Electric Cooler
In the case of the T200, 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.
06858E DCN7057